WO2012089706A1

WO2012089706A1 - Conformal particle therapy system

Info

Publication number: WO2012089706A1
Application number: PCT/EP2011/074048
Authority: WO
Inventors: Damien Prieels; Jonathan HUBEAU
Original assignee: Ion Beam Applications
Priority date: 2010-12-27
Filing date: 2011-12-26
Publication date: 2012-07-05

Abstract

Particle therapy system (100) comprising a charged particle beam (6) generator (3), a beam transport system (4) and a beam delivery system (5) for irradiating the particle beam (6) according to a main beam axis (Z) to a target volume (1) to be treated. The system comprises an energy modulation filter (10), interposed transversally in the beam path between the generator (3) and the target volume (1) and comprising a plurality of individual filtering elements (21, 22, 23,... 31, 32, 33,...) which are arranged according to a two-dimensional (X,Y) grid. At least a first and a second filtering element (21, 22) are arranged according to the X direction and are adapted to generate respective distributions of particle energies at their respective outputs which are totally independent from each other. Furthermore, at least a third and a fourth filtering element (31, 32) are arranged according to the Y direction and are adapted to generate respective distributions of particle energies at their respective outputs which are totally independent from each other. Accordingly, a better depth-conformal irradiation of the target volume (1) can be achieved. Preferably, the therapy system (100) comprises scanning means (40) for scanning the particle beam (6) in the X and Y directions over the individual filtering elements of the energy filter (10). Hence a better lateral-conformal irradiation of the target volume (1) can be achieved.

Description

Conformal particle therapy system Field of the invention The invention relates to a charged particle therapy system.

More particularly, the invention relates to a therapy system for irradiating a target volume within a patient with a charged particle beam, comprising a charged particle beam generator, a beam transport system for transporting the charged particle beam, an irradiation device for delivering the charged particle beam to the target volume, said irradiation device having a main beam axis (Z) and comprising an energy filter placed across the main beam axis (Z), said energy filter comprising a plurality of filtering elements arranged in a transversal plane (XY) to the main beam axis (Z), each filtering element being adapted to deliver a specific distribution of particle energies at its output after being crossed by the charged particle beam.

In the context of the present invention, a "distribution of particle energies at the output of a filtering element" is generally to be understood as a probability density

function of mean particle energies, which function gives, for each mean particle energy value, the ratio of the number of particles having said mean particle energy value at the output of the filtering element to the total number of particles crossing the filtering element.

The invention also relates to a method for irradiating the target volume. Description of prior art

Charged particle therapy systems are well known in the art. Their function is to destroy unwanted cells in a particular 3D region (hereafter "the target volume") of a living being (hereafter "the patient") by irradiating the target volume with a beam of charged particles such as a beam of protons, ions, etc... . There currently exist several irradiation techniques for irradiating the target with the particle beam. These techniques can roughly be classified into scattering techniques and scanning techniques. In the first case, a broad scattered beam is irradiated to the target volume as a whole, whereas in the second case a narrow beam is scanned over the target volume. In the case of the scanning technique, the particle beam is generally scanned according to the X and Y directions.

Whatever the irradiation technique, an aim has always been to reduce unwanted irradiation of cells of the patient lying outside of the target volume, both laterally (X,Y) and in depth (Z) . This aim is often referred to as

"improving conformal irradiation".

For obtaining a certain degree of depth conformity with the target volume (thus in the Z direction), several solutions have been proposed, such as the placement of energy

modulation filters in the path of the particle beam for instance . An example of a therapy system comprising such an energy modulation filter is disclosed in American patent number US-6087672. According to such a known system, the energy modulation filter (sometimes also called "ridge filter" or "energy filter") is placed across the beam path between the beam generator and the target volume. A beam spreading device (sometimes called a "scatterer") , located upstream of the energy filter, spreads the particle beam over the entire surface of the energy filter. The ridge filter is made up of a plurality of elongated filter elements (ridge components) arranged in parallel with each other in the transversal plane, each filter element having a cross section in the form of a staircase. When charged particles pass through such a ridge component, a specific

distribution of particle energies is generated at the output of the ridge component, and this specific energy distribution will result in a corresponding specific

Spread-Out Bragg Peak profile (SOBP) in a corresponding region of the target volume when it is irradiated by the particle beam through the ridge component. As is well known, the distribution of particle energies at the output of a ridge component will depend on the geometry of the ridge component, more particularly on the various widths and heights of its staircase steps. The height of a

staircase step will determine the mean particle energy level at its output, whereas the width of a staircase step will determine the particle ratio. According to a first embodiment of this known ridge filter, the geometry of a ridge component is designed in such a way that a more or less flat SOBP will be obtained in a

corresponding region of the target volume and in such a way that a width of said SOBP corresponds to the thickness of said corresponding region of the target volume in its depth direction (Z) . Additional shields are placed on top of some parts of some of the ridge components in order to reduce the width of the SOPB for a corresponding region of the target volume having a smaller thickness. Said in other words, these shields do cut-off lower energy components in the distribution of particle energies.

