Evaluation of dosimetric characteristics of diodes and ionization chambers in small megavoltage photon field dosimetry

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. 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. 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. 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.