RESUMO
Background: modern radiation therapy such as intensity modulated radiation therapy [IMRT] and volumetric modulated arc therapy [VMAT] has resulted in using small therapeutic photon beams. The scope of this work is to investigate the variation in efficiency of different ionization chambers and semiconductor diodes in small multi leaf collimator [MLC] defined fields of ARTISTE linear accelerator
Materials and Methods: dose distributions measurements was done in field sizes ranging from 0.5×0.5 to 10×10 cm2 combining with Monte Carlo [MC] simulation. The treatment head of linac and the detectors were simulated by means of BEAMnrc/DOSXYZnrc of EGSnrc MC. The parameters such as output ratio [OR], penumbral width, dosimetric field size and the percentage surface dose in small radiation fields was evaluated using ionization chambers and semiconductor dosimeters
Results: ORs and beam profiles resulting from various measurements showed significant difference between ionization chambers and diodes in small fields. Derivation of less than 2% was observed between EDGE and Diode P. ORs vary by more than 30% for 1×1 cm2 field size but in larger field sizes differences was less than 1%. Penumbra underestimation was seen in Semiflex relative to pinpoint ionization chamber. No difference was seen in the pattern of surface dose among the applied detectors
Conclusion: dosimetric characteristics of different detectors showed significant differences in small photon beams. Profiles and ORs analysis with different dosimeters showed that not only water equivalency of detectors, but also dosimeter active volume is important factors for determination of dosimetric behavior in small photon beams
RESUMO
The objective of this study was to measure the radiation dose absorbed by testes in those men receiving pelvic radiotherapy in order to provide appropriate shielding. Thermo luminescence dosimeter [TLD] which is in small tablet forms was used for this study. Patients were selected and were advised of the procedure. The TLD tablets were positioned in the appropriate places over the gonads and then the absorbed dose was determined. In this study the correlations between the absorbed dose of the gonads and factors such as the radiotherapy system energy, variation in the source skin distance [SSD] of the instrument, the pelvic area and the radiation direction were determined. Results showed significant association of the radiotherapy system energy, SSD variation, and pelvic area with the absorbed dose of the gonads [P < 0.05]. Our findings show that in pelvic radiotherapy the gonads are exposed to high radiation energy. This is an important issue for young men in order to protect their fertility. Therefore, it is very important to protect the gonads from the ionizing radiation with the best possible shielding