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Computer assisted dosimetry of scanned electron and photon beams for radiation therapy
Radiotherapyand Oncology,2 (1984) 261 269 Elsevier
261
RTO00072
Computer assisted dosimetry of scanned electron and photon beams for radiation therapy Peder N/ifstadius, Anders Brahme and Bo Nordell Department of Radiation Physics, Karolinska Institute and UniversiO, of Stockholm, Box 60204, S-104 Ol Stockholm, Sweden
(Received 10 February 1984,revisionreceived24 May 1984, accepted28 May 1984)
Key words: Scannedbeams; Computeddosimetry;Electrontherapy;Photon therapy;Electroncontaminationin photon beams.
Summary A computer controlled beam forming system for energies up to 50 MeV has been developed in order to produce high quality electron and photon beams for radiation therapy. The desired radiation field shape and dose distribution are achieved by programming the scanning pattern of a narrow and unfiltered electron or photon beam. The computer that controls the scanning pattern also performs dosimetric analyses in the resultant radiation beams. The system allows real time display of the measured dose distributions at a rate of up to five discrete dose values per second for a 15 cm square field. Measurements in scanned as well as in stationary electron and photon beams at energies of 10, 20 and 50 MeV are presented. Finally, the consequences of photon generated electrons in the very broad high energy photon beams that can be produced by a scanning system are illustrated and discussed.
Introduction A computer controlled beam forming system, for peak energies up to 50 MeV, has been developed in order to produce high quality electron and photon beams for radiation therapy. The system controls the direction of an elementary narrow unfiltered electron or photon beam [3], by means of computer controlled scanning magnets. These magnets are arranged in such a way that a bremsstrahlung target can be placed close to the scanning centre of the emerging beam. The scanned elementary electron beam can in this way be converted into a scanned elementary photon beam. By appropriate choice of 0167-8140/84/$03.00 9 1984ElsevierSciencePublishersB.V.
the target material the photon beam can be made narrow, especially at high energies. This provides high flexibility in the shaping of the absorbed dose distribution, both for electron and photon beams. Real time dose measurements in scanned electron and photon beams for radiation therapy involve special problems. Most of the dosimetric devices available today for on-line dose measurements in stationary beams are inapplicable in scanned beams. Dosimetry with integrating detectors is possible but more difficult, particularly if the radiation field is not scanned an integer number of complete cycles. To avoid problems of this nature, some type of intelligent integrating system is needed so that
262 the correct average contribution from different parts of the radiation field reaches the point of measurement. In the present system this problem has been solved by allowing the same microprocessor to control the scanning of the elementary beam and to perform dosimetric measurements in the resultant therapeutic beams.
Scanning system The electron beam is generated in a racetrack microtron with a maximum energy of 55 MeV [11]. The scanning system [3] is located at the end of a horizontal accelerator beam transport system. It consists of two scanning magnets with a 97 ~ bending magnet in between, as shown in Fig. 1. The first
scanning magnet deflects the beam in the horizontal plane. The second scanning magnet deflects the beam in the vertical plane. The bending magnet operates with a constant homogeneous field during the measurements. Its main function is to bend the beam into the treatment head of a future, gantry, and at the same time image the scanning center of the first horizontal scanning magnet on the corresponding center of the vertical scanning magnet. With this configuration the emerging beam appears to originate from a point source at the center of the last scanning magnet as seen in Fig. 1. This arrangement makes it possible to direct the beam towards an arbitrary point in the transversal plane, where a phantom or patient can be placed, by using the independent control of the beam direction provided by the scanning magnets in the two perpendicular planes orthogonal to the central axis of the beam.
Accelerator interface
P o w e r supply Hor. V e r t .
Diod control
Dosimetry system
I I Water phantom
~
Fig. 1. System design and experimental set-up. The water phantom was rotated 90 ~ relative to the horizontal plane (x-y plane) for clarity. The water phantom is provided with a 10 mm wide slit covered by a thin mylar film (0.1 mm) in the horizontal direction of the front surface (x axis).
A movable silicon diode was used as field detector for relative dosimetry in a 60 x 60 x 50 cm 3 water phantom (RFA 3, Therados). The pn junction of the silicon diode was of very small cross-section (0.5 mm), and drifted into the silicon to give a good spatial resolution and negligible angular dependence [6]. The reference monitor was a parallel plate transmission ionisation chamber (cf. Ref. 5) placed close to the scanning center of the last scanning magnet. The active area of the monitor covered almost the entire cross-section of the elementary beam at any deflection angle. A current limiting resistor, in series with the monitor chamber, was needed to make the signal level of the reference chamber compatible with that of the reference semiconductor diode normally used by the dosimetry system in stationary therapy beams. The positioning of the field detector was controlled by the computer in three independent, mutually perpendicular directions. For each accelerator pulse, the ratio of the field and reference signals was determined by the divider circuit of the dosi-
263 metry system, in order to make dose measurements independent of variations in the pulse contents of the macro pulse from the accelerator. All the dose ratios from a predetermined number of accelerator pulses, normally one or two times the number of grid points in a complete scan, are integrated to give the absorbed dose signal at a given position of the field detector. The dose ratio integrator is an external circuit which is connected to the analogue output of the diod dosimetry system. When the computer has completed a predetermined number of scans the content of the integrator is read by a sample-and-hold circuit which, in turn, is connected to an analogue recorder and to the analogue-to-digital converter. During the integration the field detector is stationary, and it is rapidly moved to its new position immediately after a complete integration cycle has been terminated. When using this technique the measured dose distributions will consist of a large number of discrete dose values corresponding to the predetermined field detector positions. The fact that the field detector during a short interval is moved at the same time as the beam is scanned across the field grid could cause slight errors in the measured dose value. However, if the number of grid points in a field and the number of complete scans per integration are high and the detector transport time short, the integration time will be much larger than the detector transport time and the error negligible. The maximum position error is simply half the distance the detector was moved multiplied by the ratio of the transport time to the scan time. The error can be further decreased by using scan grid patterns that start the irradiation along the periphery of the field. In this way most of the dose contributions to the moving detector will be practically zero during the transport time. The position error is generally less than a few tenths of a millimeter. The total nonlinearity of the dose integrator, sample-and-hold circuit and the D/A-converter is approximately 0.05%. At present the ratios between the field and reference signals are determined by an analogue divider circuit. For small field detector signals, of the order of 1% and below, the
nonlinearity and offset of the analogue divider circuit could be a problem. This is the case in scanned electron beams of high energy (> 25 MeV) where the elementary beam is very narrow (see Fig. 4) and the integrated dose value consists of a large number of small contributions in addition to the few large ones. This problem is especially important outside the irradiated field and in the photon background. With an accurately adjusted divider circuit the error is normally less than 1%. For these reasons it would be better to determine the ratios and perform the integration numerically. Such a system is presently under development.
