Approximation of skin dose for selected setup parameters during external beam radiotherapy at Uganda Cancer Institute

Abstract

Skin dose is unavoidable during radiation treatment. However, it should be within tolerance. Excessive cases can lead to radiation skin injuries. Investigation of radiation dose distributions to the skin surface and build-up region is vital for treatment planning against these effects. This study aimed at determining the skin dose and evaluating the accuracy of the Eclipse treatment planning system (TPS) within the build-up region under a range of clinical set-up parameters including field size, gantry angle, source-to-surface distance (SSD), multileaf collimator (MLC), and enhanced dynamic wedge angles. The study was conducted at the Uganda Cancer Institute, where no previous experimental assessment of skin dose and TPS accuracy had been reported. The EBT3 film was used for the skin dose measurements while CC13 ionisation chamber was used for verification of the TPS dose calculations at 1 cm depth within the build-up region. These measurements were done with a plastic phantom (SP34 slab phantom, IBA dosimetry, Germany) using two Varian TrueBeam linear accelerators; Linac 1 and Linac 3, each with three beam energies; 6 MV, 10 MV and 15 MV. Theresults show that the skin dose increased with field size, both for open fields and MLC fields but the MLC yielded more skin dose compared to open field, with the highest deviation being 5.1%. Skin doses for oblique beam incidences were more than for normal beams and the larger the gantry angle, the more the deviations. Skin dose reduced with increasing beam energy and SSDs. For the SSD, the effect was more significant for larger fields (difference was within 6.6% for field size more than 5 ×5 cm2)thanfor smaller fields (difference was within 3.5% for field size up to 5 × 5 cm2). The TPScalculations deviated from measured doses by up to ± 5.50% and ± 9.41% in Linac 1 and Linac 3, respectively, with the largest discrepancies observed for the 10 MV beam and the smallest for 6 MV. It is concluded that, field size, beam angle, energy, and SSD significantly influence skin dose in megavoltage photon therapy. Larger fields, lower beam energies and more oblique beam angles reduce the skin-sparing benefit of megavoltage beams. Their applications for treatments should be considered with precautions. Furthermore, TPS dose predictions in the build-up region, particularly at higher energies, are less reliable and should be supplemented with direct dosimetric verification for accurate skin dose assessment.