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Dosimetry of independent collimators for 6 MV photon energy
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218
PRIMARY AND HEAD SCATTER COMPONENTS IN PRESENCE OF ATTENUATORS FOR A 4 MV X-RAY BEAM
POTENTIAL APPLICAnONS OFNEW X-RAY SOURCES IN RADIOTHERAPY
M. E. CASTELLANOSI,2.]. C. ROSENWALD I IInstitut Curie, Servicede PhysiqueMedicale,Paris, France 2InstitutNationalde Cancerologie,Bogota, Colombie In the calculation of dose, contribution of "primary" and "scattered" radiation are commonly separated. The primary radiation includes scatter from several elements including target, flattening filter and collimators and its measure can be affected I by the radiation backscattered from collimator jaws into the. dose monitor chamber. In radiotherapy, the radiation beam is sometimes shaped or attenuated by the use of attenuators introduced into the beam. : These attenuators alter the "primary" as well as the "scattered" radiation components of the beam and there is at present no accurate method of dose calculation under them. Measurements designed to separate the head scatter components contribution of the primary dose for these situations were made on the central axis of a radiation beam from a 4 MV linear accelerator. In order to isolate the "primary" photon fluence we used a miniphantom in the shape of a column of square cross section large enough to ensure lateral electronic equilibrium. The' primary photon fluence was measured at 5 cm depth for square fiel sizes from 5x5 cm2 to 40x40 cm2 at a distance from the source of 100 em, for open and uniformlyattenuatedfields. The results indicate that backscattered radiation in the monitor chamber contributes to less than 1% and the scatter from collimator jaws accounts for less than 2,5% of the "primary" f1uence. The magnitude of the beam modifier scatter' contribution was calculated to be of several percent of the transmitted primary dose in a 20x20 cm2 field for 1 and 2 cm thick brass attenuators. Although collimator jaws scatter is of low magnitude its contribution to the dose behind the attenuator covering part of the radiation field is found to be , important. A calculation method is presented for the prediction of the "primary" under these attenuators.
F.Scarlat, E.Popescu*, Rodica Scarlat*, N.Yarga** Institute of Atomic Physics, Bucharest, Romania *University Hospital, Bucharest, Romania **"COLTEA" ClinicalHospital, Bucharest, Romania
It is well known thatthe incoherent photon beams with peak
energy ranging between 0.3 MeV and 20 MeV supplied by electron accelerators are used in radiotherapy. The occurrence of new coherent radiation sources, in the frame of the classification made by Gover and Yariv, 1978 and Pantell, 1980, namely: slow wave radiators, periodic bremsstrahlung radiators and transverse bending radiators, constitute a potential application of these new coherent radiation sources in radiotherapy. In this paper, we refer to Smith-Purcell radiation, transient radiation, channelling radiation, coherent bremsstrahlung and magnetic bremsstrahlung free electron laser. The main beam characteristics, interesting for radiotherapy, are also presented.
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220
DOSIMETRY OF INDEPENDENT COLLIMATORS FOR 6 MV PHOTON ENERGY
DOSIMETRY AND DOSE CALCULATION PROCEDURES FOR MULTILEAF COLLIMATOR TREATMENTS
G.ISIN, G. ARSLAN, S.GURDALLI, I. L.ATAHAN Hacettepe University, Radiation Onkology Department, Ankara, TURKEY
G. Ghiso, R. Martinelli, F.Foppiano, G. Paoli, R. Ansebni, L. Andreucci Biophysical Laboratory, National Institote for Cancer Research, Genova, ITALY.
Asymmetric collimators have many clinical advantages. Some linear accelerators have collimators which can move independently from each other. Before clinical implementation, it is important to evaluate the dosimetry of this nonstandard treatment delivery technique. We have studied the dosimetry ofan independent jawsystem inthe Philips SL 25 Linac. Dose distributions of the asymmetric beam were analyzed with ionometric water scans, ion chamber studies, and film dosimetry. Dose profiles and Percent Depth Doses (PDD) were measured at depths of 1.5 (dmax), 5, 10, 20em for various asymmetric fields. Offaxis factors for asymmetric field sizes were determined at 5, 10, 15 em offaxis distances for 1.5 (dmax) and 5 em depths bynormalizing to central axis readings. These offaxis factors ranged from 1.000 at central axis toa maximum of 1.060 at 15 em offaxis at dmax. Asymmetric and symmetric PODs were compared and we found that, inasymmetric case, POD is decreasing for theincreasing values ofoffaxis distance and this effect is increasing with depth. Amaximum of6%decrease was observed for the PDD.