Computer hardware In order to simplify dosimetry, and ideally make it as straightforward as in radiation fields originating from conventional stationary beams, the same microprocessor controls the scanning of the beam, the positioning of the field detector and the processing of the dosimetry signal. The CPU is a Zilog Z80A microprocessor and it is connected to a number of input/output devices such as D/A and A/D converters. The accuracy of conversion is 12 bits. The bipolar power supplies that drive the scanning magnets, the dosimetry control unit and the dose ratio integrator are connected to the computer as shown in Fig. 1. The accelerator is also interfaced to the computer for beam on/off switching. This is achieved by controlling the microwave oscillator which drives the klystron. Measured data are continuously displayed in colour on a graphical video monitor with a resolution of 240 x 240 pixels.
Program structure The program that scans the elementary radiation beams, and the program that controls and displays the dose measurements in the scanned beams, are executed in real time mode. The system also performs real time display of the measured data points. The software is structured in two main program
264 blocks: the first block contains all service routines that are needed for the design of scan matrixes and data processing. The second block contains the scan and dosimetry control programs operating during the measurements. The tasks controlled by the latter scan and dosimetry programs are executed with the following priorities: Priority 1. The magnetic fields of the two scanning magnets are set in accordance with the desired location of next beam pulse. This is performed in the time interval between two consecutive accelerator pulses. Priority 2. After a predetermined number of complete scans the integrated dose ratio is read, stored and the detector is placed in its next position. During the short transport time of the detector, the beam is still scanned according to the predetermined pattern to improve the reproducibility in the positioning of the beam pulses in the grid. Priority 3. Presentation of the last measured dose value on the graphical display9 The trigger pulse which initiates the acceleration of the electrons is used to interrupt the processor in order to synchronize the program execution (see Fig. 3). The dosimetry routine (priority 2) has been given a lower priority than the scan routine (priority 1) since it has to operate during the pauses between repeated executions of the scan routine. The time sequence of the different routines is shown in Fig. 2.
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Both the scanning and the dosimetry routines are executed in parallel by the computer. The program flow during measurements are illustrated in Fig. 3. It is possible to run the scan routine independently of the measuring routine. This allows the detector
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Fig. 2. Time sequence for execution of the scan and dosimetry routines. The program switching is controlled by the trigger pulse that initiates the electron beam pulse. Each trigger pulse causes an interruption in the dosimetry routine. The scan routine starts immediately after the dosimetry routine has been interrupted. The dosimetry routine is continued from the point where it was interrupted after a complete execution of the scan routine.
Fig. 3. Main steps in the scanning and dosimetry routines. Both routines are executed intermittently according to their priorities. The program switching is controlled by the interrupt control. The scan interlock must be released before scanning can take place. The routines are executed according to Fig. 2. Beam scanning is interlocked by the dosimetry routine after the last dose value has been integrated and stored in the computer memory. The common register is needed for communication between the two routines.
265 system to be operated manually or under the control of the diod scanner even with scanned beams. With an accelerator pulse repetition frequency of 250 Hz, as many as five discrete dose values are obtained per second for a field with 50 grid points. The on/off control of the accelerator itself has also been interfaced to the computer system. This facilitates dosimetry with integrating detector systems in beams scanned an integer number of times. In this mode the computer is programmed to switch on the beam and perform a predetermined number of scans across the field. When the last scan has been completed the beam is switched off. In this way the mean energy imparted to the phantom is accurately known. The computer is also provided with programs for the design of scan matrixes and for data processing. The maximum field that can be scanned is enclosed in a rectangular matrix with a resolution of 256 x 256 pixels. The distance between two adjacent pixels corresponds to about 2 mm at the phantom surface. Regular or irregular scan matrix patterns can easily be generated and modified by means of a graphical editor. Dosimetry data are displayed as curves which can be scaled to fit a given normalisation level and be moved folded, added or subtracted. These possibilities greatly facilitate comparison of dose distributions measured in scanned beams using different grids with measurements in stationary beams regarding factors such as uniformity, depth dose and symmetry.
