- Email: [email protected]
An on-line data acquisition system for radiation therapy: External photon and electron beams
COMPUTERS
AND
An On-Line
BIOMEDICAL
16, 169-175 (1983)
Data Acquisition External Photon
KAM-SHING Departmenr
RESEARCH
of Radiation
LAM,
System for Radiation and Electron Beams
CRAIG TABORSKY,
Oncology,
AND WING-CHEE
The Johns Hopkins Maryland 21205
Medical
Institutions,
Therapy:
LAM Baltimore,
Received May 19, 1982
An on-line data acquisition system for external beam treatment planning has been developed. The hardware consists of a Tektronix 4054 Computer, an SHM Nuclear 3-D scanning water phantom, interfaces for 16 channels of ADC and 32 channels of digital I/O, and two detector probes for measuring dose distribution and monitoring fluctuations in machine output. The program to control the movement of the scanning probe and to collect data is written in BASIC. Results of the measurements are plotted in real time on the Tektronix screen. The beam data format consists of a set of percentage depth doses along the central axis and five sets of off-axis ratios at five different depths. In the electron mode, when ionization chambers are used, data points are averages of two measurements, one with positive bias and the other with negative bias. The beam data are stored on disk which can be accessed by the Capintec treatment planning system.
INTRODUCTION
Before treating patients in radiation therapy using an external beam, computer simulation is used in some cases to optimize absorbed dose distributions around the tumor volume, a process known as computer treatment planning. In addition to patient data, such as the patient’s external contour, tumor volume, and internal structures, beam data are also required by the computer. Beam data are usually measured when the therapy machine is installed. Very often the measured data are prepared either in graphic form and digitized into the computer or in numerical form and keyed into the computer. The data are then stored on a disk or tape, ready for use in a treatment planning program. Either process is laborious and error prone. This article describes an on-line beam data acquisition system that has been designed around the Tektronix 4054 computer and an SHM Nuclear 3-D water phantom. EQUIPMENT
The beam data are usually measured with a probe in a water phantom, and the probe is moved to specified positions to scan the beam. A cobalt-60 machine does not give a fluctuating radiation beam and only a single detector 169 OOlO-4809/83 $3.00 Copyright 0 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.
170
LAM, TABORSKY,
AND LAM
probe is required to measure the beam data. However, for measurements on an accelerator, an additional probe called the reference probe is required to monitor fluctuations in the machine output. The reference probe is set at a fixed position for each beam measurement. In addition to outputs from the two probes, the voltages of the position encoders for X, y. and z coordinates of the detector probe will also be required. Together, they form five channels of analog data, which can go through a multiplexer and an analog-to-digital converter (ADC) before being input to the computer. After receiving all the data from the five channels, the computer sends out a signal in the form of a series of pulses to move the detector probe to another position, and takes another set of data from the ADC. Such a direct data acquisition system can be constructed using readily available components. Figure 1 shows the schematic layout of the equipment. We have designed the system around a Tektronix 4054 computer (used in the Capintec treatment planning system), which has convenient graphic display facilities, and an SHM 3-D scanning water phantom. The Tektronix computer is equipped with 4907 Option 30 (twin) disk drives, and has four ROM pack slots available. It only accepts BASIC as its computer language. Inputs from its keyboard and outputs from the computer are displayed on its screen. It can be programmed to display data in graphic form. The probes can be either ionization chambers or diode detectors. PTW microchambers with a volume of 0.1 cm3 are used in the percent design. The amplifiers are high input impedance current amplifiers. The multiplexer and analog-to-digital converter are combined as a single unit available from Trans Era as an ADC 652/752 ROM pack. It has 16 differential input channels with 12 bits (4096 parts) resolution over an input voltage range of f 10 volts.
TEKTRONIX COMPUTER
EINARY/EICD 110 (TRANS ERA 632/732 I
FIG.
