Study of the response of ionization chambers in photon beams for off-axis point dose

Abstract

Numerous challenges associated with small-field dosimetry compared with standard dosimetry, primarily because of the different irradiation conditions than that of the standard case. These challenges include concerns regarding reliability in the fields of small-field stereotactic radiotherapy and radiosurgery with respect to radiation detection. Furthermore, the use of intensity modulated radiotherapy (IMRT) has increased for the treatment of difficultly situated carcinomas. Therefore, while employing IMRT, measurements should include small fields as well as off-axis measurements. In this study, we investigated the response of small-volume ionization chambers and Farmer-type chambers for dose profile measurements at different measurement depths. We found that the response of the ionization chamber decreased at maximum dose depth when moving from the central axis to the 15 cm offaxis position. Until now, only a 0.1 cm3-volume iionization chamber has been investigated. The Farmer-type ionization chamber investigated herein behaved similarly to the smallvolume ionization chamber. The response of for Farmer-type ionization chamber decreased by 3.1% and 3.5% for the 4 and 14 MV beams, respectively. Furthermore, the response both Farmer-type ionization chamber and small-volume ionization chamber increased with decreasing nominal photon energy. This under response could be the result of the variation of the wall correction factor.