Measurements The performance of the dosimetry system and the principal dosimetric properties of scanned electron and photon beams are illustrated in Figs. 4/9. Electrons. Figure 4 shows the lateral dose profiles measured when the elementary electron beam was directed along the central axis of the scanning system and incident perpendicularly on the phantom surface at energies of 10, 20 and 50 MeV. In this case the beam was made stationary simply by switching off the power supplies to the scanning
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Fig. 4. Lateral dose profiles obtained in 10, 20 and 50 MeV elementary electron beams at an SSD of 100 cm. The curves were obtained at the depths 15, 25 and 50 m m , respectively.
magnets. In this mode the beam properties can be regarded as similar to those achieved by conventional beam techniques. Each profile in Fig. 4 was measured both with the integrating dosimetric system used in the scanning mode and with the conventional dosimetric system for stationary beams. The measurements were made at discrete points with a spacing of 2 mm. Very good agreement between the two methods was obtained down to 0.5% of the maximum dose level. The decreased width of the elementary beams as the energy is increased is due to the decreased effect of multiple scatter in the vacuum window, monitor chamber and air, but is also due to the decrease in the emittance of the beam from the accelerator. Broad uniform electron beams can be produced when the elementary electron beams are scanned in an appropriate way over the field as shown in Fig. 5. The dotted line in Fig. 5 shows the cross beam distribution of a broad scanned 20 MeV beam consisting of 5 x 5 elementary beams on a square grid, with a spacing of 7 cm measured at a depth of 25 ram. It can be seen that the dose distribution of a field collimated down to 20 x 20 cm 2 at the surface is far from uniform at this fairly wide grid spacing. The dashed curve corresponds to a decreased grid spacing consisting of 6 x 6 beams at a spacing of 5.8 cm. This distribution still contains small fluc-
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Fig. 5. Cross beam profiles for a 20 MeV scanned field collimated to 20 • 20 cm 2 at the phantom surface. The dose profiles are measured in fields with a regular square grid of different spacings. The solid line curve is the scanned elementary beam, which was focused slightly stronger by quadropoles than in Fig. 4.
tuations but less than 3% in amplitude. The dashed curve for a 7 • 7 grid and a spacing of 5 cm is very smooth with almost no visible fluctuations due to the discrete beams. This should be expected from our previous analyses [4], because the dose fluctuations decrease significantly when the grid spacing approaches the root mean square spread of the elementary beam which was 4 cm in the present case. For comparison one o f the elementary beams has been included as the solid line curve. In Fig. 6 the depth dose curves corresponding to a 7 • 7 grid are shown for electron beam energies
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Fig. 6. Depth-dose curves for scanned 10, 20 anti 50 MeV electron beams. The field size is 20 • 20 cm 2 at 100 cm SSD.
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Fig. 7. The lateral dose profile (solid line, left scale) in a 50 MeV wedge field produced by a non-uniform grid spacing. The contributions from each of the six elementary beams are also indicated (dashed lines, right scale).
of 10, 20 and 50 MeV. The collimated field size is 20 • 20 cm 2 as in Fig. 5. The normalized dose gradient G [2] is quite high (about 3.0-3.1) for energies from 10 MeV up to about 30 MeV, but decreases from 30 MeV due to the short source to surface distance and the increased influence of range and energy straggling in the phantom. Photons. Due to the width of the intrinsic bremsstrahlung process and the influence of multiple scattering in the target, the width of the elementary photon beam is generally larger than that of electron beams [4,9]. Many representative lateral dose profiles have already been published in the references cited. We shall therefore concentrate on illustrating the shaping of a wedge field by modifying the scan pattern of the elementary bremsstrahlung beam. This is shown in Fig. 7, where the dashed curves correspond to the lateral profiles of each elementary photon beam, and the solid curve represents the total dose, Dtot, of all elementary beams assuming their weight factors at dose maximum, Dm,el, to be the same. It can be seen that even with a small number of elementary beams a very distinct wedge effect can be achieved. In Fig. 8 the central axis depth dose curves are shown for the elementary unfiltered bremsstrahlung beams from a 2 mm tantalum target, followed by 4 cm aluminium as electron stopper. The incident electron beam energies were again 10, 20 and 50
267
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Fig. 8. Central axis depth-dose curves for 10, 20 and 50 MeV elementary photon beams at an SSD of 100 cm. MeV. Since the beams are unflattened the curves are comparable to the depth doses in small uniform fields of 10, 20 and 50 MeV photon beams. The curves have not been corrected for the depth of the radiation-sensitive layer beyond the front surface of the detector (about 1 mm in Figs. 6, 8 and 9). Figure 9 shows the corresponding depth dose curves for 20 x 20 cm uniform photon beams obtained by scanning the elementary photon beams over a very wide area ( ~ > 50 cm). A comparison o f Figs. 8 and 9 shows that the surface dose has increased considerably and, at the same time, the dose maximum has moved closer to the phantom surface by 4, 7 and 15 mm, respectively. However, the almost exponential dose fall-off portions of the 1.0
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Fig. 9. Central axis depth dose curves obtained in uniformly scanned 10, 20 and 50 MeV photon fields. The field size is 20 x 20 cm at 100 cm SSD. The additional electroncontamination compared to an elementarybeam is given in the lower left-hand corner.
