SE518811C2 - Radiation detector for calibrating and verifying radiosurgical equipment, contains several detector elements each producing individual induced currents for creating multi dimensional radiation dose distribution - Google Patents

Abstract

The detector (20) includes several adjacent detector elements each defining a limited detection area and each being configured to produce an individual current dependent upon the radiaiton dose in each respective area. A means (34-36) is provided for measuring each individual current to obtain data used to create an expression over the dose distribution in at least two dimensions. A radiation detector unit for determining the radiation dose distribution or rate from at least one radiation source (2) comprises a detector in which a current induced by the radiation source is dependent on the dose, and a means for measuring the induced current. The detector includes several adjacent detector elements each defining a limited detection area and each being configured to produce an individual current dependent upon the radiaton dose in each respective area. A means is provided for measuring each individual current to obtain data used to create an expression over the dose distribution in at least two dimensions. Independent claims are also included for (a) a radiation dose detection system for verification or calibration of radiosurgical equipment, comprising one or more radiation sources, a control system, a means for registering radiation doses, a means for combining the doses to generate a dose distribution, and the above detector unit, and (b) a method for generating a three-dimensional radiation dose distribution from at least one radiation source. INSTRUMENTS AND TESTING - CLAIMED METHOD: The detector unit is moved from a starting position to a region near the radiation source, where it is exposed to radiation from the source for a given time period, and then the unit is returned to its starting position. A set of radiation dose values are built up for a number of detection areas and combined into a three-dimensional distribution.

Description

20 25 30 518 811 2 collimator helmet. The helmet is best described as a substantially hemispherical shell with 201 positions, one sat each beam. The collimators in the helmet can be replaced with blind plugs that prevent special rays from reaching the patient, to, for example, prevent the eyes from being irradiated.

The stereotactic frame, which is mechanically attached to the patient's skull, is mounted on the inside of the helmet by means of two adjusters. The position of the frame is adjusted so that the preselected target point exactly coincides with the intersection point when the couch is in the treatment position.

Depending on the nature of the radiosurgical treatment described above, the calibration of the dose rate at the intersection point and the verification of the spatial dose spread are very important.

The calibration of the dose rate is performed by means of a small ionization chamber centrally located in a spherical polystyrene phantom, 160 mm in diameter, which simulates a human head.

The verification of the spatial dose distribution is normally performed by using the polystyrene phantom, in which an mlm is used as a detector. The phantom, with the strap arranged in the middle, is fixed to the stereotactic frame and moved to the treatment position, where the strap is exposed to radiation for a predetermined period of time. The phantom is removed from the radiation sources, after which the film is developed. To determine the dose spread in a two-dimensional plane, the optical density of the developed film is then measured by a laser.

The only way to obtain a dose spread in three dimensions is to repeat the procedure of exposing a ch in different places relative to the point of intersection. A common way to do this is to rotate the phantom, and thus the plane, around two vertical axes in the collimator helmet, thereby simplifying the measurement of the spatial dose spread in another plane, see Fig. 2.

Measurement of more than two strips to obtain a dose spread in each plane is not feasible due to the time consuming procedure of each strip. Furthermore, the limited dynamic range regarding the film density is a problem.

Another way to detect spatial doses is to use a detector with a diamond as a detector medium, which is unfortunately expensive. This detector has, however, a good spatial resolution due to a high signal-to-volume ratio. This allows a small sensitive volume. In addition, diamond is crystalline carbon and therefore ideal in terms of energy response and dispersion properties. In other radiation detection areas, for example in mammography, another type of detector is used, which consists of a silicon diode. The detector limits a detection area, where a current is induced by the radiation. This current depends on the radiation dose.

SUMMARY OF THE INVENTION The object of the invention is to provide a faster way of obtaining a more accurate expression of dose scattering in two or three dimensions when measuring radiation doses from at least one radiation source.

Another object of the invention is to provide a measuring system which can be used both in determining the spatial dose spread and the dose rate.