According to a second embodiment, the ridge filter

comprises two kinds of elongated ridge components with respectively two different heights. Such geometry allows the generation of SOBPs with two different widths in function of one transversal direction only, namely the direction which is perpendicular to the elongated ridge components, thereby improving the depth conformity with a target volume presenting two different thicknesses in function of said one transversal direction. In this second embodiment, additional shielding elements are placed downstream of the lower height ridge components in order to reduce the height of the corresponding SOPBs, and hence to reduce the dose rates in the corresponding regions of the target volume having smaller thicknesses.

Another example of a known therapy system comprising such an energy modulation filter is disclosed in American patent application number US-2003/0160189.

The ridge filter disclosed in this patent application also comprises a plurality of elongated filter elements (ridge components) arranged in parallel with each other in the transversal plane, each filter element having a cross section in the form of a staircase. Furthermore, a pre- filter is arranged upstream of this ridge filter. Said pre- filter is made up of two perpendicularly arranged patterns of elongated ribs running parallel with each other, thereby defining an (X,Y) grid of pre-filtering elements. When the particle beam crosses such a pre-filtering element, a first distribution of particle energies will appear at the output of the pre-filtering element and said first distribution is then further distributed by the ridge filter in order to obtain a final distribution which more or less corresponds to what is needed to obtain conformity to the shape of target volume.

Although such known systems work well, there is a wish to further improve the conformity of the irradiation to the three-dimensional (3D) shape of the target volume. Summary of the invention

It is therefore an object of the invention to provide a therapy system which is adapted to irradiate the target volume with a better conformity to its three-dimensional shape.

To this end, the therapy system according to the invention is characterised in that the filtering elements are

individually arranged in the transversal plane according to an (X,Y) grid and comprise:

- at least a first filtering element and a second filtering element arranged on said grid along a first direction (X) , said first filtering element being adapted to deliver at its output a first distribution of particle energies which is totally independent of a second distribution of particle energies which the second filtering element is adapted to deliver at its output, and

- at least a third filtering element and a fourth filtering element arranged on said grid along a second direction (Y) which is different from the first direction (X), said third filtering element being adapted to deliver at its output a third distribution of particle energies which is totally independent of a fourth distribution of particle energies which the fourth filtering element is adapted to deliver at its output.

Preferably, the first direction (X) is perpendicular to the second direction (Y) and the (X,Y) grid is an orthogonal grid .

In a known therapy system according to US-6087672 for example, any elongated ridge component, whether or not partially shielded on its top part, presents lower

staircase steps (or slopes) which are geometrically

identical (both in step width and height) for any two locations taken along a direction of elongation of said ridge component. Hence, the two distributions of particle energies outputted at these two different locations will necessarily present identical higher energy components (both in mean energy values - because the step heights are the same - and in particle ratios - because the step widths are the same) . So, even if considering that these two locations correspond to two individual filtering elements, these two filtering elements are not adapted to deliver totally independent distributions of particle energies at their output. The same holds for a known therapy system according to US- 2003/0160189, in which the filtering elements deliver distributions of particle energies energy which are

identical for all X,Y grid positions. With a therapy system according to the invention, the distributions of particle energies respectively delivered by the first and the second filtering elements are totally independent from each other, so that better depth conformity can be achieved to an XZ section of the target volume, whatever the shape of this XZ section.

The same holds for the third and the fourth filtering elements, so that better depth conformity can further also be achieved to an YZ section of the target volume, whatever the shape of this YZ section.

As a result, it becomes possible to achieve a better depth conformation of the irradiation to the 3D shape - and preferably also to the physiological properties - of the target volume, whatever the 3D shape and/or these

physiological properties are.

Preferably, the first filtering element is adapted to deliver at its output a first distribution of particle energies comprising a first particle ratio (PRminl) at a first minimum energy (Eminl) and a second particle ratio (PRmaxl) at a first maximum energy (Emaxl) ,the second filtering element is adapted to deliver at its output a second distribution of particle energies comprising a third particle ratio (PRmin2) at a second minimum energy (Emin2) and a fourth particle ratio (PRmax2) at a second maximum energy (Emax2), the third filtering element is adapted to deliver at its output a third distribution of particle energies comprising a fifth particle ratio (PRmin3) at a third minimum energy (Emin3) and a sixth particle ratio (PRmax3) at a third maximum energy (Emax3), the fourth filtering element is adapted to deliver at its output a fourth distribution of particle energies comprising a seventh particle ratio (PRmin4) at a fourth minimum energy (Emin4) and a eighth particle ratio (PRmax4) at a fourth maximum energy (Emax4), such that PRmaxl is different from PRmax2, and Emaxl is different from Emax2, and PRmax3 is different from PRmax4, and Emax3 is different from Emax4. With such a preferred therapy system, a better conformal irradiation can be achieved in case the target volume presents any kind of varying maximum depths (in the Z direction) both in the XZ plane and in the YZ plane.

More preferably the said first, second, third and fourth filtering elements are adapted to deliver at their

respective outputs respectively the first, second, third and fourth distributions of particle energies such that

(Emaxl - Eminl) is different from (Emax2 - Emin2), and such that (Emax3 - Emin3) is different from (Emax4 - Emin4) . With such a more preferred therapy system, a better

conformal irradiation can be achieved in case the target volume presents any kind of varying thicknesses (in the Z direction) both in the XZ plane and in the YZ plane.

