Measurement of Photoneutron Dose in the Linear Accelerator at the Radiation Therapy Section of Seyed-Al-Shohada Hospital, Isfahan, Iran

Radiation therapy with linear accelerators (Linac) is one of the efficient techniques for cancer treatment. However, the interaction between high energy photons and various materials on the Linac's head produces photoneutron contamination. Total flux of neutrons per unit of radiation in the isocenter is the sum of thermal, scattered and direct neutrons fluxes. In this study, the neutron contamination at an energy level of 18 MeV was investigated in 15 points of the Linac's room at Seyed-Al-Shohada Hospital, Isfahan, Iran. For calculation of the dose of the neutrons, 30 thermoluminescence dosemeters (TLD-600 and TLD-700) were randomly distributed in 15 points of the treatment room. In order to use TLDs, both thermal and gamma calibrations are required. While thermal calibration was performed with a 60Co source at the hospital, gamma calibration was conducted with an AM-Be source at the secondary standard dosimetry lab (SSDL) in Karaj, Iran. The exposure of the detectors was performed during the treatment of a patient at a dose of 100 cGy during 111 Monitor Unit. The results of the TLD detectors were all in the range of millisievert (mSv) and less. The measured doses in detectors 1, 2, 3, 4 and 15 were 0.378, 0.652, 0.693, 0.0605, and 0.671 mSv, respectively. Based on our findings, non-zero values of photoneutron contamination exist in the treatment room which are not always considered in the treatment planning of patients.