1. Block diagram of equipment.
DATA ACQUISITION
SYSTEM
FOR EXTERNAL
BEAM
171
Output pulses from the computer control the movement of the three stepping motors which move the detector probe in three orthogonal directions. These pulses go through a Trans Era 632/732 binary/BCD I/O ROM pack computer interface. The stepping motors and position decoders are part of the SHM 3-D water phantom. Both Trans Era ROM packs are plugged into the computer ROM pack slots. COMPUTER SOFTWARE
We have written an interactive program in BASIC to control the movement of the detector probe and to accept data from the two probes and the position decoders for the detector probe. The program provides options for reviewing, editing, or measuring central axis or profile (off-axis ratio) data for each beam. Prior to the measurement, the initial position of the detector probe is manually set and its x, y, and z coordinates are entered into the computer memory. This position is constantly monitored in subsequent scans. The detector probe can scan in either X, y, or z direction under program control. To minimize effects of noise, the measurement at each point is made by sampling the outputs of measuring probes a fixed number of times. Normally, 160 samples are used, a number which is large enough to smooth out statistical fluctuations and yet requires a reasonable amount of time for each measurement. The number of samples recorded at each point can be changed at the beginning of each scan. For measurements on an accelerator, to smooth out fluctuations from the machine output, the outputs of the two probes are sampled from the ADC, and the value for the detector probe is digitally divided by that for the reference probe. The dose variation as a function of the position of the scanning detector, after correcting for machine fluctuations, is displayed in graphic form on the screen in real time. The profiles are measured at five different depths, each having 47 points lying on fan lines (see, e.g., Milan and Bentley (I)). During the measurement, facilities are provided for rescanning, for plotting a previously measured profile at the same depth for the same beam stored on a disk, for halting the measurement, or for aborting the measurement and getting a new beam. In the photon mode, the central axis data are measured at 30 points, which are then used for fitting an empirical function to obtain the coefficients required by the treatment planning program. In the electron mode, since ionization chambers are used to obtain the central axis data, two measurements are required-one with positive bias and another with negative bias on the probes. The data points are taken as the average of the two measurements. Eighty data points are made from a depth of 5 mm to a depth equal to twice the estimated extrapolated range of the electron energy under investigation. The effective measurement point is not at the center of the ionization chamber but at a point displaced 9 radius towards the electron source (see, e.g., Almond (2)). The ionization data are normalized so
LAM, TABORSKY,
172
$
AND LAM
80-
F cf N
,z w 2 5 ,"
60-
40-
0
2
4
6
8 OEPTH
JO
12
14
" 16
'
'
(cm)
Fm. 2. PIot of percentage ionization vs depth. The two straight lines are the least squares linearly fitted lines. The lower line is for the background due to X rays. The upper line is for the ionization due to electrons in the falloff region. The depth at which the two fitted lines intersect is the calculated practical range of the electrons. Energy = 20 MeV, cone size = 10 cm, source to water phantom surface distance (SSD) (cm) = 100, D,,, (cm) = 0.5, depth interval (cm) = 0.5.
FIG. 3. Plot of percentage depth dose vs depth. The central axis data due to an electron beam in Fig. 2 is converted into percentage depth dose. Here D maxis the point where measurements start. Energy = 20 MeV, cone size = 10 cm, SSD (cm) = 100, D,, = 0.5, depth interval (cm) = 0.5.
DATAACQUKITIONSYSTEMFOREXTERNALBEAM
173
that the maximum value is 100%. These raw data are stored as a permanent record of the ionization which can be analyzed immediately or at a later time. In the analysis of electron central axis data, the last 20 points are taken to be pure background due to X rays and are used for a linear least squares fit. The fitted line is assumed to give the ionization due to X rays alone at any depth. When this X-ray ionization is subtracted from the total ionization, the result is the ionization due to electrons alone. A linear least squares fit is made to the falloff region which includes those data points 20 to 80% above the X-ray background. The depth at which the two fitted lines intersect is the calculated practical range rP (Fig. 2). The energy E0 of the electrons at the surface is then calculated from the well-established range-energy relationship (see, e.g., ICRU Report 2 1) (3) &, = (RP + 0.376)/0.521. RESULTS AND DISCUSSION
