Matching extended-SSD electron beams to multileaf collimated photon beams in the treatment of head and neck cancer.

Purpose: Matching the penumbra of a 6 MeV electron beam to the penumbra of a 6 MV photonbeam is a dose optimization challenge, especially when the electron beam is applied from an extended source-to-surface distance (SSD), as in the case of some head and neck treatments. Traditionally low melting point alloy blocks have been used to define the photonbeam shielding over the spinal cord region. However, these are inherently time consuming to construct and employ in the clinical situation. Multileaf collimators(MLCs) provide a fast and reproducible shielding option but generate geometrically nonconformal approximations to the desired beam edge definition. The effects of substituting Cerrobend® for the MLC shielding mode in the context of beam matching with extended-SSD electron beams are the subject of this investigation. Methods: Relative dosebeam data from a Varian EX 2100 linear accelerator were acquired in a water tank under the 6 MeV electron beam at both standard and extended-SSD and under the 6 MV photonbeam defined by Cerrobend® and a number of MLC stepping regimes. The effect of increasing the electron beam SSD on the beam penumbra was assessed. MLC stepping was also assessed in terms of the effects on both the mean photonbeam penumbra and the intraleaf dose-profile nonuniformity relative to the MLC midleaf. Computational techniques were used to combine the beam data so as to simulate composite relative dosimetry in the water tank, allowing fine control of beam abutment gap variation. Idealized volumetric dosimetry was generated based on the percentage depth-dose data for the beam modes and the abutment geometries involved. Comparison was made between each composite dosimetry dataset and the relevant ideal dosimetry dataset by way of subtraction. Results: Weighted dose-difference volume histograms (DDVHs) were produced, and these, in turn, summed to provide an overall dosimetry score for each abutment and shielding type/angle combination. Increasing the electron beam SSD increased the penumbra width (defined as the lateral distance of the 80% and 20% isodose contours) by 8 – 10 mm at the depths of 10 – 20 mm . Mean photonbeam penumbra width increased with increased MLC stepping, and the mean MLC penumbra was ≈ 1.5 times greater than that across the corresponding Cerrobend® shielding. Intraleaf dose discrepancy in the direction orthogonal to the beam edge also increased with MLC stepping. Conclusions: The weighted DDVH comparison techniques allowed the composite dosimetry resulting from the interplay of the abovementioned variables to be ranked. The MLCdosimetry ranked as good or better than that resulting from beam matching with Cerrobend® for all except large field overlaps ( − 2.5 mm gap). The results for the linear-weighted DDVH comparison suggest that optimal MLC abutment dosimetry results from an optical surface gap of around 1 ± 0.5 mm . Furthermore, this appears reasonably lenient to abutment gap variation, such as that arising from uncertainty in beam markup or other setup errors.