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Digital measurement system for radiation dose distribution (for therapy treatment planning)
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1975 Phys. Med. Biol. 20 314 (http://iopscience.iop.org/0031-9155/20/2/014) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 137.149.200.5 This content was downloaded on 18/08/2015 at 16:43
Please note that terms and conditions apply.
PHYS. MED. BIOL.,
1975, VOL. 20, NO. 2, 314-317. 0 1975
Technical Note
Digital Measurement System for Radiation Dose Distribution V. J. BLACKMORE,
MSC.
and G. P. NAYLOR,
B.SC., M.INST.P.
Physics Department, University Hospital of South Manchester, Manchester M20 9BX, England
Received 11 April 1974
1. Introduction
Computer treatment planning systems require the beam data to be either calculated from generating formulae or obtained from measurements on the beam itself. The latter method requires digital measurement of the radiation dose distribution and this may be achieved by coupling the computer directly to anionization chamber moving in a water phantom (Glenn, Bodicky,Feldman and Glenn 1969). However, if the radiotherapy machine and the computer are not in close proximity, it is more convenient to obtain the data off-line and a system has thereforebeen constructed for obtaining transverse and central axis dose data on punched paper tape. The system consists of a fixed monitor chamber,aremotelycontrolledmeasuringchamber,adigitalvoltmeterfor displaying the dose rate ratio and chamber position and a standard ASR-33 teletype and punch. The data are automatically printed on the teletype and punched inASCII format on the paper tape which may subsequently be input to a computer. 2. Data measurements
The fixed chamber, together with the measuring chamber and its associated movement drive and sensing components, is mounted for use on a rectangular perspex water tank or phantom, into which the beam of radiation is directed. For obtaining transverse data, the monitor chamber is fixed near the point of maximum dose andthe measuring chamber remotely moved along the transverse axis. An analogue voltage proportional to the position of the measuring chamber relative to the central axis is produced by measuring the voltagedrop across a slide wire parallel to the transverseaxis.Movement along the central axis is not required t o be continuous as interpolation is possible, and is therefore limited to well defined positions a t 2, 5 , 10, 15 and 20 cm from the phantom face. To obtain depth dose data, the phantom is rotated throughninety degrees and the monitorchambermoved to the new dose maximum, thereby allowing continuous scanning of the central axis. (DVM) in which the operating Thedisplay device isadigitalvoltmeter principle is to vary the gating period of an internal clock oscillator in direct
Measurement System for Radiation Dose Distribution
315
proportion to the signal under measurement. The level of the voltage input determines the number ofclock pulses passed during the gating period and these are totalized in a ‘33’ digit display. This mode is used to measure the position signal but when the dose rate signals are measured the internal clock is disabled and thereference chamber output is used to generate clock pulses at a frequency inversely proportional to its amplitude. The measuring chamber output is simultaneously applied to the DVM input and the display shows the ratio of the dose rates at the two chambers. 3. Complete system
The output from the DVM is in parallel binary coded decimal (BCD) and it is necessary to interface the voltmeter to a standard ASR-33 teletype. Fig. 1 shows a diagram of the system. The DVM is normally set by relays RL1 and RL2 to measure the dose rate ratio and is sampled by an external oscillator
-
Tank Ref.
Motor control
Fig. 1
Fig. 2
Fig. 1. Block diagram of the complete digital measurement system. Fig. 2. Output format from a ‘quick check’ transverse scan of an 8 MV X-ray beam at 5 cm deep with jaw settings 10 x 10.
about ten times a second. The 33 digit reading appears on 13 output lines as three 4-bit BCD numbers with the last line determining whether the fourth and most significant digit is ‘0’or ‘l’. The requirement is to print both the dose rate ratio and the corresponding chamber position side by side on the teletype, separating the two numbers by two ‘spaces’ and terminating each line with a ‘carriage return’ and ‘line feed’. The action of the interface or serializeris therefore to :
V . J. Blackmore and G . P.Naylor
316
(a) Convert the BCD into the ASCII code required by the teletype and to provide the necessary parity. The teletype had previously been modified to accept parallel input data. (b) Interspace the printingof the datawith the control characters ‘line feed’, ‘carriage return’ and ‘space’. (c) Produce the timing for switching the measuring system from reading the dose rate ratio toreading the position. (d) Provide the sampling pulses required by the DVM. The action is produced by the control logic which steps through the following sequence : State 0 1 2
3 4 5
6 7 8 9 10 11 12
Action ‘Normal’ state. ‘Ratio’ continuously sampled. Arrest sampling. Read and print 1stdigit of ratio value. Read and print 2nd digit of ratio value. Read and print 3rd digit of ratio value. Read and print 4th digit of ratio value. Print ‘space’. Change to reading ‘position’. Print ‘space’. Sample ‘position’ signal. Read and print 1st digit of position value. Read and print 2nd digit of position value. Read and print 3rd digit of position value. Read and print 4th digit of position value. Print ‘carriage return’. Print ‘line feed’ and return to state0.
