- Email: [email protected]
674 Poster Monte Carlo simulation of a neck phantom with a TLD-tube
Posters
Wednesday/Thursday, 18-19 September 2002 $205
metric verifications of the system delivering performances and development of appropriate quality assurance programs, Material and methods: ionimetric and film dosimetry measurements have been largely employed to characterise the 1D modulating system [PMB 40: 221-240, 1995] in terms of transmission and relative homogeneity, positioning accuracy, speed calibration, reproducibility of modulation factors at different speeds or gantry/collimator angles and verification of the agreement between the expected/calculated fluence profile and the measured one. A quality assurance program for tissue-deficit clinical implementation includes either pre-treatment checks to verify the correspondence between the planned incident modulated fluence profile and the measured one and in-vivo dose measurements by diodes and portal film dosimetry during the treatment. A more advanced application of the Single Absorber modulation technique in 3D conformal radiotherapy to spare critical organs needs appropriate corrective algorithms to counteract inertia effects, Results: A positioning accuracy within 2 mm (at the isocenter level) and a satisfactory reproducibility (SD~0.1%) for flat speed profiles and for single maximum/minimum profiles have been found during preliminary check& The agreement between calculated/expected and measured modulation factors of various speed profiles have been found within ± 3% for slow gradients, suitable for tissue-deficit compensation aims. The results obtained, either with homogeneous phantom and in clinical practice, confirm the possibility of homogenising in a satisfactory way the dose distribution in the whole "midline" isocentric plane. Deviations became increasingly relevant as the absorber bar executed steeper gradient for 3D conformal appiications, where appropriate correction algorithms should be used: however, by applying the proposed corrections, Single Absorber performances were improved to a satisfactory level, comparable to that one obtained for simple
most efficient technique to achieve desired beam profile is by scanning with elementary electron and photon beams combined with dynamic multi-leaf collimation. Such sophisticated treatment requires more accurate verification of the dose delivery. In this work a verification procedure for IMRT using scanned beams is developed. The position, and dose for each elementary beam pulse is measured with a segmented transmission ion-chamber mounted in the treatment head of a racetrack microtron. The ion-chamber is made of two separated layers, each devided in eight strips.The idea is to build a database, a lookup table, of known strips' responses which are unique for each scanned beam position. The signals for an unknown position are then compared with the known ones. The most similar one will represent the position of the beam. The lookup table, that will eventually will be build in to a dose planning program, consists of sixteen chamber strip signals for each possible scan position, that is a set of unique fingerprints. Although a rather simple least-square scoring function to determine the position of a beam was used the results are promising and the method will be developed.
applications. Conclusions: Intensity modulated fields can be efficaciously and accurately delivered using the Single Absorber 1D beam modulator; a rigorous and dedicated quality assurance program guarantees its accuracy and reproducibility,
Purpose: To evaluate the technical characteristics of the MOSFET-TNRD50 dosimetry system, and the relevant correctionfactors for routine in vivo dosimetry of photon beams. Materials and methods: Measurements were performed on an Elekta SLi15 linear accelerator. The reproducibility of the MOSFET-system was determined with a 6 MV photon beam, and compared with diodes and TLD's, Nine measurements were performed, delivering 200 ME's under standard conditions, i.e. a fieldsize of 10x10 cmz, the dosimeters positioned at the isocenter, the MOSFET's and TLD's covered with 1,5 cm build-up material. Angular-, fieldsize-, SSD- dependency and the influence of wedge and carbon fiber insert were established by comparison of MOSFET and ionisation chamber measurements. The relevant correction factors for daily practice were determined. Because of large deviations of the MOSFET readings, obtained without build-up material under increasing angles of beam incidence, the correction factors were determined under build-up at dmax. Results:The MOSFET-system shows a reproducibility of ± 1.8 % (1 SD) of the mean; for TLD's and Diodes we obtained respectively ± 2.9 % and ± 0.04 %. Correction factors for gantry angle, fieldsize, SSD, carbon fiber table top and wedges are listed in table 1.
