PHOTONEUTRONS FROM HIGH-ENERGY MEDICAL LINEAR ACCELERATORS - MEASUREMENT OF THE SPECTRUM AND DOSE USING A MINIATURE PROPORTIONAL COUNTER

Purpose: A new method of measuring photoneutron dose to the patient du ring treatment with high energy photon or electron beams is presented. This method has the advantage of providing not only the dose, but the microdosimetric spectrum at the same time. Methods and Materials: A m iniature cylindrical gas proportional counter (0.5 mm diameter by 0.5 mm height) has been used to measure scatter radiation from a 20 MV tel etherapy photon beam, At atmospheric pressure, filled with propane bas e tissue equivalent gas, this counter simulates a unit density tissue region of approximately 0.9 mu m. We present here single event microdo simetric spectra measured outside the primary beam 1.4 m from the targ et. This technique allows a single measurement to determine the scatte red dose due to gammas and photoneutron contamination, as well as the quality factor of the photoneutrons. Results: Spectral components from scattered photons and the photoneutrons are easily separated, and dos e contributions can be estimated, The ratio of photoneutron dose measu red by the proportional counter to photon dose at isocenter is 0.75 X 10(-4). Conclusions: Neutron dose was also measured using a bubble neu trometer. The proportional counter and neutrometer agree within experi mental errors, This type of instrument is shown to be a viable techniq ue for determination of exposure of patient and also personnel to phot oneutrons, providing not only a dose determination, but also a spectru m that can be used to estimate quality factors for equivalent dose. It s main drawback is that it requires a lengthy (several hours) measurem ent because of low count rate of the neutrons.