Novel 6 MV X-ray photoneutron detection and dosimetry of medical accelerators

Purpose: Dosimetry of fast, epithermal and thermal photoneutrons in 6 MV X-ray beams of two medical accelerators were studied by novel dosimetry methods. Methods: A Siemens ONCOR and an Elekta COMPACT medical accelerators were used. Fast, epithermal and thermal photoneutron dose equivalents in 10 cm x 10 cm 6 MV X-rays fields were determined in air and on surface of a polyethylene phantom in X and Y directions. Polycarbonate dosimeters as bare or with enriched B-10 convertors (with or without cadmium covers) were used applying a 50 Hz-HV electrochemical etching method. Results: Fast, epithermal and thermal photoneutron dose equivalents were efficiently determined respectively as similar to 1145.8, similar to 45.3 and similar to 170.6 mu Sv in air and similar to 1888.5, similar to 96.1 and similar to 640.6 mu Sv on phantom per 100 Gy X-rays at the isocenter of Siemens ONCOR accelerator in air. The dose equivalent is maximum at the isocenter which decreases as distance from it increases reaching a constant level. Tissue-to-air ratios are constants up to 15 cm from the isocenter. No photoneutrons was detected in the Elekta COMPACT accelerator. Conclusions: Fast, epithermal and thermal photoneutron dosimetry of 6 MV X-rays were made by novel dosimetry methods in a Siemens ONCOR accelerator with sum dose equivalent per Gy of similar to 0.0014% mSv with similar to 0.21 MeV mean energy at the isocenter; i. e. similar to 150 times smaller than that of 18 MV X-rays. This observation assures clinical safety of 6 MV X-rays in particular in single-mode machines like Elekta COMPACT producing no photoneutrons due to no "beryllium exit window" in the head structure. (C) 2017 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.