Practical considerations for reporting surface dose in external beam radiotherapy: a 6 MV X-ray beam study.

In this study, we assessed the accuracy of surface doses determined by direct measurement and treatment planning system (TPS) calculations, relative to benchmark Monte Carlo (MC) doses calculated at 70 μm for a 6 MV, 10 × 10 cm clinical radiotherapy beam. In a homogeneous phantom with both open and fixed wedged fields, we found that the relative dose measured with an Attix chamber underestimates the MC calculated surface dose by 2.9 %, while the relative dose measured with EBT2 Gafchromic film overestimates the MC surface dose by 0.9 %. There was a significant over-response of up to 20 % in doses calculated at < 2 mm depth with the Eclipse analytic anisotropic algorithm (AAA) compared to corresponding MC doses for an open field. This drops to < 2 % at 2 mm depth. In a heterogeneous phantom, EBT2 film overestimates relative dose by up to 3.1 % compared to the MC calculated surface dose. The AAA relative dose calculated in a heterogeneous phantom at 2 mm depth agrees to within 1.5 % with the MC doses calculated at the same depth, but overestimates the MC surface dose (at 70 μm) by up to 2.5 %. Our results suggest that TPS doses evaluated near the surface be reported with a depth that should be at least 2 mm and this should be taken into consideration in the planned target volume for treatments where surface dose is a constraining factor. Our study demonstrates the usefulness of EBT2 film for measuring surface dose: under homogeneous conditions, the effective point of measurement of EBT2 film can be considered equivalent to the clinical skin depth of 70 μm.