Film dosimetry for IMRT fields in the near-surface buildup region

Abstract

Film dosimetry provides fast, convenient 2-D dose distributions, but is challenged by the dependence of film response on scatter conditions (i.e. energy dependence).  Verification of delivered dose in the surface buildup region is important for intensity modulated radiation therapy (IMRT) when volumes of interest encroach on these regions (e.g. head/neck, breast).  The current work demonstrates that film dosimetry can accurately predict the dose in the buildup region for IMRT, since 1) film dosimetry can be performed with sufficient accuracy for small fields and 2) IMRT is delivered by “small” segments (SMLC) or dynamic gaps (DMLC).  This work evaluates the accuracy of XV and EDR film for measurements from 2 mm to 10 cm depths for small fields and clinical IMRT beams.   Film measurements have been compared to single point measurements made with a stereotactic diode (STD) and parallel plate ionization chamber (P11) and thermoluminescent dosimeters (TLD) at various depths for square (STD, P11) and IMRT (STD, TLD) fields.  Film calibration was performed using an 8-field step exposure on a single film at 5cm depth, which has been corrected to represent either small field or large field depth dependent film calibration techniques.  Up to 10% correction for film response variation as a function of depth was required for measurements in the buildup region.  A depth-dependent calibration can improve the accuracy sufficiently for IMRT calculation verification (i.e. ≤5% uncertainty).  A small-field film calibration technique was most appropriate for IMRT field measurements. Improved buildup region dose measurements for clinical IMRT fields promotes improved dose estimation performance for (inverse) treatment planning and allows more quantitative treatment delivery validation.