The therapy system according to the invention is preferably characterised in that it further comprises scanning means for scanning the particle beam over the energy filter according to the first direction (X) and according to the second direction (Y) .

With such a preferred therapy system it becomes indeed possible to scan the target volume with the particle beam without the beam going beyond an outer contour of said volume (from a beam-eye point of view) . Hence, sane cells lying outside of this outer contour will not (or less) be irradiated. With prior art systems using a broad beam scattered over the whole target volume, this will not be the case unless additional means are being used to avoid that the beam laterally reaches these sane cells - such as individualized collimators for instance - in which case these additional means present a.o. the drawback of delivering more unwanted neutrons to the target as well as representing additional costs and being more bulky.

Furthermore, when comparing with prior art systems

comprising beam scanning means and irradiating the target layer by layer with a different overall beam energy for each layer (thus without using a ridge filter), this preferred system according to the invention presents the advantage of making it possible to irradiate the target volume in a reduced number of scans, preferably in a single scan. This reduces treatment times and also reduces the negative effects of organ motion in the course of the irradiation .

It is a further object of the invention to provide a therapy system which is capable of achieving more

homogeneous doses distributions in the target volume.

To this end, the therapy system according to the invention is preferably characterised in that it further comprises control means for modulating a particle fluence at a frontal surface of each of the plurality of filtering elements in function of an (X,Y) position of the filtering element in the (X,Y) grid. By "frontal surface", it is to be understood a surface as seen from an incident particle beam ( = surface as seen from a beam-eye view) .

With such a preferred therapy system, it becomes indeed possible to achieve a better dose uniformity in the target volume by controlling the therapy system in such a way that the particle fluence (expressed in number of particles per unit of frontal surface) will be set to values which are in better proportion to the thickness (in the beam direction) and/or to the physiological properties (such as the

sensitivity to the irradiation) of the target volume at each corresponding filtering element. Such a system will for example be capable of irradiating regions in the target volume having a small thickness with a smaller fluence than regions having a larger thickness, thereby achieving better dose uniformity in all regions of the target.

Short description of the drawings

These and further aspects of the invention will be

explained in greater detail by way of example and with reference to the accompanying drawings in which:

Fig. 1 shows a schematic view of a therapy system

according to the invention;

Fig. 2 shows a schematic view of an energy filter of a

therapy system according to the invention;

Fig. 3 shows various possible geometries for a filtering element of the energy filter of Fig.2;

Fig. 4a shows a schematic view of a preferred therapy

system according to the invention;

Fig. shows exemplary dose distributions along various beam directions in an XZ plane when using the therapy system of Fig.4a; Fig. 4c shows exemplary particle energy distributions along various beam directions in said XZ plane when using the therapy system of Fig.4a; Fig. 4d shows another view of the therapy system of Fig.4a;

Fig. 4e shows further exemplary dose distributions along various beam directions in the YZ plane when using the therapy system of Fig.4a;

Fig. 4f shows further exemplary particle energy

distributions along various beam directions in the YZ plane when using the therapy system of Fig.4a; Figs. 5a, 5b and 5c show examples of a preferred filtering element of an energy filter of a therapy system according to the invention, as well as a

corresponding SOBP profile in the target volume; Figs. 6a and 6b show two views of a preferred energy filter of a therapy system according to the invention;

Figs. 7a and 7b show two views of a more preferred energy filter of a therapy system according to the invention.

Unless otherwise indicated, the figures are not drawn to scale. Generally, identical components are denoted by the same reference numerals in the figures. Detailed description of preferred embodiments

Fig. 1 shows a schematic view of a therapy system (100) according to the invention. The system comprises a charged particle beam generator (3) (such as a cyclotron or a synchrotron for instance) for generating a beam of charged particles (such as protons or carbon ion particles or any other type of charged particle), a beam transport system (4) for transporting the charged particle beam from the generator (3) to an irradiation device (5) (sometimes called a nozzle) . The irradiation device (5) has a main beam axis (Z) and is adapted for delivering the charged particle beam (6) in an appropriate form to a target volume (1) within a patient (the patient not shown here) . The system also comprises an energy filter (10) which is transversally interposed in the beam path between the generator (3) and the target volume (1) . In this example, the energy filter (10) is placed between the irradiation device (5) and the patient but it may also be integrated into the irradiation device (5) . A typical distance between the irradiation device (5) and the target volume (1) is around 200 cm, whereas a typical distance between the energy filter (10) and the target volume (1) is around 20 cm (both distances taken along the main beam axis) .

Such a therapy system may apply various target irradiation techniques such as beam scattering, beam wobbling, beam scanning, or other methods. The irradiation device (5) may be mounted on a gantry for rotation of said device about an isocenter or it may be of the fixed beam line type or of any other type. Such systems are well known in the art and will therefore not be described in further detail. Of interest here is the energy filter (10), in the prior art sometimes also called a "ridge filter", which comprises a plurality of individual filtering elements (21, 22, ...) arranged in a transversal plane (XY plane) to the main beam axis (Z) . Preferably, the transversal plane is

perpendicular to the main beam axis (Z) . For the ease of the description, the transversal plane is here considered to be a flat surface, but it must be understood that the transversal plane may also be any other two-dimensional surface. In particular, the transversal plane may be a spherical surface, for example a spherical surface whose radius substantially corresponds to the distance along the main beam axis between the beam source (5) and the energy filter (10) so that the filtering elements are always perfectly facing the beam source (5) .

