Assessment and Measurement of the Photoneutron Field Produced in the Varian Medical Linear Accelerator

Abstract

This paper evaluates the doses and spectra of photoneutrons produced in a medical linear accelerator with photon energies of 10 and 15 MV. The Monte Carlo code, MCNPX, was used to simulate the transport of these photoneutrons in the plane of a patient and around the upper part of the head. The fully described geometry of the varian accelerator head was used in this calculation. The photoneutron energy spectra and doses for various photon field sizes were calculated at 20 positions for each field. The results indicate that the maximum dose equivalents are observed in the 20 × 20 cm² photon field size. It was found that the average neutron energy at the isocenter for a 0 × 0 cm² field is 0.38 MeV for 10 MV and 0.45 MeV for 15 MV. The neutron doses at 10 positions around the head in the treatment room of the operation facilities with energies of 10 and 15 MV were measured using bubble detectors. Measurements were compared with the calculations using the same geometry in the experiment. It was found that the majority of the calculated results based on the same dose equivalent conversion factors as those of bubble detectors agreed within the standard deviations of the measurements. The photoneutron spectrum for 15 MV was measured with a bubble spectrometer as well. It was shown that the calculated fluences in the neutron energy bins were higher than the measured ones.

These results can be applied in the verification of the maximum allowed neutron leakage percent of treatment dose defined by the International Electrotechnical Commission (IEC). We have been employing them to derive the empirical formula for neutron dose equivalent level at the maze entrance of medical accelerator treatment rooms in the current study.

KEYWORDS:

• photoneutron
• medical linear accelerator
• dose equivalent
• MCNPX
• bubble detector and spectrometer