GB2137747A

GB2137747A - Apparatus for measuring radiation beam intensity

Info

Publication number: GB2137747A
Application number: GB08408776A
Authority: GB
Inventors: J R N Symonds-Tayler
Original assignee: TOWER HAMLETS HEALTH AUTHORITY
Current assignee: TOWER HAMLETS HEALTH AUTHORITY
Priority date: 1983-04-05
Filing date: 1984-04-05
Publication date: 1984-10-10
Also published as: GB8408776D0

Abstract

In an apparatus for measuring radiation beam intensity in radiotherapy equipment in which a sealed ionisation chamber is subjected to the beam being measured, an electrometer module is connected to receive the output from the ionisation chamber and gives an amplified output to an output module which gives a signal indicating whether the radiation level is within or outside acceptable limits. The ionisation chamber may comprise a body 1 of insulation material housing a central plate electrode 7 surrounded by a cup-shaped guard electrode 2 and surmounted by a planar polarising electrode 14. <IMAGE>

Description

SPECIFICATION Apparatus for measuring radiation beam intensity The invention relates to apparatus for measuring the beam intensity of radiation from radiotherapy equipment. The radiation may be, for example, X-ray or electron beam radiation.

It is customary to check the radiation levels of radiotherapy equipment by mounting a radiation detector in a "Phantom" of material which simulates the tissue to be subjected to the radiation. The detector is conventionally an ionisation chamber-based radiation dosemeter such as that known as the Farmer instrument. This system is very accurate but has the disadvantage that the equipment is expensive and sensitive.

Furthermore, the readings are dependent upon the atmospheric density at the time and place of measurement. Therefore, additional measurements must be made of atmospheric temperature and pressure in order to effect compensation of the readings taken. Such additional measurements add to the complication of the system and give rise to potential errors.

An object of the present invention is to provide beam monitoring equipment of a more rugged kind suitable for everyday use.

According to the invention there is provided apparatus for measuring radiation beam intensity in radiotherapy equipment, the apparatus comprising a sealed ionisation chamber which in use is subjected to the beam being measured, an electrometer module connected to receive the output from the ionisation chamber and which provides an amplified output, and an output module adapted to receive the amplified output from the electrometer module and provide an output which indicates whether the radiation level is within or outside acceptable limits.

Preferably the output module gives an indication by virtue of three lamps; the first lamp indicating a radiation level which is too high; the second lamp indicating a radiation level which is within limits; and the third lamp indicating a radiation level which is too low. The lamps are illuminated by a driver circuit which receives input from two comparators -- a "high" comparator and a "low" comparator. The comparators compare the input level from the amplified electrometer signal with respective preset levels indicative of the limits of the "window" which is representative of the acceptable radiation level.

Preferably the ionisation chamber comprises a body of insulation material which houses three electrodes insulated from each other. A central plate electrode is mounted within a cup-shaped guard electrode and this pair of electrodes is mounted beneath a planar polarising electrode.

The active volume of the ionisation chamber is the space between the electrodes which is fiiled with a suitable gas. In use, the beam being measured is directed to the active volume through the polarising electrode. A coaxial cable makes connection with the electrodes, having an outer braid which is connected to the guard electrode; and a central conductor which is connected to the central electrode. The electrodes may be mounted within the insulating body to be removable and sealing is effective by resilient O-rings. This arrangement is particularly rugged and insensitive to physical abuse.

The invention will further be described with reference to the accompanying drawings, of which: Figure 1 is a cross-sectional view of an ionisation chamber for use in conjunction with the present invention; Figure 2 is a circuit diagram of the electrometer module which is connected to the ionisation chamber of Figure 1, and Figure 3 is a circuit diagram of the output module which is connected to the output of the electrometer module.

Referring to Figure 1 there is shown in crosssection an ionisation chamber which is contained in a body 1 of perspex. The body is formed with a cavity which houses a cup-shaped aluminium guard electrode 2. This electrode is sealed with respect to the body by means of an O-ring 3.

Within the guard electrode is mounted an inner body of perspex 4 which is sealed to the guard electrode by means of an O-ring seal 5. The body is mounted by three screws, one of which is shown at 6. Within the body 4 is mounted a central electrode 7 which is sealed to body 4 by means of an O-ring seal 8. An annular space 9 is defined between the central electrode 7 and the guard electrode. A top cover 10 is held in place by twelve screws, one of which is shown at 11 and is sealed to the body 1 by means of an O-ring seal 12. Two plates 13 and 21 of perspex are fitted in place at the top and bottom of the unit.