674
Poster
Monte Carlo simulation o f a neck phantom w i t h a T L D - t u b e P. Harald~son 1,2, T. /(nOds1,3, P. EngstrOm1, H. Nystr6m I 1Copenhagen University Hospital, Radiation Physics, Copenhagen, Denmark 2Malm5 University Hospital, Radiation Physics, Malm5, Sweden 3Lund University Hospital, Radiation Physics, Lund, Sweden Purpose: The purpose of the study was to compare Monte Carlo simulations of a neck phantom with a TLD-tube inserted into a cylindrical air cavity, with data from a treatment planning system (TPS) and measurements. Methods: A neck phantom with a TLD-tube in a cylindrical air cavity have been defined in the EGS4 based XYZDOS Monte Carlo code. The code has been modified to include a poly energetic source as well as divergence of the beam. Simulations were performed with an open 10 x 10 cmz 6 MV-field and the TLD-tube placed at different positions within the cylindrical air cavity. The phantom was irradiated with the same geometries using 6 MV photons from a Varian 2300CD accelerator and the TLDs were then evaluated using a Harshaw 5500 TLD reader. CT-scanning of the phantom was done and imported into a TPS (CadPlan). Calculated dose distributions were compared with the Monte Carlo simulations as well as with measurements, especially With regard to the absorbed dose in the TLD-tube. Results: Differences between TPS calculated and MC simulated absorbed doses to the TLD-tube were found to be small compared to the uncertainty in the TLD measurement. Furthermore, dose profiles and depth doses through the cylindrical air cavity were used in order to investigate the absorbed dose in the interface regions between air and the TLD-tube. As expected, the TPS overestimated the dose in these regions, Conclusions: This study shows that for most situations using an open symmetric field, Monte Carlo simulations of TLD-measurements using a tube in a cylindrical air cavity gives results comparable to the treatment planning
system. 675
Poster
Optimized scanned beam dose delivery, real time dose mortitoring and treatment verification in radiation therapy A. Zapotoczna, R. Svensson Karo/inska /nstitute, Dept. of Medical Radiation Physics, Stockholm, Sweden Radiobiological optimized treatment planning shows that the radiotherapy outcome is heavily increased by use of intensity modulated beams. The
676 Poster C o m m i s s i o n i n g o f a MOSFET-TNRD-50 dosimetry system for routine measurements o f 6 MM a n d 10 MM external photon
beam radiation treatments 14/.du Bois, E. Bloemen-van Gurp, P. Visser, I. Bruinvis, D. Jafink, J. Hermans, P. Lambin, A. Minken RTIL /Academic Hospital Maastricht, Radiotherapy, Heerlen, The Nether. lands
Table 1.: correction factors for mosfet readings gantry angle *) t5 ° 3O° 45° 6 MV 1.01 1.00 1,02 10 MV 1.01 1.02 1.02
6O° 5x5 1.03 1.00 1.00 1,01
*) A correctionfactor of 1.01 per 25° full wedge 6 MV 1.05 I0 MV 1.02
fieldsize (cm2) 10 x 10 2o x 2O 3o x 3o 1.00 1.01 1.00 1.00
for both 6 and 10 MV beamsis used. carbon tabletop SSD 80 cm 1.02 1.01 1.02 1.00
Conclusion:The reproducibility of the MOSFET-TNRD-50 dosimetry system compares favourably with TLD's and meets the requirement for routine in vivo dosimetry. For 6 and 10 MV photon beams, the readings need only few and small corrections, provided that the dosimeters are placed under under build-up at dmax. 677
Poster
In-vivo dosimetry in electron beam radiotherapy with silicon diodes; the impact of the build-up cap thickness M. LOOt1, N. Joiner2, V. Ra-Betf3, P. Carrasco de Fez2, H. Nystrom 1 1The Finsen Centre, Copenhagen University Hospital, Radiation Physics dep., Copenhagen, Denmark 2Hospital de/a Sta Creu i St Pau, Radiation Physics dep, Barcelona, Spain 3Norfolk and Norwich University Hospital, Radiotherapy Physics dep, Norwich, England The sensitive volume of a diode intended for entrance dose measurements,
Wednesday/Thursday, 18-19 September 2002 $205
