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

Background: modern radiation therapy such as intensity modulated radiationtherapy (imrt) and volumetric modulated arc therapy (vmat) has resulted in usingsmall therapeutic photon beams. the scope of this work is to investigate thevariation in efficiency of different ionization chambers and semiconductor diodes insmall multi-leaf collimator (mlc) defined fields of artiste linear accelerator.materials and methods: dose distributions measurements was done in field sizesranging 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 ofbeamnrc/dosxyznrc of egsnrc mc. the parameters such as output ratio (or),penumbral width, dosimetric field size and the percentage surface dose in smallradiation fields was evaluated using ionization chambers and semiconductordosimeters. results: ors and beam profiles resulting from variousmeasurements showed significant difference between ionization chambersand diodes in small fields. derivation of less than 2% was observed betweenedge and diode p. ors vary by more than 30% for 1×1 cm2 field size but, inlarger field sizes differences was less than 1%. penumbra underestimationwas seen in semiflex relative to pinpoint ionization chamber. no differencewas seen in the pattern of surface dose among the applied detectors.conclusion: dosimetric characteristics of different detectors showed significantdifferences in small photon beams. profiles and ors analysis with differentdosimeters showed that not only water equivalency of detectors, but alsodosimeter active volume is important factors for determination of dosimetricbehavior in small photon beams.