In the last years the Multileaf Collimator (MLC) has been developed for some linear accelerators; itallows toconform the target volume inaneasier way with respect to the beam shaping blocks. Recently, a linear accelerator Varian 2100CIO, provided with the MLC, has beeninstalled in ourInstitute. In order to beable to use this accessory in the clinical practice, a work project has been elaborated: we were interested in the knowledge of the absorbed dose per M.U. at the treatment depth, in different geometrical configurations, for any MLC field, positioned in an arbitrary zone of the irradiating field. The work we carried outcould bedivided schematically inthree points: I) Atheoretical model for dose calculation: Starting from a model which calculates the dose values for irregularly shaped fields, an algorithm specific for the MLC has been elaborated. The formula we thought isideally divided intwo components: the first takes into account the dose variation due to the primary collimators, which usually define an asymmetric field with respect to the central axis; the last component of the formula introduces some factors which take into account ofthe changes produced bythe MLC. 2)MLC dosimetry: Specific measurements for the MLC have been performed. Their aim is to display the effect of the MLC onthedose delivered per M.U..In particular we studied both the scattering towards the measurement point and towards the monitor chambers and the influence ofthe primary collimating system on the scattered radiation. We also investigated the effectofthe reduction ofthe scattering volume due totheMLC insertion inthe irradiation field. 3)Dosimetric data elaboration: The experimental measurements were elaborated trying to emphasize the impact ofthe measured quantities onthe dose delivered per M.U.. Moreover it was necessary, inorder tomodify as less aspossible the algorithms inuse before the introduction of the MLC, to refer the different parameters, specific of the new configuration, to thestandard geometrical one used for the linac calibration.
218
PRIMARY AND HEAD SCATTER COMPONENTS IN PRESENCE OF ATTENUATORS FOR A 4 MV X-RAY BEAM
POTENTIAL APPLICAnONS OFNEW X-RAY SOURCES IN RADIOTHERAPY
M. E. CASTELLANOSI,2.]. C. ROSENWALD I IInstitut Curie, Servicede PhysiqueMedicale,Paris, France 2InstitutNationalde Cancerologie,Bogota, Colombie In the calculation of dose, contribution of "primary" and "scattered" radiation are commonly separated. The primary radiation includes scatter from several elements including target, flattening filter and collimators and its measure can be affected I by the radiation backscattered from collimator jaws into the. dose monitor chamber. In radiotherapy, the radiation beam is sometimes shaped or attenuated by the use of attenuators introduced into the beam. : These attenuators alter the "primary" as well as the "scattered" radiation components of the beam and there is at present no accurate method of dose calculation under them. Measurements designed to separate the head scatter components contribution of the primary dose for these situations were made on the central axis of a radiation beam from a 4 MV linear accelerator. In order to isolate the "primary" photon fluence we used a miniphantom in the shape of a column of square cross section large enough to ensure lateral electronic equilibrium. The' primary photon fluence was measured at 5 cm depth for square fiel sizes from 5x5 cm2 to 40x40 cm2 at a distance from the source of 100 em, for open and uniformlyattenuatedfields. The results indicate that backscattered radiation in the monitor chamber contributes to less than 1% and the scatter from collimator jaws accounts for less than 2,5% of the "primary" f1uence. The magnitude of the beam modifier scatter' contribution was calculated to be of several percent of the transmitted primary dose in a 20x20 cm2 field for 1 and 2 cm thick brass attenuators. Although collimator jaws scatter is of low magnitude its contribution to the dose behind the attenuator covering part of the radiation field is found to be , important. A calculation method is presented for the prediction of the "primary" under these attenuators.
