Exhibit Hall | Forum 3
Purpose: To report and quantify the thermal neutron fluence (Φth) produced within the field for various photon energies in different irradiation conditions for Elekta Versa HD linac. To compare the Φth production between the flattened beam (FF) and flattening filter-free (FFF) beam.
Methods: Indium foils were irradiated inside water phantom for 10 MV FF, 10 MV FFF beam, and 15 MV beams. The induced activity of the activated foil was measured using a calibrated Kromek™ GR1-A+ CZT semiconductor portable gamma spectrometer and was correlated with Φth. Measurements were carried out for field sizes ranging from 5×5 cm²2 to 40×40 cm² and depths ranging from 0 cm to 15 cm. To study the variation of Φth across the field, foils were kept at equidistant positions from the center to the field edge along both X and Y axes.
Results: The Φth at the phantom surface for 15 MV, 10 MV, and 10 MVFFF beams were found to be 8.93×105, 4.35×105, 2.21×105 neutrons cm⁻² Gy⁻¹, respectively. Maximum Φth was found to be 3.02×106 neutrons cm−2 Gy−1 for 15MV beam at 3cm depth and 20×20 cm² field size. With depth from surface, Φth increased till 3cm, and then reduced gradually. Φth increased with field size from 5×5 cm² and achieved maximum value at 20×20 cm² then decreased till 40×40 cm². The Φth decreased from the center to the field edge along both X and Y axes. The FFF beams produced lesser Φth compared to FF beams with an average difference of 57% with depth and 54% with field size.
Conclusion: The in-phantom Φth values for Elekta Versa HD linac were reported for three-photon energies. The effect of depth and field size on Φth was studied. FFF beams can be used instead of FF beams for reducing the neutron dose to patients.