Dose to radiation therapists from activation at high-energy accelerators used for conventional and intensity-modulated radiation therapy

The increased beam-on times which characterize intensity-modulated radiation therapy (IMRT) could lead to an increase in the dose received by radiation therapists due to induced activity. To examine this, gamma ray spectrometry was used to identify the major isotopes responsible for activation at a representative location in the treatment room of an 18 MV accelerator (Varian Clinac 31EX). These were to found to be Al-28, Mn-56, and Na-24. The decay of the dose rate measured at this location following, irradiation was analyzed in terms of the known half-lives to field Saturation dose rates of 9.6, 12.4. and 6.2 muSv/h, respectively. A formalism was developed to estimate activation dose (muSv/week) due to successive patient irradiation cycles. characterized by the number of 18 MV fractions per week. F, the number of MU per fraction, M, the in-room time between fractions, t(d) (min). and the treatment delivery time t'(r). (min). The results are represented by the sum of two formulas. one for the dose from Al-28 approximate to 1.8 X 10(-3) F M ( 1 - e-(0.3t'r))/t'(r) and one for the dose from the other isotopes approximate to 1.1 x 10(-6)F(1.7)Mt(d). For conventional therapy doses are about 60 mu Sv/week for an IS MV workload of 60000 MU/week. Irradiation for QA purposes can significantly increase the dose. For IMRT as currently practiced, lengthy treatment delivery times limit the number of fractions that can be delivered per week and hence limit the dose to values similiar to those in conventional therapy. However for an IMRT regime designed to maximize patient throughput. doses up to 330 muSv/week Could be expected. To reduce dose it is recommended that IMRT treatments should be delivered at energies lower than 18 MV. that in multienergy IMRT, high-energy treatments should be scheduled in the latter part of the day. and that equipment manufacturers should Strive to minimize activation in the design of high-energy accelerators. (C) 2002 American Association of Physicists in Medicine.