The sequence is started by pressing the START button and the lastvalue of the dose rate ratio sampled is held by the DVM. The interface expects a four bit number from the DVM and these four bits are passed directly to the output buffers as bits one to four of the ASCII code. When reading the 4 digit, i.e. states 1 and 7 above, the first three ASCII bits are automatically set to zero. The switching logic senses the control state and either sets up the required control character, or,whenever a number isbeing read, provides the remaining bits,bits five to seven. Theeighthbit is produced by aseparateparity generator circuit. The teletype gives a signal when a character has been printedlpunched and this signal is used to advance the control logic by one step. The rate of printout is therefore determined by the maximum teletype speed of ten characters a second. The format of the output is shown in fig. 2. The dose rate at the measuring chamberis expressed asa 3-figure percentage of the monitor chamber dose rate; the leading digit is always zero. The position is given in tenths of a millimetre from the central axis, A polarity digit is available from the DVM but it was felt that the printing of the sign is unnecessary. Annotations to the data may be made from the teletypekeyboard whenever the serializer is in its ‘normal’ state.
Measurement System for Radiation Dose Distribution
317
Summary The system has the following features: (a) Remote positioning of the measuring chamber. (b) Digital display of the dose rate ratio: The sampling rate is adjustable up to 20 Hz allowing a fast response when the measuring position is changed. (c) Digital display of the measuring chamber position, either momentarily during the print-out or as a continuously sampled display by manually over-riding the automatic control. (d) Print-out in just over one second on an ASR-33 teletype andpunch of the dose rate ratio and the chamber position. Data are in computer compatible ASCII code with parity. The logic functions areobtainedwithstandardtransistor-transistor logic integrated circuits. The system has been in use for a number of months now with the excellent reliability usually associated with such circuits.
4.
REFERENCE GLENN, W. R., BODICKY,A. L., FELDMAN, A., and GLENN, R., 1969, Proc. 55th Meeting of the Rad.Soc. of North America.
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My IOPscience
Digital measurement system for radiation dose distribution (for therapy treatment planning)
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1975 Phys. Med. Biol. 20 314 (http://iopscience.iop.org/0031-9155/20/2/014) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 137.149.200.5 This content was downloaded on 18/08/2015 at 16:43
Please note that terms and conditions apply.
PHYS. MED. BIOL.,
1975, VOL. 20, NO. 2, 314-317. 0 1975
Technical Note
Digital Measurement System for Radiation Dose Distribution V. J. BLACKMORE,
MSC.
and G. P. NAYLOR,
B.SC., M.INST.P.
Physics Department, University Hospital of South Manchester, Manchester M20 9BX, England
Received 11 April 1974
1. Introduction
Computer treatment planning systems require the beam data to be either calculated from generating formulae or obtained from measurements on the beam itself. The latter method requires digital measurement of the radiation dose distribution and this may be achieved by coupling the computer directly to anionization chamber moving in a water phantom (Glenn, Bodicky,Feldman and Glenn 1969). However, if the radiotherapy machine and the computer are not in close proximity, it is more convenient to obtain the data off-line and a system has thereforebeen constructed for obtaining transverse and central axis dose data on punched paper tape. The system consists of a fixed monitor chamber,aremotelycontrolledmeasuringchamber,adigitalvoltmeterfor displaying the dose rate ratio and chamber position and a standard ASR-33 teletype and punch. The data are automatically printed on the teletype and punched inASCII format on the paper tape which may subsequently be input to a computer. 2. Data measurements
The fixed chamber, together with the measuring chamber and its associated movement drive and sensing components, is mounted for use on a rectangular perspex water tank or phantom, into which the beam of radiation is directed. For obtaining transverse data, the monitor chamber is fixed near the point of maximum dose andthe measuring chamber remotely moved along the transverse axis. An analogue voltage proportional to the position of the measuring chamber relative to the central axis is produced by measuring the voltagedrop across a slide wire parallel to the transverseaxis.Movement along the central axis is not required t o be continuous as interpolation is possible, and is therefore limited to well defined positions a t 2, 5 , 10, 15 and 20 cm from the phantom face. To obtain depth dose data, the phantom is rotated throughninety degrees and the monitorchambermoved to the new dose maximum, thereby allowing continuous scanning of the central axis. (DVM) in which the operating Thedisplay device isadigitalvoltmeter principle is to vary the gating period of an internal clock oscillator in direct
Measurement System for Radiation Dose Distribution
315
proportion to the signal under measurement. The level of the voltage input determines the number ofclock pulses passed during the gating period and these are totalized in a ‘33’ digit display. This mode is used to measure the position signal but when the dose rate signals are measured the internal clock is disabled and thereference chamber output is used to generate clock pulses at a frequency inversely proportional to its amplitude. The measuring chamber output is simultaneously applied to the DVM input and the display shows the ratio of the dose rates at the two chambers. 3. Complete system
The output from the DVM is in parallel binary coded decimal (BCD) and it is necessary to interface the voltmeter to a standard ASR-33 teletype. Fig. 1 shows a diagram of the system. The DVM is normally set by relays RL1 and RL2 to measure the dose rate ratio and is sampled by an external oscillator
-
Tank Ref.
