IN-VIVO NEUTRON DOSIMETRY DURING HIGH-ENERGY BREMSSTRAHLUNG RADIOTHERAPY

Purpose: A new technique is presented for in vivo measurements of the dose equivalent from photoneutrons produced by high-energy radiotherap y accelerators. Methods and Materials: The dosimeters used for this pu rpose are vials of superheated halocarbon droplets suspended in a tiss ue-equivalent gel. Neutron interactions nucleate the formation of bubb les, which can be recorded through the volume of gel they displace fro m the detector vials into graduated pipettes, These detectors offer in herent photon discrimination, dose-equivalent response to neutrons, pa ssive operation, and small sensitive size. An irt vivo vaginal probe w as fabricated containing one of these neutron detector vials and a pho ton-sensitive diode. Measurements were carried out in patients undergo ing high-energy x-ray radiotherapy and were also repeated in-phantom, under similar irradiation geometries. Results and Conclusion: Neutron doses of 0.02 Sv were measured in correspondence to the cervix, 50 cm from the photon beam axis, following a complete treatment course of 46 .5 Gy with an upper mantle field of 18-MV x-rays, This fraction of dos e from neutrons is measured reliably within an intense photon backgrou nd, making the technique a valid solution to challenging dosimetry pro blems such as the determination of fetal exposure in radiotherapy. The se measurements can be easily carried out with tissue-equivalent phant oms, as our results indicate an excellent correlation between in vivo and in-phantom dosimetry. (C) 1998 Elsevier Science Inc.