Evaluation of the Undesired Neutron Dose Equivalent to Critical Organs in Patients Treated by Linear Accelerator Gamma Ray Therapy

Abstract

A careful theoretical and experimental evaluation has been made in order to estimate the neutron dose equivalent for critical organs of patients treated by gamma ray therapy. The machine employed in these studies is the linear electron accelerator MD Class Mevatron, Siemens, installed at the S. Giovanni Battista A.S. Hospital in Torino (Italy) and used for tumour therapy. A calculational method was developed, using two suitable 3D Monte Carlo computer codes: the EGS4 code (Electron Gamma Shower), which evaluates bremsstrahlung gamma rays and photo-neutron production in the accelerator head and the MCNP code (Monte Carlo Neutron and Photon Transport), which simulates the neutron transport and evaluates the organ dose equivalents. In addition, two sets of measurements were made during treatment sessions with 2 Gy therapeutic gamma dose: (a) by means of thermoluminescence dosemeters TLD-600 and TLD-700 placed at various depths inside an anthropomorphic phantom in positions corresponding to the organs; (b) in vivo by albedo neutron personal dosemeters UD-802, National Panasonic, positioned on the patients according to the organs. The agreement between experimental and theoretical data is good. The neutron organ dose equivalents are by no means negligible; for instance HT varies from 2.65 mSv on the thyroid to 42.02 mSv on testes, if normalised to one Gy of maximum absorbed dose in the pelvis, of the primary gamma beam if 15 MeV maximum energy (10 cm × 10 cm field size and source-skin distance of 1 m).