Such an energy filter (10) has a function of - and is specially designed to selectively reduce the energy of incident particles, so that a specific distribution of particle energies will be present at the output of the energy filter. When these particles penetrate the target volume (1) after having crossed the energy filter (10), a corresponding distribution of Bragg Peaks will be generated in the target volume (1), the combination of which will result in a so called "Spread Out Bragg Peak" (SOBP) of an appropriate shape. In themselves, the function and basic operation of such an energy filter are well known in the art and will therefore also not be described further.

Fig.2 shows a more detailed view of the energy filter (10) of a therapy system according to the invention. As can be seen on this figure, the energy filter (10) comprises a plurality of individual filtering elements (21, 22, 23, 31, 32, 33, ...) which are individually arranged in the transversal plane according to an (X,Y) grid,

preferably on a support plate (15) . For the sake of clarity, not all filtering elements are shown on the figure. Preferably, as shown in this example, the X and Y directions are perpendicular to each other and the

filtering elements are arranged according to an orthogonal grid (shown with dotted lines), but other arrangements may of course also be used, such as non-orthogonal grids for example .

Taking into account the direction of the incident particle beam (6) (indicated by a set of arrows on the left side of the figure), the skilled person will understand that each filtering element will correspond to a particular region in the target volume (1) . For a given filtering element (23), the corresponding region (60) in the target volume (1) is that part of the target volume (1) which will be irradiated with those particles having passed through said filtering element ( 23 ) . Each individual filtering element has a specific geometry which is adapted to deliver a specific distribution of particle energies at its output after being crossed by the charged particle beam (6) . A corresponding specific dose distribution (SOBP profile) in the depth direction (Z direction) will hence be obtained in the corresponding region of the target volume.

The filtering elements more particularly comprise:

- at least a first filtering element (21) and a second filtering element (22) arranged on said grid along a first direction (X), said first filtering element (21) being adapted to deliver at its output a first distribution of particle energies which is totally independent of a second distribution of particle energies which the second

filtering element (22) is adapted to deliver at its output, and

- at least a third filtering element (31) and a fourth filtering element (32) arranged on said grid along a second direction (Y) which is different from the first direction (X), said third filtering element (31) being adapted to deliver at its output a third distribution of particle energies which is totally independent of a fourth

distribution of particle energies which the fourth

filtering element (32) is adapted to deliver at its output.

Fig. 3 shows various possible three-dimensional ("3D") shapes for an individual filtering element (21, 22, 23, 31, 32, 33, ...) of the energy filter (10) of Fig.2. As shown on Fig.3, an individual filtering element (23) may for example have the shape of a 3D pyramid, or of a 3D staircase, or of a 3D cone, each of which having either stepped or continuous lateral slopes. A filtering element (23) may also have a more complex 3D shape, such as the shape shown in the bottom right part of Fig.3 for example.

For a given 3D shape of a filtering element, for example a 3D pyramid, the detailed geometrical dimensions ("the geometry") of said filtering element, such as the number of steps as well as the height and the width (frontal surface) of each step, are determined in advance in function of the desired SOBP profile in the corresponding region (60) of the target volume (1) which is to be irradiated and hence in function of the desired energy distribution at the output of said filtering element. To this end, an

analytical transfer function of a filtering element may be used and an optimization loop may make several iterations with this transfer function until obtaining the desired SOBP profile in the corresponding region of the target volume (1) . Such methods are known from the skilled person for calculating the known elongated ridge filters for example.

Accordingly, to each region (60) in the target volume (1) is associated a corresponding specific filtering element with a corresponding specific geometry. Having determined the specific geometries of all individual filtering

elements, a specific energy filter (10) dedicated to a specific target volume (i.e. to a specific patient) can be built, for example by stereolithography or by selective laser sintering. As will now be clear, when the target volume (1) has a shape (thickness and/or depth in the direction of the beam) and/or physiological properties which vary both according to the X and Y position, the respective geometries of the corresponding filtering elements will also vary accordingly and both according to the X and Y position of the filtering element on the (X,Y) grid.

Preferably, the energy filter (10) has lateral dimensions (in the XY plane) which substantially correspond to a frontal surface of the target volume (1) . For a target volume having for example maximum outer dimensions of 10cm x 10cm x 10cm (according to X, Y, Z), the energy filter may for example have overall outer dimensions of 10cm x 10cm (according to X, Y) .

The number of filtering elements arranged in the

transversal plane as well as their respective dimensions may be freely chosen and will depend on the required accuracy of the depth conformity. Examples of dimensions of filtering elements will be given herein below.