Electrical connections are made to the central electrode, the guard electrode, and an aluminium plate electrode 1 4 which is situated beneath the cover plate. These electrodes are connected to a coaxial lead which comprises an outer insulator 15, an outer braided conductor 16, an inner insulator 17; an inner braid conductor 18; an inermost insulator 19, and a central conductor 20.

The central electrode is connected to the innermost conductor, the guard electrode is connected to the inner braid, and the polarising electrode is connected to the outer braid.

Referring now to Figure 2 there is shown the electrometer module to which the output from the ionisation chamber is connected. The outermost braid of the lead is connected to the body 22 of the module which is earthed. The inner braid is connected to the non-inverting input of a differential amplifier 23 which is held at approximately + 200 volts with respect to earth, to cause the collection on the electrodes of any radiation induced ions within the sensitive volume.

The innermost conductor is connected to the inverting input of the amplifier 23 via an input resistor 24. Feedback is provided on the amplifier by an RC circuit consisting of a resistor 25 and a capacitor 26. A reset contact RL1 is operated by a relay RL which is energised manually by application of a voltage to a reset line 27. The output of the electrometer module is provided by a line 28 and represents an amplified signal corresponding to the ionisation charge collected on the central electrode 7 (Figure 1).

Referring now to Figure 3 there is shown the output from the electrometer module of Figure 2.

The signal from the electrometer module is applied to a lead 30 which is the input to a buffer amplifier 31. The output from amplifier 31 is applied through a manually controllable gain potentiometer 32 to an input amplifier 33.

Amplifier 33 is provided with an RC feedback circuit 34. The output from amplifier 33 is applied simultaneously to the non-inverting inputs of two differential amplifiers 35 and 36. Amplifier 35 is a "high" comparator and amplifiers being provided by a reference voltage obtained by a zener diode 38 via resistors 39 and 40. The setting of variable resistors 46 and 47 in the feedback net-work of the comparators defines a "window" and there are respective output lines which are energised according to whether the input from the electrometer is above an upper limit or below a lower limit respectively. If only the output of the "low" comparator is energised then the output from the electrometer is within the required range.

A driver circuit 42 receives the outputs from the comparators and energises three lamps via output connectors 43, 44 and 45. The lamps are respectively "high", "low" and "fine" lamps and according to the output from the comparators, the appropriate lamp is illuminated to indicate whether the dosage being detected by the ionisation chamber is too high, too low or correct.

The invention is not restricted to the details described above. For example, the configuration of the ionisation chamber could be altered in some respects. The materials could be different, for example, the body parts could be made from other plastics than perspex, perhaps from water or tissue radiation equivalent materials. The polarising electrode could be a conductive plating on the top cover 10 (Figure 1), or the electrodes could be made from electrically conducting plastics. Various of the O-rings could be replaced by suitable sealing adhesives, or the electrodes moulded into a cast chamber body.

Of the electronics features, there could be an automatic reset facility coupled with the on/off switch of the system and there may be a zero check circuit incorporated to ensure that the reset circuitry is working correctly. Either instead of or in addition to a visual display, the display could be audible. An interlock could be provided so that therapy is inhibited until a correct measurement has been taken.

Claims

1. Apparatus for measuring radiation beam intensity in radiotherapy equipment, the apparatus comprising a sealed ionisation chamber which in use is subjected to the beam being measured, an electrometer module connected to receive the output from the ionisation chamber and which provides an amplified output, and an output module adapted to receive the amplified output from the electrometer module and provide an output which indicates whether the radiation level is within or outside acceptable limits.

2. Apparatus as claimed in claim 1 wherein the output module gives an indication by virtue of three lamps; the first lamp indicating a radiation level which is too high; the second lamp indicating a radiation level which is within limits; and the third lamp indicating a radiation level which is too low.

3. Apparatus as claimed in claim 2 wherein the lamps are illuminated by a driver circuit which receives input from two comparators-a "high" comparator and a "low" comparator, comparators compare the input level from the amplified electrometer signal with respective preset levels indicative of the limits of the "window" which is representative of the acceptable radiation level.

4. Apparatus as claimed in any of the preceding claims wherein the ionisation chamber comprises a body of insulation material which houses three electrodes insulated from each other, a central plate electrode is mounted within a cup-shaped guard electrode is mounted beneath a planar polarising electrode, the active volume of the ionisation chamber is the space between the electrodes which is filled with a suitable gas.

5. Apparatus as claimed in claim 4 wherein the electrodes are mounted within the insulating body to be removable and sealing is effective by resilient O-rings;

6. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.