A scanning system is essential for beam flattening at energies higher than 25 MeV because conventional flattening techniques, such as the use of electron scattering foils and highly attenuating photon filters, have considerable influence on the beam quality, especially when broad beams have to be produced. In addition, the benefits of a scanning system can be exploited at any available accelerator energy without using additional hardware such as scattering foils or flattening filters. Beam flattening, both for photons and electrons, is thus achieved solely by means of the computer software. An important advantage of the present beam forming system is that the scanning pattern can easily be varied to compensate for the field size dependency of the scattered photon dose contribution. In this way optimum beam flattening can be obtained for any depth of the target volume and any desired field size or even field shape. Similarly, a large number of special filter techniques such as wedge filters and compensators for body contours and inhomogeneities can be accomplished simply by modifying the scanning software. This could be achieved by allowing the scan step and/or the dose per pulse to be continuously variable, in order to generate the desired dose distribution. The physical advantages with scanned electron beams have long been known (cf. Ref. 2) and need no further discussion here. The most surprising resuits of this investigation are instead seen in the properties of the scanned photon beams. Very broad uniform high energy photon beams have not really been available for dosimetry before. (Em,x = 50 MeV; Z = 50 cm). The new result is that the well-known electron contamination problem in low megavoltage beams is a major problem also in these broad high energy beams. The causes of the observed shifts in the dose maximum and the descending depth doses for photon beams are well known:
268 the beam is contaminated by electrons and positrons produced by photons mainly through Compton and pair producing interactions with materials in the beam [8]. The principal sources are, however, quite different in the three cases presented here. At energies below about 15 MV the air volume is the principal source since most electrons from the target are lost due to multiple scatter in the air. At energies higher than about 25 MV, however, the situation is reversed since the Compton and pair electrons and positrons from the target or filter have such a high energy and small scattering power that they easily penetrate the air volume to reach the phantom. Due to the simultaneous long range of the electrons, the air contribution is negligible because about 50 m are needed to obtain a complete fluence build-up in air at these energies. At intermediate energies from 15 to 25 MV both these sources are present but with a smaller net contribution. Due to the wide energy and angular spread of the photon generated electrons their depth dose is almost triangular, as shown in the lower left corner of Fig. 9 by taking the difference between the elementary and broad beam depth-dose curves. The mean electron energy is about one quarter of the monochromatic electron beam energy incident on the target. It can thus be concluded that the presence of a good dose build-up in low and intermediate energy photon beams is a fortunate effect of the presence of scatter filtering of low energy electrons in the air [1]. This effect is not important in high energy beams, and the target or filter electrons are generally the dominating cause of depth dose degradation in high energy photon beams. The influence of atomic number and thickness of flattening filters and targets on the photon spectrum is therefore often not the principal factor affecting the shape of the depth-dose curve. It can be seen by inspecting the pair and Compton cross sections that the net electron contamination from an aluminium filter is generally smaller than from a lead filter, even though the higher scattering power of lead reduces the clinically observed electron contamination [7]. The effects of the electron contamination on the shape of the depth-dose curve was overlooked in
previous discussions on target and filter designs [2,10]. The electron contamination at low energies can be reduced considerably by using hydrogen or helium instead of air in the gap between source and phantom [8]. At high energies, however, the only possibility is to use a purging magnet to remove the high energy electrons and positrons in order to get a very clean photon beam. As seen by comparing Figs. 8 and 9, such a magnet is a necessity for producing high energy photon beams of high quality. This explains why photon energies above approximately 25 MV so far have been of little additional clinical use. The effect of these different methods to reduce the electron contamination in high energy photon beams will be discussed in more detail in forthcoming publications.
Acknowledgement Financial support from the Swedish Cancer Society and technical assistance from the accelerator group at the Royal Institute of Technology are gratefully acknowledged.
References 1 Brahme, A. Electrontransport phenomena and absorbed dose distributions in therapeutic electron beams. Paper presented at 14th Int. Congr. Radiol. Rio de Janeiro, Brazil, 1977. 2 Brahme, A. and Svensson, H. Radiation beam characteristics of a 22 MeV microtron. Acta Radiol. 18: 244~272, 1979. 3 Brahme,A., Kraepelien,T. and Svensson,H. Electronand photon beam characteristics from a 50 MeV Racetrack Microtron. Acta Radiol. 19: 305-~319, 1980. 4 Brahme, A.., Lax, I. and Andreo, P. Electronbeam dose planning using discrete Gaussian beams. Acta Radiol. Oncol. 20:147 158, 1981. 5 Burmester, U., Muller, U., Rosenow, U. and Wustenfeld, H. Die Verwendung des Monitorsignals des Elektronenbeschleunigers Philips SL 75/20 als Referenzsignal f/Jr das ferngesteuerte Wasserphantom. Therados RFA-3. Strahlentherapie 159: 483~484, 1983. 6 Lax, I., Brahme, A. and Andreo, P. Electronbeam dose planning using Gaussian beams: Improved radial dose pro-
269 files. Proceedings of the symposium on computerized electron beam dose planning, Stockholm, 1982. Acta Radiol. suppl. 364: 49-59, 1983. 7 Nilsson, B. Electron contamination from filters in high energy photon beams. Phys. Med. Biol., submitted, 1984. 8 Nilsson, B. and Brahme, A. Absorbed dose from secondary electrons in high energy photon beams. Phys. Med. Biol. 24: 901-912, 1979. 9 Nordell, B. and Brahme, A. Angular distribution and yield from bremsstrahlung targets. Phys. Med. Biol. 29:797 810, 1984.
10 Podgorsak, E. B., Rawlinson, J. A., Glavinovic, M. I. and Johns, H.E. Design of X-ray targets for high energy linear accelerators in radiotherapy. Am. J. Roentgenol. 121: 873882, 1974. 11 Rosander, S., Sedlacek, M., Wernholm, O. and Babic, H. The 50 MeV racetrack microtron at the Royal Institute of Technology, Stockholm. Nucl. Instrum. Methods 204:1 20, 1982.