Fig. 4a shows a more detailed schematic view of a therapy system (100) according to the invention. It substantially comprises the same components as the system of Fig. 1, including an energy filter (10) as described in relation to Fig.2 and Fig.3, Fig.4a more particularly shows a cross-section of a

particular target volume (1) in an XZ plane, as well as a corresponding cross-section of the energy filter (10) in the same XZ plane. In this XZ plane, the particle beam (6) may for instance follow a first beam direction (Zlx) intercepting a first region of the target volume (1) delimited in depth by two first points (Alx, Blx) . A corresponding first filtering element (21) has a geometry which is designed to produce a first SOBP (SOBP-Zlx) whose profile (essentially width, height and depth position) substantially corresponds to a desired dose distribution in said first region when the particle beam (6) follows the first beam direction (Zlx) . The desired dose profile along the first beam direction (Zlx) as well as the desired first SOBP (SOBP-Zlx) is shown in the graph of Fig. 4b in which the horizontal axis Zix represents a beam direction such as Zlx (or Z2x or Z3x : see hereafter) .

The desired first distribution of particle energies to be produced at the output of the first filtering element (21) is schematically shown in Fig. 4c wherein the horizontal axis indicates mean particle energies (E) on a linearly graduated scale (tick marks) and wherein the vertical axis indicates the ratio (PR) of the number of particles having a mean particle energy at the output of the filtering element to the total number of particles crossing the filtering element.

As shown on Fig. 4c, said first energy distribution

comprises a first particle ratio (PRminl) at a first minimum energy (Eminl) and a second particle ratio (PRmaxl) at a first maximum energy (Emaxl) . The first minimum energy (Eminl) and the first maximum energy (Emaxl) respectively correspond to the depth of first point (Alx) and to the depth of the second point (Blx) in the target volume (1) .

From this desired first distribution of particle energies, the specific geometry of the first filtering element (21) can be designed according to known methods as already explained previously. In case the first filtering element has the shape of a stepped pyramid or of a stepped

staircase for example (see Fig.3), the frontal surface and height of the lowest step of the staircase will

respectively correspond to the second particle ratio

(PRmaxl) and to the first maximum energy (Emaxl) .

Figs. 4a, 4b and 4c also show a second filtering element (22) and a further filtering element (23) arranged in the X direction on the (X,Y) grid and the corresponding desired dose profiles and SOBPs ( S0BP-Z2x, S0BP-Z3x) as well as desired second and further energy distributions when the particle beam (6) respectively follows a second beam direction (Z2x) or a further beam direction (Z3x) in the XZ plane. As can be seen on Fig. 4c, the second distribution of particle energies comprises a third particle ratio

(PRmin2) at a second minimum energy (Emin2) and a fourth particle ratio (PRmax2) at a second maximum energy (Emax2) . The second minimum energy (Emin2) and the second maximum energy (Emax2) respectively correspond to the depth of another first point (A2x) and to the depth of the another second point (B2x) in the target volume (1) .

From these desired second and further distributions of particle energies, the geometries of the second and further filtering elements can be designed.

In case the second filtering element has the shape of a stepped pyramid or of a stepped staircase for example (see Fig.3), the frontal surface and height of the lowest step of the staircase will respectively correspond to the fourth particle ratio (PRmax2) and to the second maximum energy ( Emax2 ) .

Fig.4d shows another view of the therapy system (100) of Fig.4a. Fig.4d more particularly shows a cross-section of the same target volume (1) in an YZ plane, as well as a corresponding cross-section of the energy filter (10) in the same YZ plane. By analogy with Figs.4a, 4b and 4c, Figs. 4d, 4e and 4f show a third (31), a fourth (32) and a further (33) filtering element arranged in the Y direction on the (X,Y) grid and the corresponding desired dose profiles and SOBPs ( SOBP-Zly, SOBP-Z2y, SOBP-Z3y) as well as the desired energy distributions when the beam

respectively follows three further beam directions (Zly, Z2y, Z3y)in the YZ plane.

From Fig. 4f, one can see that the third energy

distribution to be produced by the third filtering element (31) comprises a fifth particle ratio (PRmin3) at a third minimum energy (Emin3) and a sixth particle ratio (PRmax3) at a third maximum energy (Emax3) and that the fourth energy distribution to be produced by the fourth filtering element (32) comprises a seventh particle ratio (PRmin4) at a fourth minimum energy (Emin4) and a eighth particle ratio (PRmax4) at a fourth maximum energy (Emax4) . From these desired distributions of particle energies, the geometries of the third, fourth and further filtering elements can be designed.

In case the third filtering element has the shape of a stepped pyramid or of a stepped staircase for example (see Fig.3), the frontal surface and height of the lowest step of the staircase will respectively correspond to the sixth particle ratio (PRmax3) and to the third maximum energy ( Emax3 ) .

In case the fourth filtering element has the shape of a stepped pyramid or of a stepped staircase for example (see Fig.3), the frontal surface and height of the lowest step of the staircase will respectively correspond to the seventh particle ratio (PRmax4) and to the fourth maximum energy (Emax4) .

As can be seen from the example on Fig.4a and Fig.4d, the target volume presents varying maximum depths (in the beam direction) and varying physiological properties (not shown), both in an XZ plane (points Blx, B2X, B3x) and in an YZ plane (points Bly, B2y, B3y) .

Hence the first (21), second (22), third (31) and fourth (32) filtering elements are preferably designed in such a way that PRmaxl is different from PRmax2, and Emaxl is different from Emax2, and PRmax3 is different from PRmax4, and Emax3 is different from Emax4.

