[Influence on measurements of pre-irradiation due to differences in ionization chamber shape or frequency in use].

Ionization chamber measurements in radiation therapy should be repeatedly performed until a stable reading is obtained. Ionization chambers exhibit a response which depends on time elapsed since the previous irradiation. In this study, we investigated the response of a set of two Farmer-style, one Plane parallel, and seven small ionization chambers, which are exposed to 4, 6, 10, and 14 MV. The results show that Farmer-style and Plane parallel ionization chambers settle quickly within 9-20 min. On the other hand, small ionization chambers exhibit settling times of 12-33 min for 6, 10, and 14 MV. It will take longer for a settling time of 4 MV. The settling time showed time dependent irradiation. The first reading was up to 0.76% lower in the Farmer-style and Plane parallel ionization chambers. The small ionization chambers had a 2.60% lower first reading and more gradual response in reaching a stable reading. In this study, individual ionization chambers can vary significantly in their settling behavior. Variation of the responses on ionization chambers were confirmed not only when radiation was not used for a week but also when it was halted for a month. Pre-irradiation of small ionization chambers is clearly warranted for eliminating inadvertent error in the calibration of radiation beams.