261
RTO00072
Computer assisted dosimetry of scanned electron and photon beams for radiation therapy Peder N/ifstadius, Anders Brahme and Bo Nordell Department of Radiation Physics, Karolinska Institute and UniversiO, of Stockholm, Box 60204, S-104 Ol Stockholm, Sweden
(Received 10 February 1984,revisionreceived24 May 1984, accepted28 May 1984)
Key words: Scannedbeams; Computeddosimetry;Electrontherapy;Photon therapy;Electroncontaminationin photon beams.
Summary A computer controlled beam forming system for energies up to 50 MeV has been developed in order to produce high quality electron and photon beams for radiation therapy. The desired radiation field shape and dose distribution are achieved by programming the scanning pattern of a narrow and unfiltered electron or photon beam. The computer that controls the scanning pattern also performs dosimetric analyses in the resultant radiation beams. The system allows real time display of the measured dose distributions at a rate of up to five discrete dose values per second for a 15 cm square field. Measurements in scanned as well as in stationary electron and photon beams at energies of 10, 20 and 50 MeV are presented. Finally, the consequences of photon generated electrons in the very broad high energy photon beams that can be produced by a scanning system are illustrated and discussed.
Introduction A computer controlled beam forming system, for peak energies up to 50 MeV, has been developed in order to produce high quality electron and photon beams for radiation therapy. The system controls the direction of an elementary narrow unfiltered electron or photon beam [3], by means of computer controlled scanning magnets. These magnets are arranged in such a way that a bremsstrahlung target can be placed close to the scanning centre of the emerging beam. The scanned elementary electron beam can in this way be converted into a scanned elementary photon beam. By appropriate choice of 0167-8140/84/$03.00 9 1984ElsevierSciencePublishersB.V.
the target material the photon beam can be made narrow, especially at high energies. This provides high flexibility in the shaping of the absorbed dose distribution, both for electron and photon beams. Real time dose measurements in scanned electron and photon beams for radiation therapy involve special problems. Most of the dosimetric devices available today for on-line dose measurements in stationary beams are inapplicable in scanned beams. Dosimetry with integrating detectors is possible but more difficult, particularly if the radiation field is not scanned an integer number of complete cycles. To avoid problems of this nature, some type of intelligent integrating system is needed so that
262 the correct average contribution from different parts of the radiation field reaches the point of measurement. In the present system this problem has been solved by allowing the same microprocessor to control the scanning of the elementary beam and to perform dosimetric measurements in the resultant therapeutic beams.
Scanning system The electron beam is generated in a racetrack microtron with a maximum energy of 55 MeV [11]. The scanning system [3] is located at the end of a horizontal accelerator beam transport system. It consists of two scanning magnets with a 97 ~ bending magnet in between, as shown in Fig. 1. The first
scanning magnet deflects the beam in the horizontal plane. The second scanning magnet deflects the beam in the vertical plane. The bending magnet operates with a constant homogeneous field during the measurements. Its main function is to bend the beam into the treatment head of a future, gantry, and at the same time image the scanning center of the first horizontal scanning magnet on the corresponding center of the vertical scanning magnet. With this configuration the emerging beam appears to originate from a point source at the center of the last scanning magnet as seen in Fig. 1. This arrangement makes it possible to direct the beam towards an arbitrary point in the transversal plane, where a phantom or patient can be placed, by using the independent control of the beam direction provided by the scanning magnets in the two perpendicular planes orthogonal to the central axis of the beam.
Accelerator interface
P o w e r supply Hor. V e r t .
Diod control
Dosimetry system
I I Water phantom
~
Fig. 1. System design and experimental set-up. The water phantom was rotated 90 ~ relative to the horizontal plane (x-y plane) for clarity. The water phantom is provided with a 10 mm wide slit covered by a thin mylar film (0.1 mm) in the horizontal direction of the front surface (x axis).
A movable silicon diode was used as field detector for relative dosimetry in a 60 x 60 x 50 cm 3 water phantom (RFA 3, Therados). The pn junction of the silicon diode was of very small cross-section (0.5 mm), and drifted into the silicon to give a good spatial resolution and negligible angular dependence [6]. The reference monitor was a parallel plate transmission ionisation chamber (cf. Ref. 5) placed close to the scanning center of the last scanning magnet. The active area of the monitor covered almost the entire cross-section of the elementary beam at any deflection angle. A current limiting resistor, in series with the monitor chamber, was needed to make the signal level of the reference chamber compatible with that of the reference semiconductor diode normally used by the dosimetry system in stationary therapy beams. The positioning of the field detector was controlled by the computer in three independent, mutually perpendicular directions. For each accelerator pulse, the ratio of the field and reference signals was determined by the divider circuit of the dosi-
263 metry system, in order to make dose measurements independent of variations in the pulse contents of the macro pulse from the accelerator. All the dose ratios from a predetermined number of accelerator pulses, normally one or two times the number of grid points in a complete scan, are integrated to give the absorbed dose signal at a given position of the field detector. The dose ratio integrator is an external circuit which is connected to the analogue output of the diod dosimetry system. When the computer has completed a predetermined number of scans the content of the integrator is read by a sample-and-hold circuit which, in turn, is connected to an analogue recorder and to the analogue-to-digital converter. During the integration the field detector is stationary, and it is rapidly moved to its new position immediately after a complete integration cycle has been terminated. When using this technique the measured dose distributions will consist of a large number of discrete dose values corresponding to the predetermined field detector positions. The fact that the field detector during a short interval is moved at the same time as the beam is scanned across the field grid could cause slight errors in the measured dose value. However, if the number of grid points in a field and the number of complete scans per integration are high and the detector transport time short, the integration time will be much larger than the detector transport time and the error negligible. The maximum position error is simply half the distance the detector was moved multiplied by the ratio of the transport time to the scan time. The error can be further decreased by using scan grid patterns that start the irradiation along the periphery of the field. In this way most of the dose contributions to the moving detector will be practically zero during the transport time. The position error is generally less than a few tenths of a millimeter. The total nonlinearity of the dose integrator, sample-and-hold circuit and the D/A-converter is approximately 0.05%. At present the ratios between the field and reference signals are determined by an analogue divider circuit. For small field detector signals, of the order of 1% and below, the
nonlinearity and offset of the analogue divider circuit could be a problem. This is the case in scanned electron beams of high energy (> 25 MeV) where the elementary beam is very narrow (see Fig. 4) and the integrated dose value consists of a large number of small contributions in addition to the few large ones. This problem is especially important outside the irradiated field and in the photon background. With an accurately adjusted divider circuit the error is normally less than 1%. For these reasons it would be better to determine the ratios and perform the integration numerically. Such a system is presently under development.
