Estimation of surface dose from therapeutic 6 MV photon beams

Abstract

Accurate assessment of surface dose from therapeutic photon beam can lead to predict the skin reaction and design the radiation treatment technique. Many common dosimeters have been used to estimate the surface dose. However, they are known to over-response in the near surface region. In this study, the surface dose at the central axis of 6 MV photon beams from Varian Clinac 23EX medical linear accelerator was investigated using both measurement and simulation. The photon beams are in square and rectangular shape with the side ranging from 3 to 25 cm defined by the collimators. The measurements were taken using four detectors: TLD chip, PFD[superscript 3G] diode, Markus parallel-plate and CC13 cylindrical ionization chamber. The EGS4nrc Monte Carlo (MC) code was used in the simulation. The surface dose was obtained from an extrapolation of absorbed dose in the near surface region. It was found to be significantly increased almost linearly with the increasing square field size from both measured and simulated results. As expected, the surface dose from the measurement was higher than the simulated result. The lowest and highest over-responses in the surface dose measurement, compared with MC simulation, were found with the TLD chip and the CC13 chamber, respectively. Using the MC result as a gold standard, the correction factors for each dosimeter for estimating the surface dose from square photon beam with the side between 5 to 20 cm were introduced. For the rectangular field, its surface dose can also be obtained from that of the square field using the equivalent square approach. Specifically, the side of the equivalent square of rectangular field was computed using the area-to-perimeter formula. In the comparison between the percentage surface dose of rectangular field and that of the relevant equivalent square, the difference was found to be clinically negligible. Moreover, the influence of collimator scatter on the surface dose was also found to be insignificant. Hence, the surface dose of various rectangular fields from therapeutic photon beam can be predicted reliably using the approach proposed in this study.