For electron beam measurements, the percentage depth dose along the central axis required for generating isodose curves is obtained from the ionization curve (Fig. 2) by taking into account the perturbation correction factor (P,,,) due to the ionization chamber at each depth and the conversion factor for absorbed dose in water (Cs) for electrons of initial energy E0 (3). Thirty points at intervals specified by the user are generated at the end of the analysis. The results are then plotted on the screen which shows the percentage depth dose as a function of depth (Fig. 3). In the photon mode, the maximum ionization is taken to be 100%, and the display of the percentage depth dose along the central axis looks similar to that of the electron mode (Fig. 3) except that only 30 points are measured. For off-axis ratio measurements, the operations of both the electron mode and photon mode are identical. At each depth, the ionization along the central axis is first measured and normalized to unity. Then the detector probe is moved pass the edge to 1.3 times the specified field size. The measurements obtained are normalized to the central axis measurement. The data points, joined together by straight lines, are displayed in real time. Figure 4 shows an example of the profile obtained at the end of an off-axis ratio measurement of one of the five depths. The vertical lines define the center of the beam and the geometric edges. The abscissa tick marks correspond to the positions where the probe samples the radiation level. After all five profiles have been measured, the value at the 24th fan line, which is the center of each profile, is normalized to unity. Then all five profiles are displayed on the screen (Fig. 5). A typical measurement consisting of 160 samples per point for both central axis and profile data takes about 10 minutes for each beam. The program used in this data acquisition system is coded in BASIC with low program development cost. With off-the-shelf products, the hardware interfacing is simple to construct. It can be implemented very inexpensively as a data
174
LAM, TABORSKY,
AND LAM
FIG. 4. A typical display during profile scan. User definable keys CUD Key) are used in the program to interrupt a profile scan. The geometric beam width is defined by the two middle lines of the three vertical lines on each side. Each tick mark on the horizontal axis represents the position of the fan line where measurement is made. Level 1, wedge #6: field size = 21 cm, UD key: 12 = Rescan, 13 = Plot-O, 14 = Halt, 15 = Abort.
FIG. 5. Display at the end of profile scans. Each tick mark on the vertical axis represents 10% of the ionization. The ionization at the center of each profile is normalized to 100%. Wedge #6: field size = 21 cm.
DATA ACQUISITION
SYSTEM FOR EXTERNAL
BEAM
I75
acquisition package for the Capintec treatment planning system. The coding can be modified to generate data for other treatment planning systems using a different beam data format. REFERENCES 1. MILAN, J., AND BENTLEY, R. E. The storage and manipulation of radiation dose data in a small digital computer. Er. J. Raa’iol. 47, 115 (1974). 2. ALMOND, P. R. Radiation physics of electron beams. In “Clinical Applications of the Electron Beam” (Norah duV. Tapley, Ed.), pp. 7-80. Wiley, New York, 1976. 3. ICRU Report 21, “Radiation dosimetry: electrons with initial energies between 1 and 50 MeV,” 1972.
AND
An On-Line
BIOMEDICAL
16, 169-175 (1983)
Data Acquisition External Photon
KAM-SHING Departmenr
RESEARCH
of Radiation
LAM,
System for Radiation and Electron Beams
CRAIG TABORSKY,
Oncology,
AND WING-CHEE
The Johns Hopkins Maryland 21205
Medical
Institutions,
Therapy:
LAM Baltimore,
Received May 19, 1982
An on-line data acquisition system for external beam treatment planning has been developed. The hardware consists of a Tektronix 4054 Computer, an SHM Nuclear 3-D scanning water phantom, interfaces for 16 channels of ADC and 32 channels of digital I/O, and two detector probes for measuring dose distribution and monitoring fluctuations in machine output. The program to control the movement of the scanning probe and to collect data is written in BASIC. Results of the measurements are plotted in real time on the Tektronix screen. The beam data format consists of a set of percentage depth doses along the central axis and five sets of off-axis ratios at five different depths. In the electron mode, when ionization chambers are used, data points are averages of two measurements, one with positive bias and the other with negative bias. The beam data are stored on disk which can be accessed by the Capintec treatment planning system.
INTRODUCTION
Before treating patients in radiation therapy using an external beam, computer simulation is used in some cases to optimize absorbed dose distributions around the tumor volume, a process known as computer treatment planning. In addition to patient data, such as the patient’s external contour, tumor volume, and internal structures, beam data are also required by the computer. Beam data are usually measured when the therapy machine is installed. Very often the measured data are prepared either in graphic form and digitized into the computer or in numerical form and keyed into the computer. The data are then stored on a disk or tape, ready for use in a treatment planning program. Either process is laborious and error prone. This article describes an on-line beam data acquisition system that has been designed around the Tektronix 4054 computer and an SHM Nuclear 3-D water phantom. EQUIPMENT
The beam data are usually measured with a probe in a water phantom, and the probe is moved to specified positions to scan the beam. A cobalt-60 machine does not give a fluctuating radiation beam and only a single detector 169 OOlO-4809/83 $3.00 Copyright 0 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.