metric verifications of the system delivering performances and development of appropriate quality assurance programs, Material and methods: ionimetric and film dosimetry measurements have been largely employed to characterise the 1D modulating system [PMB 40: 221-240, 1995] in terms of transmission and relative homogeneity, positioning accuracy, speed calibration, reproducibility of modulation factors at different speeds or gantry/collimator angles and verification of the agreement between the expected/calculated fluence profile and the measured one. A quality assurance program for tissue-deficit clinical implementation includes either pre-treatment checks to verify the correspondence between the planned incident modulated fluence profile and the measured one and in-vivo dose measurements by diodes and portal film dosimetry during the treatment. A more advanced application of the Single Absorber modulation technique in 3D conformal radiotherapy to spare critical organs needs appropriate corrective algorithms to counteract inertia effects, Results: A positioning accuracy within 2 mm (at the isocenter level) and a satisfactory reproducibility (SD~0.1%) for flat speed profiles and for single maximum/minimum profiles have been found during preliminary check& The agreement between calculated/expected and measured modulation factors of various speed profiles have been found within ± 3% for slow gradients, suitable for tissue-deficit compensation aims. The results obtained, either with homogeneous phantom and in clinical practice, confirm the possibility of homogenising in a satisfactory way the dose distribution in the whole "midline" isocentric plane. Deviations became increasingly relevant as the absorber bar executed steeper gradient for 3D conformal appiications, where appropriate correction algorithms should be used: however, by applying the proposed corrections, Single Absorber performances were improved to a satisfactory level, comparable to that one obtained for simple
most efficient technique to achieve desired beam profile is by scanning with elementary electron and photon beams combined with dynamic multi-leaf collimation. Such sophisticated treatment requires more accurate verification of the dose delivery. In this work a verification procedure for IMRT using scanned beams is developed. The position, and dose for each elementary beam pulse is measured with a segmented transmission ion-chamber mounted in the treatment head of a racetrack microtron. The ion-chamber is made of two separated layers, each devided in eight strips.The idea is to build a database, a lookup table, of known strips' responses which are unique for each scanned beam position. The signals for an unknown position are then compared with the known ones. The most similar one will represent the position of the beam. The lookup table, that will eventually will be build in to a dose planning program, consists of sixteen chamber strip signals for each possible scan position, that is a set of unique fingerprints. Although a rather simple least-square scoring function to determine the position of a beam was used the results are promising and the method will be developed.
applications. Conclusions: Intensity modulated fields can be efficaciously and accurately delivered using the Single Absorber 1D beam modulator; a rigorous and dedicated quality assurance program guarantees its accuracy and reproducibility,
Purpose: To evaluate the technical characteristics of the MOSFET-TNRD50 dosimetry system, and the relevant correctionfactors for routine in vivo dosimetry of photon beams. Materials and methods: Measurements were performed on an Elekta SLi15 linear accelerator. The reproducibility of the MOSFET-system was determined with a 6 MV photon beam, and compared with diodes and TLD's, Nine measurements were performed, delivering 200 ME's under standard conditions, i.e. a fieldsize of 10x10 cmz, the dosimeters positioned at the isocenter, the MOSFET's and TLD's covered with 1,5 cm build-up material. Angular-, fieldsize-, SSD- dependency and the influence of wedge and carbon fiber insert were established by comparison of MOSFET and ionisation chamber measurements. The relevant correction factors for daily practice were determined. Because of large deviations of the MOSFET readings, obtained without build-up material under increasing angles of beam incidence, the correction factors were determined under build-up at dmax. Results:The MOSFET-system shows a reproducibility of ± 1.8 % (1 SD) of the mean; for TLD's and Diodes we obtained respectively ± 2.9 % and ± 0.04 %. Correction factors for gantry angle, fieldsize, SSD, carbon fiber table top and wedges are listed in table 1.