Computer hardware In order to simplify dosimetry, and ideally make it as straightforward as in radiation fields originating from conventional stationary beams, the same microprocessor controls the scanning of the beam, the positioning of the field detector and the processing of the dosimetry signal. The CPU is a Zilog Z80A microprocessor and it is connected to a number of input/output devices such as D/A and A/D converters. The accuracy of conversion is 12 bits. The bipolar power supplies that drive the scanning magnets, the dosimetry control unit and the dose ratio integrator are connected to the computer as shown in Fig. 1. The accelerator is also interfaced to the computer for beam on/off switching. This is achieved by controlling the microwave oscillator which drives the klystron. Measured data are continuously displayed in colour on a graphical video monitor with a resolution of 240 x 240 pixels.
Program structure The program that scans the elementary radiation beams, and the program that controls and displays the dose measurements in the scanned beams, are executed in real time mode. The system also performs real time display of the measured data points. The software is structured in two main program
264 blocks: the first block contains all service routines that are needed for the design of scan matrixes and data processing. The second block contains the scan and dosimetry control programs operating during the measurements. The tasks controlled by the latter scan and dosimetry programs are executed with the following priorities: Priority 1. The magnetic fields of the two scanning magnets are set in accordance with the desired location of next beam pulse. This is performed in the time interval between two consecutive accelerator pulses. Priority 2. After a predetermined number of complete scans the integrated dose ratio is read, stored and the detector is placed in its next position. During the short transport time of the detector, the beam is still scanned according to the predetermined pattern to improve the reproducibility in the positioning of the beam pulses in the grid. Priority 3. Presentation of the last measured dose value on the graphical display9 The trigger pulse which initiates the acceleration of the electrons is used to interrupt the processor in order to synchronize the program execution (see Fig. 3). The dosimetry routine (priority 2) has been given a lower priority than the scan routine (priority 1) since it has to operate during the pauses between repeated executions of the scan routine. The time sequence of the different routines is shown in Fig. 2.
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Both the scanning and the dosimetry routines are executed in parallel by the computer. The program flow during measurements are illustrated in Fig. 3. It is possible to run the scan routine independently of the measuring routine. This allows the detector
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Fig. 2. Time sequence for execution of the scan and dosimetry routines. The program switching is controlled by the trigger pulse that initiates the electron beam pulse. Each trigger pulse causes an interruption in the dosimetry routine. The scan routine starts immediately after the dosimetry routine has been interrupted. The dosimetry routine is continued from the point where it was interrupted after a complete execution of the scan routine.
Fig. 3. Main steps in the scanning and dosimetry routines. Both routines are executed intermittently according to their priorities. The program switching is controlled by the interrupt control. The scan interlock must be released before scanning can take place. The routines are executed according to Fig. 2. Beam scanning is interlocked by the dosimetry routine after the last dose value has been integrated and stored in the computer memory. The common register is needed for communication between the two routines.
265 system to be operated manually or under the control of the diod scanner even with scanned beams. With an accelerator pulse repetition frequency of 250 Hz, as many as five discrete dose values are obtained per second for a field with 50 grid points. The on/off control of the accelerator itself has also been interfaced to the computer system. This facilitates dosimetry with integrating detector systems in beams scanned an integer number of times. In this mode the computer is programmed to switch on the beam and perform a predetermined number of scans across the field. When the last scan has been completed the beam is switched off. In this way the mean energy imparted to the phantom is accurately known. The computer is also provided with programs for the design of scan matrixes and for data processing. The maximum field that can be scanned is enclosed in a rectangular matrix with a resolution of 256 x 256 pixels. The distance between two adjacent pixels corresponds to about 2 mm at the phantom surface. Regular or irregular scan matrix patterns can easily be generated and modified by means of a graphical editor. Dosimetry data are displayed as curves which can be scaled to fit a given normalisation level and be moved folded, added or subtracted. These possibilities greatly facilitate comparison of dose distributions measured in scanned beams using different grids with measurements in stationary beams regarding factors such as uniformity, depth dose and symmetry.