Motor control
Fig. 1
Fig. 2
Fig. 1. Block diagram of the complete digital measurement system. Fig. 2. Output format from a ‘quick check’ transverse scan of an 8 MV X-ray beam at 5 cm deep with jaw settings 10 x 10.
about ten times a second. The 33 digit reading appears on 13 output lines as three 4-bit BCD numbers with the last line determining whether the fourth and most significant digit is ‘0’or ‘l’. The requirement is to print both the dose rate ratio and the corresponding chamber position side by side on the teletype, separating the two numbers by two ‘spaces’ and terminating each line with a ‘carriage return’ and ‘line feed’. The action of the interface or serializeris therefore to :
V . J. Blackmore and G . P.Naylor
316
(a) Convert the BCD into the ASCII code required by the teletype and to provide the necessary parity. The teletype had previously been modified to accept parallel input data. (b) Interspace the printingof the datawith the control characters ‘line feed’, ‘carriage return’ and ‘space’. (c) Produce the timing for switching the measuring system from reading the dose rate ratio toreading the position. (d) Provide the sampling pulses required by the DVM. The action is produced by the control logic which steps through the following sequence : State 0 1 2
3 4 5
6 7 8 9 10 11 12
Action ‘Normal’ state. ‘Ratio’ continuously sampled. Arrest sampling. Read and print 1stdigit of ratio value. Read and print 2nd digit of ratio value. Read and print 3rd digit of ratio value. Read and print 4th digit of ratio value. Print ‘space’. Change to reading ‘position’. Print ‘space’. Sample ‘position’ signal. Read and print 1st digit of position value. Read and print 2nd digit of position value. Read and print 3rd digit of position value. Read and print 4th digit of position value. Print ‘carriage return’. Print ‘line feed’ and return to state0.
The sequence is started by pressing the START button and the lastvalue of the dose rate ratio sampled is held by the DVM. The interface expects a four bit number from the DVM and these four bits are passed directly to the output buffers as bits one to four of the ASCII code. When reading the 4 digit, i.e. states 1 and 7 above, the first three ASCII bits are automatically set to zero. The switching logic senses the control state and either sets up the required control character, or,whenever a number isbeing read, provides the remaining bits,bits five to seven. Theeighthbit is produced by aseparateparity generator circuit. The teletype gives a signal when a character has been printedlpunched and this signal is used to advance the control logic by one step. The rate of printout is therefore determined by the maximum teletype speed of ten characters a second. The format of the output is shown in fig. 2. The dose rate at the measuring chamberis expressed asa 3-figure percentage of the monitor chamber dose rate; the leading digit is always zero. The position is given in tenths of a millimetre from the central axis, A polarity digit is available from the DVM but it was felt that the printing of the sign is unnecessary. Annotations to the data may be made from the teletypekeyboard whenever the serializer is in its ‘normal’ state.
Measurement System for Radiation Dose Distribution
317
Summary The system has the following features: (a) Remote positioning of the measuring chamber. (b) Digital display of the dose rate ratio: The sampling rate is adjustable up to 20 Hz allowing a fast response when the measuring position is changed. (c) Digital display of the measuring chamber position, either momentarily during the print-out or as a continuously sampled display by manually over-riding the automatic control. (d) Print-out in just over one second on an ASR-33 teletype andpunch of the dose rate ratio and the chamber position. Data are in computer compatible ASCII code with parity. The logic functions areobtainedwithstandardtransistor-transistor logic integrated circuits. The system has been in use for a number of months now with the excellent reliability usually associated with such circuits.
4.
REFERENCE GLENN, W. R., BODICKY,A. L., FELDMAN, A., and GLENN, R., 1969, Proc. 55th Meeting of the Rad.Soc. of North America.
