EFFECTS OF FIELD SIZE AND DEPTH ON PHOTONEUTRON DOSE EQUIVALENT DISTRIBUTIONS IN AN 18 MV X-RAY MEDICAL ACCELERATOR.

Photoneutron (PN) dosimetry studies in high-energy X-ray medical accelerators are of high clinical and scientific interest in particular to protect patients undergoing radiotherapy. In this context, fast, epithermal and thermal PN dose equivalent distributions in different field sizes and depths in air and in a multilayer polyethylene phantom were studied. Polycarbonate track dosemeters in contact with a 10B convertor (with or without cadmium cover) when electrochemically etched were applied. PN dose equivalents in air and on the surface of the phantom are linear functions of field size. PN depth dose equivalents versus depth in air at the central axis are almost constant. Fast, epithermal and thermal PN dose equivalent responses versus depth in phantom peak respectively at 0.0, ~3.0 and ~3.0 cm while that of the sum PN dose equivalent value (3.32 ± 0.19 mSv·Gy-1) peaks at ~1 cm. These values confirm those of some studies but contract some others.