Measurements The performance of the dosimetry system and the principal dosimetric properties of scanned electron and photon beams are illustrated in Figs. 4/9. Electrons. Figure 4 shows the lateral dose profiles measured when the elementary electron beam was directed along the central axis of the scanning system and incident perpendicularly on the phantom surface at energies of 10, 20 and 50 MeV. In this case the beam was made stationary simply by switching off the power supplies to the scanning
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Fig. 4. Lateral dose profiles obtained in 10, 20 and 50 MeV elementary electron beams at an SSD of 100 cm. The curves were obtained at the depths 15, 25 and 50 m m , respectively.
magnets. In this mode the beam properties can be regarded as similar to those achieved by conventional beam techniques. Each profile in Fig. 4 was measured both with the integrating dosimetric system used in the scanning mode and with the conventional dosimetric system for stationary beams. The measurements were made at discrete points with a spacing of 2 mm. Very good agreement between the two methods was obtained down to 0.5% of the maximum dose level. The decreased width of the elementary beams as the energy is increased is due to the decreased effect of multiple scatter in the vacuum window, monitor chamber and air, but is also due to the decrease in the emittance of the beam from the accelerator. Broad uniform electron beams can be produced when the elementary electron beams are scanned in an appropriate way over the field as shown in Fig. 5. The dotted line in Fig. 5 shows the cross beam distribution of a broad scanned 20 MeV beam consisting of 5 x 5 elementary beams on a square grid, with a spacing of 7 cm measured at a depth of 25 ram. It can be seen that the dose distribution of a field collimated down to 20 x 20 cm 2 at the surface is far from uniform at this fairly wide grid spacing. The dashed curve corresponds to a decreased grid spacing consisting of 6 x 6 beams at a spacing of 5.8 cm. This distribution still contains small fluc-
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Fig. 5. Cross beam profiles for a 20 MeV scanned field collimated to 20 • 20 cm 2 at the phantom surface. The dose profiles are measured in fields with a regular square grid of different spacings. The solid line curve is the scanned elementary beam, which was focused slightly stronger by quadropoles than in Fig. 4.
tuations but less than 3% in amplitude. The dashed curve for a 7 • 7 grid and a spacing of 5 cm is very smooth with almost no visible fluctuations due to the discrete beams. This should be expected from our previous analyses [4], because the dose fluctuations decrease significantly when the grid spacing approaches the root mean square spread of the elementary beam which was 4 cm in the present case. For comparison one o f the elementary beams has been included as the solid line curve. In Fig. 6 the depth dose curves corresponding to a 7 • 7 grid are shown for electron beam energies
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Fig. 6. Depth-dose curves for scanned 10, 20 anti 50 MeV electron beams. The field size is 20 • 20 cm 2 at 100 cm SSD.
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Fig. 7. The lateral dose profile (solid line, left scale) in a 50 MeV wedge field produced by a non-uniform grid spacing. The contributions from each of the six elementary beams are also indicated (dashed lines, right scale).
of 10, 20 and 50 MeV. The collimated field size is 20 • 20 cm 2 as in Fig. 5. The normalized dose gradient G [2] is quite high (about 3.0-3.1) for energies from 10 MeV up to about 30 MeV, but decreases from 30 MeV due to the short source to surface distance and the increased influence of range and energy straggling in the phantom. Photons. Due to the width of the intrinsic bremsstrahlung process and the influence of multiple scattering in the target, the width of the elementary photon beam is generally larger than that of electron beams [4,9]. Many representative lateral dose profiles have already been published in the references cited. We shall therefore concentrate on illustrating the shaping of a wedge field by modifying the scan pattern of the elementary bremsstrahlung beam. This is shown in Fig. 7, where the dashed curves correspond to the lateral profiles of each elementary photon beam, and the solid curve represents the total dose, Dtot, of all elementary beams assuming their weight factors at dose maximum, Dm,el, to be the same. It can be seen that even with a small number of elementary beams a very distinct wedge effect can be achieved. In Fig. 8 the central axis depth dose curves are shown for the elementary unfiltered bremsstrahlung beams from a 2 mm tantalum target, followed by 4 cm aluminium as electron stopper. The incident electron beam energies were again 10, 20 and 50
267
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Fig. 8. Central axis depth-dose curves for 10, 20 and 50 MeV elementary photon beams at an SSD of 100 cm. MeV. Since the beams are unflattened the curves are comparable to the depth doses in small uniform fields of 10, 20 and 50 MeV photon beams. The curves have not been corrected for the depth of the radiation-sensitive layer beyond the front surface of the detector (about 1 mm in Figs. 6, 8 and 9). Figure 9 shows the corresponding depth dose curves for 20 x 20 cm uniform photon beams obtained by scanning the elementary photon beams over a very wide area ( ~ > 50 cm). A comparison o f Figs. 8 and 9 shows that the surface dose has increased considerably and, at the same time, the dose maximum has moved closer to the phantom surface by 4, 7 and 15 mm, respectively. However, the almost exponential dose fall-off portions of the 1.0
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Fig. 9. Central axis depth dose curves obtained in uniformly scanned 10, 20 and 50 MeV photon fields. The field size is 20 x 20 cm at 100 cm SSD. The additional electroncontamination compared to an elementarybeam is given in the lower left-hand corner.