170
LAM, TABORSKY,
AND LAM
probe is required to measure the beam data. However, for measurements on an accelerator, an additional probe called the reference probe is required to monitor fluctuations in the machine output. The reference probe is set at a fixed position for each beam measurement. In addition to outputs from the two probes, the voltages of the position encoders for X, y. and z coordinates of the detector probe will also be required. Together, they form five channels of analog data, which can go through a multiplexer and an analog-to-digital converter (ADC) before being input to the computer. After receiving all the data from the five channels, the computer sends out a signal in the form of a series of pulses to move the detector probe to another position, and takes another set of data from the ADC. Such a direct data acquisition system can be constructed using readily available components. Figure 1 shows the schematic layout of the equipment. We have designed the system around a Tektronix 4054 computer (used in the Capintec treatment planning system), which has convenient graphic display facilities, and an SHM 3-D scanning water phantom. The Tektronix computer is equipped with 4907 Option 30 (twin) disk drives, and has four ROM pack slots available. It only accepts BASIC as its computer language. Inputs from its keyboard and outputs from the computer are displayed on its screen. It can be programmed to display data in graphic form. The probes can be either ionization chambers or diode detectors. PTW microchambers with a volume of 0.1 cm3 are used in the percent design. The amplifiers are high input impedance current amplifiers. The multiplexer and analog-to-digital converter are combined as a single unit available from Trans Era as an ADC 652/752 ROM pack. It has 16 differential input channels with 12 bits (4096 parts) resolution over an input voltage range of f 10 volts.
TEKTRONIX COMPUTER
EINARY/EICD 110 (TRANS ERA 632/732 I
FIG.
1. Block diagram of equipment.
DATA ACQUISITION
SYSTEM
FOR EXTERNAL
BEAM
171
Output pulses from the computer control the movement of the three stepping motors which move the detector probe in three orthogonal directions. These pulses go through a Trans Era 632/732 binary/BCD I/O ROM pack computer interface. The stepping motors and position decoders are part of the SHM 3-D water phantom. Both Trans Era ROM packs are plugged into the computer ROM pack slots. COMPUTER SOFTWARE
We have written an interactive program in BASIC to control the movement of the detector probe and to accept data from the two probes and the position decoders for the detector probe. The program provides options for reviewing, editing, or measuring central axis or profile (off-axis ratio) data for each beam. Prior to the measurement, the initial position of the detector probe is manually set and its x, y, and z coordinates are entered into the computer memory. This position is constantly monitored in subsequent scans. The detector probe can scan in either X, y, or z direction under program control. To minimize effects of noise, the measurement at each point is made by sampling the outputs of measuring probes a fixed number of times. Normally, 160 samples are used, a number which is large enough to smooth out statistical fluctuations and yet requires a reasonable amount of time for each measurement. The number of samples recorded at each point can be changed at the beginning of each scan. For measurements on an accelerator, to smooth out fluctuations from the machine output, the outputs of the two probes are sampled from the ADC, and the value for the detector probe is digitally divided by that for the reference probe. The dose variation as a function of the position of the scanning detector, after correcting for machine fluctuations, is displayed in graphic form on the screen in real time. The profiles are measured at five different depths, each having 47 points lying on fan lines (see, e.g., Milan and Bentley (I)). During the measurement, facilities are provided for rescanning, for plotting a previously measured profile at the same depth for the same beam stored on a disk, for halting the measurement, or for aborting the measurement and getting a new beam. In the photon mode, the central axis data are measured at 30 points, which are then used for fitting an empirical function to obtain the coefficients required by the treatment planning program. In the electron mode, since ionization chambers are used to obtain the central axis data, two measurements are required-one with positive bias and another with negative bias on the probes. The data points are taken as the average of the two measurements. Eighty data points are made from a depth of 5 mm to a depth equal to twice the estimated extrapolated range of the electron energy under investigation. The effective measurement point is not at the center of the ionization chamber but at a point displaced 9 radius towards the electron source (see, e.g., Almond (2)). The ionization data are normalized so
LAM, TABORSKY,
172
$
AND LAM
80-
F cf N
,z w 2 5 ,"
60-
40-
0
2
4
6
8 OEPTH
JO
12
14
" 16
'
'
(cm)
Fm. 2. PIot of percentage ionization vs depth. The two straight lines are the least squares linearly fitted lines. The lower line is for the background due to X rays. The upper line is for the ionization due to electrons in the falloff region. The depth at which the two fitted lines intersect is the calculated practical range of the electrons. Energy = 20 MeV, cone size = 10 cm, source to water phantom surface distance (SSD) (cm) = 100, D,,, (cm) = 0.5, depth interval (cm) = 0.5.