According to the invention, these tasks have been solved by an inventive radiation detector device, comprising a detector, wherein the detector is divided into more than one element, each defining a limited detection area. The elements are arranged next to each other in a pattern. An individual current in each element is induced and is dependent on the radiation dose in each limited detection range. Each current is measured separately and fed into a computer, where the radiation dose can be presented immediately and a dose spread is displayed when a sufficient number of currents have been measured.

A feature of said detector is that it comprises a number of detector elements placed close to each other in a pattern in one, two or three dimensions, preferably in the form of a matrix, which provides an opportunity to determine a dose spread within the matrix without displacing or replacing the detector. .

The radiation detector device may further comprise means for moving the detector, preferably high-precision stepper motors. By using the stepper motors to move the detector closer to the radiation source, a dose spread can be determined which is greater than the volume which the detector itself comprises in its stationary position. The detector can be superficial in one, two or three dimensions.

The radiation detector device can be implemented in a phantom made of an outer shell, filled with a liquid, preferably water. By implementing the detector in said phantom, a more accurate measurement can be made, since the phantom provides a very good simulation of a mermaid shoe head.

The detector can be manufactured in a semiconductor material, preferably silicon-based, wherein the detector element is implemented in the semiconductor material. The detector element may be some kind of semiconductor component required to detect a radiation-induced current, preferably a diode.

An advantage of the present invention is that a faster determination of a dose spread in two or three dimensions can be made.

Another advantage is that the generated dose spread is more accurate due to the division of the detector into a plurality of detector elements with a limited detector range.

Another advantage is that the same measuring system can measure the spatial dose spread and the dose rate. Yet another advantage is that the present invention can be implemented within a phantom filled with water, which is recommended by national and international radiological committees for dosimetric calibration of radiological medical equipment.

The above and further objects, features and advantages of the present invention will become apparent from the detailed description in which a preferred embodiment is described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1a shows a cross section of a gamma knife at rest.

Figure 1b shows a cross section of a gamma knife in treatment position.

Figure 2 shows a schematic view of a central body of a yarn knife, with a radiation detector according to the prior art.

Figure 3a shows an embodiment of a detector according to the invention. Figure 3b shows an embodiment of a two-dimensional array of detector elements.

Figure 4 shows a measuring system for determining the radiation dose in a radiological equipment according to the invention.

Figure 5 shows a fate diagram for measuring a dose spread according to the invention.

PREFERRED EMBODIMENTS Figures 1a and 1b show cross-sectional views of a radiological equipment 1, the so-called gamma knife, in sleep mode and in treatment mode.

The gamma knife comprises 201 small radiation sources 2 of ÖOCO arranged in a central body 10.

Yarn radiation emitted from each source 2 is collimated into a thin beam with sharp edges.

All beams are arranged towards a common intersection point which is located in the center of the gamma knife. This point is defined as the intersection point UCP and is located within a treatment area 9.

To protect the patient and staff from radiation in the rest position, shown in Figure 1a, the sources 2 are enclosed by a thick protective cover 3. Protective doors 4, 5 in the cover 3 allow the patient, who is lying on a treatment table 6 on a treatment table 7, to be moved to the treatment position of the gamma knife, shown in Figure 1b.

The radiation emitted from each source is collimated by three different collimators, the two closest to the source 2 are permanently arranged in the central body 10 of the gamma knife, and the third collimator is included in a replaceable collimator helmet 8. The helmet is best described as a substantially hemispherical shell with 201 modes, one for each beam. The collimators in the helmet 8 can be replaced with blind plugs which prevent special rays from reaching the patient, in order, for example, to prevent the eyes from being irradiated.

The radiological treatment starts when the protective doors 4, 5 are open and the treatment bench 6 is transported into the treatment position, as shown in Fig. 1b. When the collimators in the helmet 8 coincide with the collimators in the central body 10, the gamma radiation emitted from the selected sources 2 is directed towards the common intersection point UCP. At this non-variable beam cutting point, a preselected target point, i.e. a radiosurgical target, is placed in treatment. When the irradiation is completed, the couch 6 with the patient slides out of the treatment area 9 and the protective doors 4, 5 close automatically.

