Identification of Induced-Radioactivity in Medical LINAC Using a NaI(Tl)-Crystal Detector

The exposure of tissues to large amount of radiation is a major concern of clinicians who use radiotherapy to treat patients for solid tumors. We assessed the amount of radioactivity induced by therapeutic x-ray beams used in radiotherapy. To measure the energy spectra of γ-rays emitted from the activated materials using a NaI(TI)-crystal detector, the detector was placed inside the heads of the medical linear particle accelerators (LINACs) of two Clinac 21EXs machines and one Clinac 21EX platinum (Varian Medical System, USA). The energy spectra were measured for one hour of data acquisition (DAQ) time, and then another spectrum was measured under the same conditions for 58∼59 hours. The ratios of full photopeak areas in the two γ-ray spectra were compared with standardized values calculated using the lifetimes of induced-isotope candidates, which were selected for their characteristic γ-ray energies and the atomic components of LINAC heads. In order to minimize errors originated from backgrounds and neighboring peaks, the photopeak areas were obtained by fitting the peak components with Gaussian functions and background components with a polynomial function. Using the energy-resolution relationships of NaI(TI)-crystal detectors, the widths of the Gaussian functions were determined. The calculated ratios of photopeak areas for the two different DAQ times were 40.6 (203Pb), 27.7 (187W), 34.6 (82Br), 28.3 (54Mn) and 34.7 (57Ni). The measured area ratios of these photopeaks tagged in γ-ray spectra were 40.3, 27.3∼29.6, 31.5∼36.9, 28.5 and 35.1∼39.0 respectively These values are consistent with each others.