F.Scarlat, E.Popescu*, Rodica Scarlat*, N.Yarga** Institute of Atomic Physics, Bucharest, Romania *University Hospital, Bucharest, Romania **"COLTEA" ClinicalHospital, Bucharest, Romania
It is well known thatthe incoherent photon beams with peak
energy ranging between 0.3 MeV and 20 MeV supplied by electron accelerators are used in radiotherapy. The occurrence of new coherent radiation sources, in the frame of the classification made by Gover and Yariv, 1978 and Pantell, 1980, namely: slow wave radiators, periodic bremsstrahlung radiators and transverse bending radiators, constitute a potential application of these new coherent radiation sources in radiotherapy. In this paper, we refer to Smith-Purcell radiation, transient radiation, channelling radiation, coherent bremsstrahlung and magnetic bremsstrahlung free electron laser. The main beam characteristics, interesting for radiotherapy, are also presented.
219
220
DOSIMETRY OF INDEPENDENT COLLIMATORS FOR 6 MV PHOTON ENERGY
DOSIMETRY AND DOSE CALCULATION PROCEDURES FOR MULTILEAF COLLIMATOR TREATMENTS
G.ISIN, G. ARSLAN, S.GURDALLI, I. L.ATAHAN Hacettepe University, Radiation Onkology Department, Ankara, TURKEY
G. Ghiso, R. Martinelli, F.Foppiano, G. Paoli, R. Ansebni, L. Andreucci Biophysical Laboratory, National Institote for Cancer Research, Genova, ITALY.
Asymmetric collimators have many clinical advantages. Some linear accelerators have collimators which can move independently from each other. Before clinical implementation, it is important to evaluate the dosimetry of this nonstandard treatment delivery technique. We have studied the dosimetry ofan independent jawsystem inthe Philips SL 25 Linac. Dose distributions of the asymmetric beam were analyzed with ionometric water scans, ion chamber studies, and film dosimetry. Dose profiles and Percent Depth Doses (PDD) were measured at depths of 1.5 (dmax), 5, 10, 20em for various asymmetric fields. Offaxis factors for asymmetric field sizes were determined at 5, 10, 15 em offaxis distances for 1.5 (dmax) and 5 em depths bynormalizing to central axis readings. These offaxis factors ranged from 1.000 at central axis toa maximum of 1.060 at 15 em offaxis at dmax. Asymmetric and symmetric PODs were compared and we found that, inasymmetric case, POD is decreasing for theincreasing values ofoffaxis distance and this effect is increasing with depth. Amaximum of6%decrease was observed for the PDD.
In the last years the Multileaf Collimator (MLC) has been developed for some linear accelerators; itallows toconform the target volume inaneasier way with respect to the beam shaping blocks. Recently, a linear accelerator Varian 2100CIO, provided with the MLC, has beeninstalled in ourInstitute. In order to beable to use this accessory in the clinical practice, a work project has been elaborated: we were interested in the knowledge of the absorbed dose per M.U. at the treatment depth, in different geometrical configurations, for any MLC field, positioned in an arbitrary zone of the irradiating field. The work we carried outcould bedivided schematically inthree points: I) Atheoretical model for dose calculation: Starting from a model which calculates the dose values for irregularly shaped fields, an algorithm specific for the MLC has been elaborated. The formula we thought isideally divided intwo components: the first takes into account the dose variation due to the primary collimators, which usually define an asymmetric field with respect to the central axis; the last component of the formula introduces some factors which take into account ofthe changes produced bythe MLC. 2)MLC dosimetry: Specific measurements for the MLC have been performed. Their aim is to display the effect of the MLC onthedose delivered per M.U..In particular we studied both the scattering towards the measurement point and towards the monitor chambers and the influence ofthe primary collimating system on the scattered radiation. We also investigated the effectofthe reduction ofthe scattering volume due totheMLC insertion inthe irradiation field. 3)Dosimetric data elaboration: The experimental measurements were elaborated trying to emphasize the impact ofthe measured quantities onthe dose delivered per M.U.. Moreover it was necessary, inorder tomodify as less aspossible the algorithms inuse before the introduction of the MLC, to refer the different parameters, specific of the new configuration, to thestandard geometrical one used for the linac calibration.