As can also be seen from Fig.4a and Fig.4d, the target volume may present varying thicknesses (in the beam direction) both in an XZ plane and in an YZ plane.

Hence the first (21), second (22), third (31) and fourth (32) filtering elements are preferably designed in such a way that (Emaxl - Eminl) is different from (Emax2 - Emin2), and that (Emax3 - Emin3) is different from (Emax4 - Emin4) .

Preferably, the therapy system according to the invention comprises scanning means (40) for scanning the particle beam (6) over the filtering elements (21, 22, 23, ... , 31, 32, 33, ...)of the energy filter (10) according to the first direction (X) and according to the second direction (Y) . Such scanning means (40) are well known in the art and may for example comprise electromagnets placed around the beam line for deviating the particle beam (6) in the X and Y directions .

Hence, when scanning a particle beam (6) having for example a fixed energy over the individual filtering elements (21, 22, 23, ... , 31, 32, 33, ...) of the energy filter (10), a depth-conformal irradiation of the target volume can be achieved, preferably in a single scan (i.e. a scan wherein the beam passes only once over each filtering element) . Lateral conformity to the target volume (1) will preferably be achieved by scanning the particle beam (6) exclusively over an area of the energy filter (10) which corresponds to a projection of the target volume (1) on the transversal plane (XY) according to the beam directions. Said

projection is represented by a dotted line (80) in the transversal plane in Fig.2.

Preferably, the therapy system (100) further comprises control means (50) for modulating a particle fluence at a frontal surface of each of the plurality of filtering elements in function of a position ((X,Y) grid coordinates) of the filtering element in the transversal plane. By "frontal surface" of a filtering element, it is to be understood a surface of the filtering element as seen from an incident particle beam (6) (i.e. from a beam-eye view of the filtering element) .

By "particle fluence at a surface", it is to be understood a number of particles going through said surface per unit of surface.

With such a preferred system, one can indeed selectively adjust the dose delivered to each region of the target volume (1), with a view to obtain an as homogeneous dose distribution as possible in the target volume. For regions having a small thickness in the beam direction, the control means (50) may for example control the therapy system (100) for achieving a small fluence at a frontal surface of a corresponding filtering element and vice versa. In the system as illustrated in Figs. 4a to 4f, the control system may for example control the therapy system (100) for obtaining a smaller fluence at a frontal surface of the first filtering element (21) than at a frontal surface of the second filtering element (22) because the first

thickness (AlX-Blx) is smaller than the second thickness (A2x-B2x) . For modulating the particle fluence at a frontal surface of a filtering element, the control system may for example act on a beam exposure time of said filtering element and/or on the beam intensity to which said filtering element is exposed. In the system as illustrated in Figs. 4a to 4f, the control system may for example control the therapy system (100) for irradiating the first filtering element (21) with the particle beam (6) for a shorter time and/or with a lower beam intensity compared to the second filtering element (22) . Controlling the exposure time can for instance be achieved by controlling the scanning means (40) and by modulating the dwell time or the scanning speed of the beam. Controlling the beam intensity can for

instance be achieved by controlling the generator (3) (the charged particle source) .

Preferably, the therapy system (100) further comprises a beam position detector (70) downstream of the scanning means (40) for detecting a lateral (Χ',Υ') beam position in the course of the beam scanning, and the control means (50) comprises an input from said beam position detector (70) for receiving lateral beam position information in function of time and means for synchronizing the fluence modulation with the lateral position of the beam.

Advantageously, all filtering elements present

substantially the same frontal surface, the therapy system (100) comprises means for creating a particle beam (6) whose size at a location of the energy filter (10)

substantially corresponds to said frontal surface, and the scanning means (40) are spot scanning means configured for positioning the particle beam (6) laterally over the frontal surface of each filtering element. By "spot

scanning means", it must be understood means which are adapted to irradiate a spot of a target with the particle beam (6) for a certain amount of time, to preferably switch-off the beam after such time has elapsed, to move the beam to another spot on the target while the beam is preferably switched off, and to repeat the preceding steps until all spots of the target have been irradiated. In themselves, spot scanning means are well known in the art and will therefore not be described further. In such a preferred system, a base of a pyramidal filtering element (23) - such as shown in Fig.3 or in Fig.5a for example - may for example have a diameter of 5 mm for a Gaussian beam spot size of 5mm at the frontal surface of the filtering element; the steps of the pyramid may for example have a few tenths of a millimetre in relative width (indicated by "Bi" in Fig.5a) and a few millimetres to a few centimetres in relative height (indicated by "Hi" in Fig.5a) . A pyramidal filtering element (23) may therefore for example have a total height between 1cm and 10cm

(depending on the number of steps) .

With such a spot scanning system, the particle fluence is preferably modulated by the control means (50) by acting on the irradiation time (or exposure time) of each spot, i.e. of each corresponding filtering element.

Preferably, at least one filter element (23) presents at least one intermediate step (23i) having a step height (Hi) which is substantially larger than each of the step heights (HI, H2, H3,...) of the other steps of said at least one filtering element (23) .