The patient's head is mechanically attached to a stereotactic frame (not visible), which is mounted on the inside of the helmet using two adjusters. The position of the head relative to the UCP is determined from previous measurements by computed tomography or magnetic resonance imaging.

Figure 2 shows a schematic view of the central body 10 of Figures 1a and 1b, comprising a radiation detector 13 according to the prior art. The sources, not visible, are arranged inside the central body and the gamma ray 11 from each source is collimated, before it hits the treatment area 9, by an apparatus 12, where the rays 11 are directed towards the common intersection point UCP.

The radiation detector is arranged inside a phantom 14, which for practical reasons is made of polystyrene. For symmetrical reasons, this phantom, which simulates the human head, is spherical in shape and has a diameter of 160 mm. Due to its good spatial resolution, an m lm 15 is used as a detector to determine the relative spatial absorbed dose spread.

When the film 15 is placed in a first position, solid line, the dose spread can be determined in two dimensions. If another m ch is placed in a second position orthogonal to the first position, shown in broken line, a second two-dimensional dose spread can be determined. A combination of these two scatterings can give an idea of what the three-dimensional scattering looks like. The method of obtaining the relative radiation doses is very time consuming and the geometric precision of the equipment can be a problem, such as a non-linear relationship between the dose and the optical density of the film.

Although ñlmer have a good spatial resolution and are excellent detectors in relative measurements, they are not suitable for dose rate calibrations. Instead, ionization chambers with small sensitive volumes must be used.

Radiological calibration of gamma knives is a simple but very important procedure. The dose rate at the point of intersection must be measured under certain special conditions.

The dose rate is currently measured with an ionization chamber arranged in the center of a polystyrene phantom, the chamber comprising a gas-filled volume between two electrodes between which a voltage is applied. Charges in the form of electrons and ions, which are created in the gas by the gamma radiation, seek out the respective electrode and the corresponding current is measured with a pike ammeter. The current is proportional to the dose rate in the volume.

Figure 3a shows an embodiment of a detector 20 according to the present invention which comprises a circuit board 21, a female contact 22, a detector unit 23 and a conductor 24 which connects the contact 22 to the detector unit 23.

The detector unit comprises a number of detector elements, each detector element defining a limited detection area and being arranged next to each other forming a matrix. The matrix can be a combination of elements in one, two or three dimensions. An embodiment of a two-dimensional matrix of the detector element is shown in Fig. 3b. Each element is connected to a respective conductor 24, which means that each element has contact with a pin in the female contact 22.

A male connector 25 may be attached to the female connector 22 in the detector device 20, which in turn is connected to external equipment via a cable 26.

Figure 3b shows an embodiment of a detector unit 23 comprising a two-dimensional array of detector elements 27 connected to the circuit board 21 in a radiation detector device 20. In this example, the detector unit 23 consists of a matrix of eight times eight detector elements.

The detector unit 23 is manufactured in a semiconductor material, where each detector element 27 is implemented as a semiconductor component, for example as a diode. Each detector element 27 is connected to the respective conductor 24 on the circuit board 21, shown in Fig. 3a, via a conductor element 28, a bonded pad 29, and a bonded wire 30. In Fig. 3b, only three conductors 24 are shown in illustration.

The detector unit can be implemented in a microchip and is preferably manufactured in silicon.

When the detector is placed in the treatment area, a current in each element 27 is induced by the gamma radiation. This current depends on the radiation dose.

The yarn radiation creates pairs of electron holes in a mining area in a PN junction in the semiconductor component. The pairs of electron holes are minority carriers and are swept away by the built-in voltage in the depletion region across the PN junction. The pairs of electron holes also constitute a current that can be measured and the current is proportional to the amount of gamma radiation and thus the radiation dose over a wide range of dose rates.