A scanning system is essential for beam flattening at energies higher than 25 MeV because conventional flattening techniques, such as the use of electron scattering foils and highly attenuating photon filters, have considerable influence on the beam quality, especially when broad beams have to be produced. In addition, the benefits of a scanning system can be exploited at any available accelerator energy without using additional hardware such as scattering foils or flattening filters. Beam flattening, both for photons and electrons, is thus achieved solely by means of the computer software. An important advantage of the present beam forming system is that the scanning pattern can easily be varied to compensate for the field size dependency of the scattered photon dose contribution. In this way optimum beam flattening can be obtained for any depth of the target volume and any desired field size or even field shape. Similarly, a large number of special filter techniques such as wedge filters and compensators for body contours and inhomogeneities can be accomplished simply by modifying the scanning software. This could be achieved by allowing the scan step and/or the dose per pulse to be continuously variable, in order to generate the desired dose distribution. The physical advantages with scanned electron beams have long been known (cf. Ref. 2) and need no further discussion here. The most surprising resuits of this investigation are instead seen in the properties of the scanned photon beams. Very broad uniform high energy photon beams have not really been available for dosimetry before. (Em,x = 50 MeV; Z = 50 cm). The new result is that the well-known electron contamination problem in low megavoltage beams is a major problem also in these broad high energy beams. The causes of the observed shifts in the dose maximum and the descending depth doses for photon beams are well known:
268 the beam is contaminated by electrons and positrons produced by photons mainly through Compton and pair producing interactions with materials in the beam [8]. The principal sources are, however, quite different in the three cases presented here. At energies below about 15 MV the air volume is the principal source since most electrons from the target are lost due to multiple scatter in the air. At energies higher than about 25 MV, however, the situation is reversed since the Compton and pair electrons and positrons from the target or filter have such a high energy and small scattering power that they easily penetrate the air volume to reach the phantom. Due to the simultaneous long range of the electrons, the air contribution is negligible because about 50 m are needed to obtain a complete fluence build-up in air at these energies. At intermediate energies from 15 to 25 MV both these sources are present but with a smaller net contribution. Due to the wide energy and angular spread of the photon generated electrons their depth dose is almost triangular, as shown in the lower left corner of Fig. 9 by taking the difference between the elementary and broad beam depth-dose curves. The mean electron energy is about one quarter of the monochromatic electron beam energy incident on the target. It can thus be concluded that the presence of a good dose build-up in low and intermediate energy photon beams is a fortunate effect of the presence of scatter filtering of low energy electrons in the air [1]. This effect is not important in high energy beams, and the target or filter electrons are generally the dominating cause of depth dose degradation in high energy photon beams. The influence of atomic number and thickness of flattening filters and targets on the photon spectrum is therefore often not the principal factor affecting the shape of the depth-dose curve. It can be seen by inspecting the pair and Compton cross sections that the net electron contamination from an aluminium filter is generally smaller than from a lead filter, even though the higher scattering power of lead reduces the clinically observed electron contamination [7]. The effects of the electron contamination on the shape of the depth-dose curve was overlooked in
previous discussions on target and filter designs [2,10]. The electron contamination at low energies can be reduced considerably by using hydrogen or helium instead of air in the gap between source and phantom [8]. At high energies, however, the only possibility is to use a purging magnet to remove the high energy electrons and positrons in order to get a very clean photon beam. As seen by comparing Figs. 8 and 9, such a magnet is a necessity for producing high energy photon beams of high quality. This explains why photon energies above approximately 25 MV so far have been of little additional clinical use. The effect of these different methods to reduce the electron contamination in high energy photon beams will be discussed in more detail in forthcoming publications.
Acknowledgement Financial support from the Swedish Cancer Society and technical assistance from the accelerator group at the Royal Institute of Technology are gratefully acknowledged.
References 1 Brahme, A. Electrontransport phenomena and absorbed dose distributions in therapeutic electron beams. Paper presented at 14th Int. Congr. Radiol. Rio de Janeiro, Brazil, 1977. 2 Brahme, A. and Svensson, H. Radiation beam characteristics of a 22 MeV microtron. Acta Radiol. 18: 244~272, 1979. 3 Brahme,A., Kraepelien,T. and Svensson,H. Electronand photon beam characteristics from a 50 MeV Racetrack Microtron. Acta Radiol. 19: 305-~319, 1980. 4 Brahme, A.., Lax, I. and Andreo, P. Electronbeam dose planning using discrete Gaussian beams. Acta Radiol. Oncol. 20:147 158, 1981. 5 Burmester, U., Muller, U., Rosenow, U. and Wustenfeld, H. Die Verwendung des Monitorsignals des Elektronenbeschleunigers Philips SL 75/20 als Referenzsignal f/Jr das ferngesteuerte Wasserphantom. Therados RFA-3. Strahlentherapie 159: 483~484, 1983. 6 Lax, I., Brahme, A. and Andreo, P. Electronbeam dose planning using Gaussian beams: Improved radial dose pro-
269 files. Proceedings of the symposium on computerized electron beam dose planning, Stockholm, 1982. Acta Radiol. suppl. 364: 49-59, 1983. 7 Nilsson, B. Electron contamination from filters in high energy photon beams. Phys. Med. Biol., submitted, 1984. 8 Nilsson, B. and Brahme, A. Absorbed dose from secondary electrons in high energy photon beams. Phys. Med. Biol. 24: 901-912, 1979. 9 Nordell, B. and Brahme, A. Angular distribution and yield from bremsstrahlung targets. Phys. Med. Biol. 29:797 810, 1984.
10 Podgorsak, E. B., Rawlinson, J. A., Glavinovic, M. I. and Johns, H.E. Design of X-ray targets for high energy linear accelerators in radiotherapy. Am. J. Roentgenol. 121: 873882, 1974. 11 Rosander, S., Sedlacek, M., Wernholm, O. and Babic, H. The 50 MeV racetrack microtron at the Royal Institute of Technology, Stockholm. Nucl. Instrum. Methods 204:1 20, 1982.