674
Poster
Monte Carlo simulation o f a neck phantom w i t h a T L D - t u b e P. Harald~son 1,2, T. /(nOds1,3, P. EngstrOm1, H. Nystr6m I 1Copenhagen University Hospital, Radiation Physics, Copenhagen, Denmark 2Malm5 University Hospital, Radiation Physics, Malm5, Sweden 3Lund University Hospital, Radiation Physics, Lund, Sweden Purpose: The purpose of the study was to compare Monte Carlo simulations of a neck phantom with a TLD-tube inserted into a cylindrical air cavity, with data from a treatment planning system (TPS) and measurements. Methods: A neck phantom with a TLD-tube in a cylindrical air cavity have been defined in the EGS4 based XYZDOS Monte Carlo code. The code has been modified to include a poly energetic source as well as divergence of the beam. Simulations were performed with an open 10 x 10 cmz 6 MV-field and the TLD-tube placed at different positions within the cylindrical air cavity. The phantom was irradiated with the same geometries using 6 MV photons from a Varian 2300CD accelerator and the TLDs were then evaluated using a Harshaw 5500 TLD reader. CT-scanning of the phantom was done and imported into a TPS (CadPlan). Calculated dose distributions were compared with the Monte Carlo simulations as well as with measurements, especially With regard to the absorbed dose in the TLD-tube. Results: Differences between TPS calculated and MC simulated absorbed doses to the TLD-tube were found to be small compared to the uncertainty in the TLD measurement. Furthermore, dose profiles and depth doses through the cylindrical air cavity were used in order to investigate the absorbed dose in the interface regions between air and the TLD-tube. As expected, the TPS overestimated the dose in these regions, Conclusions: This study shows that for most situations using an open symmetric field, Monte Carlo simulations of TLD-measurements using a tube in a cylindrical air cavity gives results comparable to the treatment planning
system. 675
Poster
Optimized scanned beam dose delivery, real time dose mortitoring and treatment verification in radiation therapy A. Zapotoczna, R. Svensson Karo/inska /nstitute, Dept. of Medical Radiation Physics, Stockholm, Sweden Radiobiological optimized treatment planning shows that the radiotherapy outcome is heavily increased by use of intensity modulated beams. The
676 Poster C o m m i s s i o n i n g o f a MOSFET-TNRD-50 dosimetry system for routine measurements o f 6 MM a n d 10 MM external photon
beam radiation treatments 14/.du Bois, E. Bloemen-van Gurp, P. Visser, I. Bruinvis, D. Jafink, J. Hermans, P. Lambin, A. Minken RTIL /Academic Hospital Maastricht, Radiotherapy, Heerlen, The Nether. lands
Table 1.: correction factors for mosfet readings gantry angle *) t5 ° 3O° 45° 6 MV 1.01 1.00 1,02 10 MV 1.01 1.02 1.02
6O° 5x5 1.03 1.00 1.00 1,01
*) A correctionfactor of 1.01 per 25° full wedge 6 MV 1.05 I0 MV 1.02
fieldsize (cm2) 10 x 10 2o x 2O 3o x 3o 1.00 1.01 1.00 1.00
for both 6 and 10 MV beamsis used. carbon tabletop SSD 80 cm 1.02 1.01 1.02 1.00
Conclusion:The reproducibility of the MOSFET-TNRD-50 dosimetry system compares favourably with TLD's and meets the requirement for routine in vivo dosimetry. For 6 and 10 MV photon beams, the readings need only few and small corrections, provided that the dosimeters are placed under under build-up at dmax. 677
Poster
In-vivo dosimetry in electron beam radiotherapy with silicon diodes; the impact of the build-up cap thickness M. LOOt1, N. Joiner2, V. Ra-Betf3, P. Carrasco de Fez2, H. Nystrom 1 1The Finsen Centre, Copenhagen University Hospital, Radiation Physics dep., Copenhagen, Denmark 2Hospital de/a Sta Creu i St Pau, Radiation Physics dep, Barcelona, Spain 3Norfolk and Norwich University Hospital, Radiotherapy Physics dep, Norwich, England The sensitive volume of a diode intended for entrance dose measurements,