FIG. 3. Plot of percentage depth dose vs depth. The central axis data due to an electron beam in Fig. 2 is converted into percentage depth dose. Here D maxis the point where measurements start. Energy = 20 MeV, cone size = 10 cm, SSD (cm) = 100, D,, = 0.5, depth interval (cm) = 0.5.
DATAACQUKITIONSYSTEMFOREXTERNALBEAM
173
that the maximum value is 100%. These raw data are stored as a permanent record of the ionization which can be analyzed immediately or at a later time. In the analysis of electron central axis data, the last 20 points are taken to be pure background due to X rays and are used for a linear least squares fit. The fitted line is assumed to give the ionization due to X rays alone at any depth. When this X-ray ionization is subtracted from the total ionization, the result is the ionization due to electrons alone. A linear least squares fit is made to the falloff region which includes those data points 20 to 80% above the X-ray background. The depth at which the two fitted lines intersect is the calculated practical range rP (Fig. 2). The energy E0 of the electrons at the surface is then calculated from the well-established range-energy relationship (see, e.g., ICRU Report 2 1) (3) &, = (RP + 0.376)/0.521. RESULTS AND DISCUSSION
For electron beam measurements, the percentage depth dose along the central axis required for generating isodose curves is obtained from the ionization curve (Fig. 2) by taking into account the perturbation correction factor (P,,,) due to the ionization chamber at each depth and the conversion factor for absorbed dose in water (Cs) for electrons of initial energy E0 (3). Thirty points at intervals specified by the user are generated at the end of the analysis. The results are then plotted on the screen which shows the percentage depth dose as a function of depth (Fig. 3). In the photon mode, the maximum ionization is taken to be 100%, and the display of the percentage depth dose along the central axis looks similar to that of the electron mode (Fig. 3) except that only 30 points are measured. For off-axis ratio measurements, the operations of both the electron mode and photon mode are identical. At each depth, the ionization along the central axis is first measured and normalized to unity. Then the detector probe is moved pass the edge to 1.3 times the specified field size. The measurements obtained are normalized to the central axis measurement. The data points, joined together by straight lines, are displayed in real time. Figure 4 shows an example of the profile obtained at the end of an off-axis ratio measurement of one of the five depths. The vertical lines define the center of the beam and the geometric edges. The abscissa tick marks correspond to the positions where the probe samples the radiation level. After all five profiles have been measured, the value at the 24th fan line, which is the center of each profile, is normalized to unity. Then all five profiles are displayed on the screen (Fig. 5). A typical measurement consisting of 160 samples per point for both central axis and profile data takes about 10 minutes for each beam. The program used in this data acquisition system is coded in BASIC with low program development cost. With off-the-shelf products, the hardware interfacing is simple to construct. It can be implemented very inexpensively as a data
174
LAM, TABORSKY,
AND LAM
FIG. 4. A typical display during profile scan. User definable keys CUD Key) are used in the program to interrupt a profile scan. The geometric beam width is defined by the two middle lines of the three vertical lines on each side. Each tick mark on the horizontal axis represents the position of the fan line where measurement is made. Level 1, wedge #6: field size = 21 cm, UD key: 12 = Rescan, 13 = Plot-O, 14 = Halt, 15 = Abort.
FIG. 5. Display at the end of profile scans. Each tick mark on the vertical axis represents 10% of the ionization. The ionization at the center of each profile is normalized to 100%. Wedge #6: field size = 21 cm.
DATA ACQUISITION
SYSTEM FOR EXTERNAL
BEAM
I75
acquisition package for the Capintec treatment planning system. The coding can be modified to generate data for other treatment planning systems using a different beam data format. REFERENCES 1. MILAN, J., AND BENTLEY, R. E. The storage and manipulation of radiation dose data in a small digital computer. Er. J. Raa’iol. 47, 115 (1974). 2. ALMOND, P. R. Radiation physics of electron beams. In “Clinical Applications of the Electron Beam” (Norah duV. Tapley, Ed.), pp. 7-80. Wiley, New York, 1976. 3. ICRU Report 21, “Radiation dosimetry: electrons with initial energies between 1 and 50 MeV,” 1972.