INVESTIGATION OF ACTIVATION PRODUCTS IN MEDICAL LINEAR ACCELERATORS

Abstract

Abstract

Study of induced activity in a medical linear accelerator room was carried out on Clinac

2100C running 15 MV at Maggiore Hospital, Trieste, Italy. Mathematical model to calculate

the induced dose rate has been derived and compared to measurements results. Both of experimental

method and mathematical model present a good agreement. The experimental

method was performed using filter papers and portable spectroscopy. The activation level

reached its practical saturation value after a 30 min continuous irradiation, corresponding

to 12000 MU at a dose rate of 240 MU/min. The filter paper method was first time used for

the purpose of this measurement. Typical radionuclides produced in the treatment room

were identified. The results obtained by this new method, consisting of filter paper, can

represent a reliable tool. Moreover, the measurements uncertainty using portable spectrometer

was decreased and determined to be 6.02%. In addition, the Clinac 2100C has

been simulated with Monte Carlo code Geant4 and the neutron fluence, as a function of

the neutron energy, has been calculated in the isocenter and outside the treatment room

to estimate the equivalent dose to technologist and patients. The ambient dose equivalent

for patient and radiotherapy technologist has been reported in this study using the above

mentioned methods. The derived data using different field sizes have been used to evaluate

the ambient dose equivalent from neutrons to a patient receiving radiation treatment. The

maximum of annual ambient dose equivalent present for 15 MV photon beam is about 1.96

mSv for the technologists, in addition to 1.032 mSv/year received by them in the control

room. The maximum of ambient dose equivalent received by patients for minimum field

size present 1.79 mSv/Gy for 20 MV photon beam. These values represent neglected doses

for technologists, but at same time cannot be ignored for patients, where they can represent

a risk for healthy tissues and contribute to secondary malignancy insurgence.