Preferably, the intermediate step (23i) has a step height (Hi) which is larger than one and a half times, more preferably two times and even more preferably tree or four or five times the step heights (HI, H2, H3,...) of the other steps .

An example of such a preferred filter element is shown in Figs.5a for a case in which the filtering element (23) has the shape of a 3D pyramid. In this example, one

intermediate step (23i) has a step height which is larger than each of the step heights of the other steps. Such a preferred geometry for a filtering element will generate two distinct SOBPs (SOBP-1 and SOBP-2) in the corresponding region of the target volume (1) when crossed by the

particle beam (6), as illustrated in Fig. 5b. Such a double SOBP profile is adequate for example for target volumes presenting a concave shape in the beam direction (Zl) . It will be clear that further intermediate steps of a

filtering element may also have a step height which is substantially larger than each of the step heights of the other steps (other intermediate steps excluded of course) . Such geometry will for example be appropriate when the target volume (1) presents multiple concavities in the beam direction and when one therefore wants to have more than two distinct SOBPs.

Fig.5c illustrates a few examples of cross sections of such a particular filter element in case the filter element has one of the other shapes illustrated in Fig.3 (shapes with continuous slopes) . The distinctive structural feature of the preferred filtering element appears more clearly here, by the presence of a discontinuity in an intermediate portion of the lateral slope of the filtering element.

Preferably, the therapy system (100) according to the invention further comprises a range compensator (75)

(sometimes referred to as a "bolus") placed across the main beam axis (Z) . In such a case, both the energy filter (10) and the range compensator (75) will contribute to the energy filtering in a cumulative manner in the beam

direction. The energy filter (10) may in such a case be designed for obtaining a desired distribution of particle energies and the range compensator (75) may be designed for further reducing the energy of all particles of a given energy distribution (i.e. of those particles having passed through a given filtering element) by a desired amount. More preferably, the energy filter (10) and the range compensator (75) are forming a single assembly. The energy filter (10) may for example be removably or not removably attached to the range compensator (75), as shown in Figs. 6a and 6b. This facilitates the manipulation and the installation of the assembly in the therapy system (100) . Alternatively, the energy filter (10) and the range

compensator (75) are more preferably forming a single continuous component made up in a single material, as shown in Figs. 7a and 7b, which facilitates manufacturing and improves accuracy. Such a single continuous component may for instance be manufactured by stereolithography or by selective laser sintering.

A set of standardized energy filters may be provided for the treatment of standard target volume shapes.

Nevertheless, the energy filter (10) according to the invention, whether with or without range compensator (75), is preferably tailor made for each specific patient (i.e. for each specific target volume) to be treated with the therapy system according to the invention. By "tailor made", it must be understood that the geometries of the individual filtering elements as well as their arrangement in the transversal plane (e.g. on the support plate) is tailor made according to the specific 3D geometry of the target volume of a particular patient. Preferably, at least the first (21), the second (22), the third (31) and the fourth (32) filtering elements are each entirely made up in a solid material such as Nylon® for example . The invention also concerns a method for irradiating a target volume (1) within a patient with a charged particle beam (6), said method comprising the steps of:

- providing a therapy system (100) as described

hereinabove and comprising the scanning means (40),

- providing a particle beam (6) at a frontal surface of the energy filter (10), and

- controlling the therapy system (100) for delivering a prescribed dose to the target volume (1) in at least one scan of the particle beam (6) over the energy filter (10) . Such method results in better conformal irradiation to the target volume (1) compared to known methods using known energy filters or using layered scannings.

Preferably, the therapy system (100) is controlled for delivering a total prescribed dose to the target volume (1) in a single scan of the particle beam (6) over the energy filter (10) .

Such a preferred method results in shorter treatment times and better conformal irradiation to the target volume compared to known methods using known energy filters or using layered scannings.

Preferably, the step of controlling the therapy system for delivering a prescribed dose to the target volume in a scan of the particle beam over the energy filter comprises the step of modulating a time during which the particle beam (6) is positioned over each filtering element (21, 22, 23, 31, 32, 33, ...) by the scanning means (40). Modulating such time may for example be performed

indirectly by time monitoring an accumulated dose at a given filtering element and moving the beam to a next filtering element once the accumulated dose has reached a prescribed dose at the given filtering element.

By modulating said time, one can adapt the dose delivered to each region of the target volume (1) and hence optimize uniformity of the dose distribution in said target volume. The present invention has been described in terms of specific embodiments, which are illustrative of the

invention and not to be construed as limiting. More

generally, it will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and/or described hereinabove. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features .

Reference numerals in the claims do not limit their

protective scope.

Use of the verbs "to comprise", "to include", "to be composed of", or any other variant, as well as their respective conjugations, does not exclude the presence of elements other than those stated.

Use of the article "a", "an" or "the" preceding an element does not exclude the presence of a plurality of such elements .

