Abstract
The present study quantified surface doses on several rectangular phantom setups and on curved surface phantoms for a 6 MV photon field using the Attix parallel‐plate chamber and Gafchromic EBT2 film. For the rectangular phantom setups, the surface doses on a homogenous water equivalent phantom and a water equivalent phantom with 60 mm thick lung equivalent material were measured. The measurement on the homogenous phantom setup showed consistency in surface and near‐surface doses between an open field and enhanced dynamic wedge (EDW) fields, whereas physical wedged fields showed small differences. Surface dose measurements made using the EBT2 film showed good agreement with results of the Attix chamber and results obtained in previous studies which used other dosimeters within the measurement uncertainty of 3.3%. The surface dose measurements on the phantom setup with lung equivalent material showed a small increase without bolus and up to 6.9% increase with bolus simulating the increase of chest wall thickness. Surface doses on the cylindrical CT phantom and customized Perspex chest phantom were measured using the EBT2 film with and without bolus. The results indicate the important role of the presence of bolus if the clinical target volume (CTV) is quite close to the surface. Measurements on the cylindrical phantom suggest that surface doses at the oblique positions of 60° and 90° are mainly caused by the lateral scatter from the material inside the phantom. In the case of a single tangential irradiation onto Perspex chest phantom, the distribution of the surface dose with and without bolus materials showed opposing inclination patterns, whereas the dose distribution for two opposed tangential fields gave symmetric dose distribution. This study also demonstrates the suitability of Gafchromic EBT2 film for surface dose measurements in megavoltage photon beams.PACS number: 87.53.Bn
Highlights
84 Nakano et al.: Surface dosimetry with EBT2 surgery and subsequent radiation therapy significantly reduces the risks of local recurrence and long-term breast cancer mortality.[2]
According to the ICRP and ICRU recommendations, skin dose should be assessed at a depth of 70 μm, which corresponds to the boundary between the dermis and epidermis layers of the skin.[3,4,5] At this depth, there is a steep dose gradient in the percentage depth dose (PDD) curve
The doses for the four different physical wedge fields were less than the open field doses, and the surface dose decreased with increasing wedge angle
Summary
84 Nakano et al.: Surface dosimetry with EBT2 surgery and subsequent radiation therapy significantly reduces the risks of local recurrence and long-term breast cancer mortality.[2]. While modern radiotherapy treatment planning systems (TPS) are, in most cases, able to accurately predict doses to a patient, a number of studies have demonstrated that surface and near-surface doses estimated by TPSs are inaccurate This is recognized by the AAPM in their document on commissioning TPSs, which suggests an acceptability criteria of up to 20% in the build-up region of X-ray depth dose curves between measurements and TPS calculations.[7] Devic et al[3] indicated that most TPS apply the beam data measured at the depth of dose maximum and beyond to calculate all parts of the dose distribution, including the build-up region, and the surface dose is estimated by extrapolating measured data toward the surface with fitting functions. Chung et al[9] reported overestimation by up to 18% of the surface dose on a semi-cylindrical phantom representing a head and neck cancer patient by the Pinnacle TPS (Philips Medical Systems, Andover, MA), compared to measurements using Gafchromic HS film
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