Abstract
Radiographic 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 a series of “small” segments (step and shoot IMRT). This work evaluates the accuracy of X‐OMAT V (XV) and Extended Dose Range (EDR) film for measurements from 2 mm to 15 mm depths for small fields and clinical IMRT beams. Film measurements have been compared to single point measurements made with a stereotactic diode and parallel plate ionization chamber (P11) and thermoluminescent dosimeters (TLD) at various depths for square (diode, P11) and IMRT (diode, TLD) fields. Film calibration was performed using an 8‐field step exposure on a single film at 5 cm 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 sufficiently improve the accuracy 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.PACS numbers: 87.53.‐j, 87.53.Dq
Highlights
88 Roberson et al.: Radiographic film dosimetry in buildup planning are essentially a collection of small fields grouped to yield a larger modulated field
Since the surface dose is strongly dependent on collimator settings, it is expected to be dependent on the treatment delivery parameters for intensity modulated radiation therapy (IMRT) fields
Task Group Report 53 [4] on quality assurance for clinical radiotherapy treatment planning systems recommends performance criteria in the buildup region that are much more forgiving compared to in-field regions at depths of dmax and greater (20% to 50% in buildup region compared to 1% to 5% in-field regions)
Summary
88 Roberson et al.: Radiographic film dosimetry in buildup planning are essentially a collection of small fields grouped to yield a larger modulated field. The switch of head and neck treatment from conformal fields to IMRT has resulted in an increase in skin toxicity.[2] This increase was attributed to increased near-surface dose due to the use of improved immobilization techniques (masks) combined with the use of multiple field IMRT techniques, effectively increasing oblique incidence on the skin. Treatment planning system dose calculations have historically been less accurate in nearsurface buildup regions. Reasons for this include the difficulty of dose calculation of accelerator-setup-dependent contamination radiation, the effect of oblique incidence on the buildup dose, and the increased challenge of measuring dose contamination and buildup dose. It has been reported that some commercial treatment planning systems over estimated the near-surface buildup region dose by 7 to 19%.(5)
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