An exact value of the radiation dose can be determined within each detector element, which makes it possible to combine these values into a dose spread. By moving the detector in the treatment area, a three-dimensional radiation dose spread can be determined.

The size of the elements and the spaces between them determine the accuracy of the dose distribution. A preferred embodiment of the detector elements are rectangular areas, more specifically a rectangle with equally long sides. An example of a detector element size is 0.3 times 0.3 mm with a gap of 0.3 mm, however, the invention should not be limited by this size.

Figure 4 shows a measuring system for determining the radiation dose in a radiological equipment, such as the gamma knife. Instead of using a polystyrene phantom, the present invention enables the use of a phantom filled with a liquid, for example water, and having the shape of a head, or any other desired shape. The detector 20, with a number of detector elements, is arranged inside the phantom and mechanically attached to a stepper motor unit 31, for example by means of an arm 32.

The position of the detector is controlled by a computer 33 via a control channel 37.

The detector unit contains a number of detector elements, where the current from each element must be measured individually. This is accomplished by connecting a switch 34 between the detector 20, via the cable 26, and a current meter 35, via a connecting cable 36. By using the computer 33 to control the switch and the current meter through the control cable 37, the current from each element can be measured separately. which creates a series of values for this mode. Once the selected current has been measured, the value is transmitted to the computer via a connection 38 and stored in a memory in the computer. The series of values in memory can be used to display the dose spread on a display, printer or any other available means of presentation 39.

Once the currents in each element have been measured, the computer initiates a movement of the detector using the stepper motors. An additional series of values are then measured and stored in the computer's memory.

This procedure is repeated until the desired number of measurements is completed.

The radiation dose can be displayed directly, which gives an opportunity to show the dose spread during the course of the measurement. This method is considerably faster than current technology, described in connection with Fig. 2, in addition it has a good geometric precision.

Alternatively, the currents from the individual detector elements can be measured with individual hearth parameters. This would speed up the procedure further as the switch, in the measuring system, would then not be needed.

The measuring system described can also be used to determine the dose rate at the point of intersection instead of, as previously mentioned, using an ionization chamber. The accuracy of the measurements increases because the correct environment around the detector is used, ie a phantom with liquid instead of one made of polystyrene.

Furthermore, only one type of radiation detector is needed to perform all the necessary calibrations and verifications of the radiation equipment.

Figure 5 shows a flow chart of a method for measuring a dose spread according to the invention.

The flow starts in box 50, where the method is initiated. In the next step, box 51, essential parameters for the fate are specified and registered. Such parameters include: - number of detector elements in the detector (n), - starting point of the detector (a, b, c), - stopping point of the detector (A, B, C), - increase of the steps in one, two or three independent dimensions ( X), (y) and (z), and - measuring the current time from each detector element.

These values are recorded for later use in the method.

Once the parameters have been registered, the detector device is placed in a position indicated in box 52.

This means that the detector in the detector device is fixed at a point inside the collimator helmet, which corresponds to the starting point (a, b, c). The collimator helmet is attached to the treatment table located outside the treatment chamber. The detector can also be arranged inside a phantom to simulate a human head.

In the next step in the fate diagram, box 53, the detector is initiated by moving the collimator helmet, with the detector device, into the treatment chamber. 5 15 20 25 518 811 10 This step is followed by putting k = 1 in box 54 and the measurement of the respective current from each detector element can be started.

This procedure starts in box 55, where the current from detector element k = 0 in the first position (a, b, c) is measured during the predetermined time (T).

The value is stored in the next step, box 56, and the value is displayed, box 57, by a presentation means, such as a display or the like.

A comparison between the value of k and the registered value of the number of detector elements k is made in box 58. If k = n, the measurement of the detector elements at this position is complete and fl fate proceeds to box 59. On the other hand, k <n, returns fl the fate to a point 60, via a box 61, where the detector element is increased by k = k + 1.