Summarized, the invention may also be described as follows: a particle therapy system (100) comprising a charged particle beam (6) generator (3), a beam transport system (4) and a beam delivery system (5) for irradiating the particle beam (6) according to a main beam axis (Z) to a target volume (1) to be treated. The system comprises an energy modulation filter (10), interposed transversally in the beam path between the generator (3) and the target volume (1) and comprising a plurality of individual filtering elements (21, 22, 23,... 31, 32, 33,...) which are arranged according to a two-dimensional (X,Y) grid. At least a first and a second filtering element (21, 22) are arranged according to the X direction and are adapted to generate respective distributions of particle energies at their respective outputs which are totally independent from each other. Furthermore, at least a third and a fourth filtering element (31, 32) are arranged according to the Y direction and are adapted to generate respective

distributions of particle energies at their respective outputs which are totally independent from each other.

Accordingly, a better depth-conformal irradiation of the target volume (1) can be achieved. Preferably, the therapy system (100) comprises scanning means (40) for scanning the particle beam (6) in the X and Y directions over the individual filtering elements of the energy filter (10) .

Claims

1. Therapy system (100) for irradiating a target volume (1) within a patient with a charged particle beam (6), comprising

- a charged particle beam (6) generator (3),

- a beam transport system (4) for transporting the charged particle beam (6),

- an irradiation device (5) for delivering the charged particle beam (6) to the target volume (1),

said irradiation device (5) having a main beam axis (Z) and comprising an energy filter (10) placed across the main beam axis ( Z ) ,

said energy filter (10) comprising a plurality of filtering elements arranged in a transversal plane (XY) to the main beam axis (Z), each filtering element being adapted to deliver a specific distribution of particle energies at its output after being crossed by the charged particle beam (6),

characterized in that said filtering elements are

filtering element (22) is adapted to deliver at its output, and

distribution of particle energies which the fourth

filtering element (32) is adapted to deliver at its output.

2. Therapy system (100) according to claim 1, in which

- the first distribution of particle energies comprises a first particle ratio (PRminl) at a first minimum energy

(Eminl) and a second particle ratio (PRmaxl) at a first maximum energy (Emaxl),

- the second distribution of particle energies comprises a third particle ratio (PRmin2) at a second minimum energy (Emin2) and a fourth particle ratio (PRmax2) at a second maximum energy (Emax2),

- the third distribution of particle energies comprises a fifth particle ratio (PRmin3) at a third minimum energy (Emin3) and a sixth particle ratio (PRmax3) at a third maximum energy (Emax3),

- the fourth distribution of particle energies comprises a seventh particle ratio (PRmin4) at a fourth minimum energy (Emin4) and a eighth particle ratio (PRmax4) at a fourth maximum energy (Emax4), characterized in that

PRmaxl is different from PRmax2, and

Emaxl is different from Emax2, and

PRmax3 is different from PRmax4, and

Emax3 is different from Emax4;

3. Therapy system (100) according to claim 2, characterized in that (Emaxl Eminl) is different from (Emax2

Emin2), and (Emax3 Emin3) is different from (Emax4

Emin4) .

4. Therapy system (100) according to any preceding claim, characterized in that it further comprises scanning means (40) for scanning the particle beam (6) over the filtering elements of the energy filter (10) according to the first direction (X) and according to the second direction (Y) ;

5. Therapy system (100) according to claim 4,

characterized in that it further comprises control means (50) for modulating a particle fluence at a frontal surface of each of the plurality of filtering elements in function of an (X,Y) position of the filtering element in the (X,Y) grid;

6. Therapy system (100) according to any of claims 4 to 5, characterized in that all filtering elements

substantially present the same frontal surface (S), in that the therapy system comprises means for creating a beam whose size at a location of the energy filter (10)

substantially corresponds to said frontal surface (S), and in that the scanning means (40) are spot scanning means adapted for positioning the particle beam (6) laterally over the frontal surface (S) of each filtering element;

7. Therapy system (100) according to claim 6,

characterized in that the control means (50) are adapted to control individually an irradiation time of each spot;

8. Therapy system (100) according to any of preceding claims, characterized in that at least one filter element presents at least one intermediate step having a total step height (Hi) which is substantially larger than each of the total step heights (HI, H2, H3,...) of the other steps of said at least one filtering element;

9. Therapy system (100) according to any of preceding claims, characterized in that the system further comprises a range compensator placed across the main beam axis (Z);

10. Therapy system (100) according to claim 9,

characterized in that the energy filter (10) and the range compensator are forming a single assembly;

11. Therapy system (100) according to claim 9,

characterized in that the energy filter (10) and the range compensator are forming a single continuous component made up in a single material;

12. Therapy system (100) according to any of preceding claims, characterized in that at least the first (21), the second (22), the third (31) and the fourth (32) filtering elements are each entirely made up in a solid material;

13. Method for irradiating a target volume (1) within a patient (2) with a charged particle beam (6), comprising the steps of :

- providing a therapy system (100) according to anyone of claims 4 to 11,

- controlling the therapy system (100) for delivering a prescribed dose to the target volume (1) in at least one scan of the particle beam (6) over the energy filter (10);

14. Method according to claim 12, wherein the therapy system (100) is controlled for delivering a total

prescribed dose to the target volume (1) in a single scan of the particle beam (6) over the energy filter (10);

15. Method according to any of claim 12 or 13, wherein the step of controlling the therapy system for delivering a prescribed dose to the target volume in a scan of the particle beam over the energy filter comprises the step of modulating a time during which the particle beam (6) is positioned over each filtering element by the scanning means ( 40 ) .