SU‐GG‐T‐324: Characteristics and Dosimetric Parameters of Small Radiosurgery Photon Beams

Purpose: The accuracy of dosimetric parameters, such as water‐to‐air stopping power ratios, directly affects the accuracy of a beam calibration. This study investigates the variation of dosimetric parameters, such as energy spectrum, water‐to‐air stopping power ratios, energy absorption coefficient, etc., as a function of field size and depth in water for small radiosurgery photon beams. Method and Materials: The Monte Carlo simulation techniques were used in this study. The accelerator head geometries of a Varian Trilogy were simulated in detail including two designs of flattening filters: one for a conventional dose rate (100–600 MU/min) and the other for a high‐dose rate (1000 MU/min) 6 MV beam. The design of the flattening filter to produce a high‐dose rate beam limits its maximum field size to 10×10 cm2. The investigated field sizes range from 0.5 cm in diameter to 10×10 cm2 and depths range from 0–50 cm in water. Results: Although there are noticeable differences in the spectrum distributions for beams with different flattening filters, the variations in calculated water‐to‐air stopping power ratios (SPR) are within 0.2%. The values of calculated water‐to‐air SPR as a function of field size and depth in a water phantom show a maximum variation of 0.4% and 1% respectively. The mean energy of photons increases as depth increases at a rate of 22 keV/cm for small fields. Conclusion: There is an insignificant variation (<0.4%) in water‐to‐air stopping power ratios between a small narrow field and a reference 10×10 cm2 field. There is negligible difference (0.2%) in water‐to‐air SPR between beams with two very different beam flattening filters. Variations of water‐to‐air SPR are not significant between small and large field. Hence correction of water‐to‐air SPR between a reference field and a small field is negligible in the dose measurement for the small field.