A new measurement of the current of the detector element k is performed (box 55), stored (box 56), and presented (box 57). A new comparison is made in box 58. This loop is repeated until all the currents from the detector elements in the detector have been measured, stored and presented at this position.

In box 59 a comparison is made between the position of the detector (X, Y, Z) and the stop point (A, B, C). If the positions do not coincide, the går fate returns to a point 62, via a box 63, where the position of the detector changes according to the incremented steps (x), (y) and (z). The procedure for measuring the currents from each detector element is repeated, as above.

If the position of the detector and the stop point coincide, fl the fate continues to box 64, where the detector device is removed from the treatment chamber and the procedure is completed, box 65.

Claims (14)

.W 10 15 20 25 30 518 811 ll PATENT REQUIREMENTS A radiation detector device for determining a radiation dose spread or dose rate from at least one radiation means (2) comprising: - a detector (20), comprising more than one detector element (27), each element defining a limited detection area and being arranged to produce a individual current, which depends on the radiation dose in the respective area, said elements being arranged next to each other, and - means (34-36) for measuring each individual current to obtain a series of values for creating an expression of the dose spread in at least two dimensions. characterized in that said device further comprises means (32) for exposing said detector (20) to obtain further series of values for said radiation dose. Radiation detector device according to claim 1 or 2, characterized in that said means (32) for fl moving said detector (20) is too fl surfaceable in at least one dimension. Radiation detector device according to claim 1 or 2, characterized in that said means (32) for fl moving said detector (20) is independent of fl surface in at least two dimensions. Radiation detector device according to any one of claims 1-3, characterized in that said means (32) for moving said detector (20) comprises at least one stepping motor (3 1). Radiation detector device according to any one of claims 1-4, characterized in that said detector (20) can be arranged inside a phantom (30), which in turn is arranged in the vicinity of said radiation means (2). . Radiation detector device according to claim 5, characterized in that said phantom (30) is comprised of an outer shell containing a liquid. Radiation detector device according to claim 6, characterized in that said liquid is water. Radiation detector device according to one of the preceding claims, characterized in that said detector (20) is partly made of semiconductor material, the detector elements (27) being implemented in said semiconductor material. Radiation detector device according to claim 8, characterized in that said semiconductor material is silicon. Radiation detector device according to one of the preceding claims, characterized in that said detector element (27) is a semiconductor diode, independently connected to said measuring means (34-36). Radiation detector device according to claim 1, characterized in that said detector (20) comprises a number of detector elements (27), and said elements are arranged in a two-dimensional matrix. A radiation dose detection system for verifying or calibrating a radiosurgical equipment, said system comprising: - at least one radiation means (2), a control system, - a means for recording radiation doses, and - a means for combining said radiation dose into a radiation dose spread, characterized in that said system further comprises a radiation detector device, comprising a detector (20), according to any one of claims 1-11, wherein said detector is used fl times to determine said radiation dose at fl your positions during verification or calibration, so as to create an expression of said dose spread in three dimensions. A radiation dose detection system according to claim 12, further comprising: - a collimator helmet (8), and - a stereotactic frame, which is fixed inside said helmet, characterized in that said detector (20) is arranged inside a phantom (30), said stereotactic frame is mechanically attached to the phantom, which is thereby fi xerated inside the collimator helmet (8). A method for generating a three-dimensional radiation dose scattering from at least one radiation means comprising the steps of: - arranging a radiation detector device, comprising a detector (20) divided into a number of individual detector elements (27) each having a detection area, in a start position, - to surface the radiation detector device closer to the radiation means (2), - to expose the detector to radiation from the radiation means for a certain period of time (T), - to remove the radiation detector device to the starting position - to measure a series of values of the radiation dose in a detection range, together the radiation dose values to the scattering, so as to generate at least one two-dimensional radiation dose scattering, characterized in that said method also comprises the following steps: - moving said detector (20) to fl your positions, measuring a number of further series of values of said radiation dose, thereby generating said radiation dose. three-dimensional dose dispersions n.