A review of photon beam skyshine studies from medical megavoltage accelerators

This review examines the literature on the assessment of photon beam skyshine dose rates in medical accelerator radiotherapy with an energy range of 4-25 MV. The selected studies focus on medical accelerators within radiotherapy facilities and employ various methodologies for calculating photon beam skyshine dose rates. Researchers utilized Monte Carlo (MC) simulations, direct measurements, and empirical methods outlined in NCRP Reports 151 and 51 for skyshine estimations. These NCRP reports have established empirical approaches for calculating skyshine dose rates in linac-based radiotherapy settings. The methodologies adopted by different researchers and their findings were compared and discussed. Among these methods, the one that aligns best with measurements across various studies was identified as the gold standard. Notably, the conclusion drawn from multiple papers indicates that NCRP 151 tends to overestimate skyshine dose rates. Various studies were conducted under different geometries and distances from the X-ray source, and comparisons with other works showed consistent results. However, some discrepancies were noted in the results of skyshine characterization studies. This review encompasses publications related to photon beam skyshine from medical accelerators, highlighting that the primary approaches, NCRP 151 empirical methods, MC simulations, and direct measurements, yield reasonable results when multiple methods are employed.