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Skin Dose Enhancement of Topical Agents and Dressings On Skin Using Unflattened and Flattened Photon Beams in Radiotherapy

J Chow

K Tse , L Morley , A Cashell , A Sperduti , M McQuestion , J Chow*, Princess Margaret Cancer Centre, Toronto, ON


SU-I-GPD-T-488 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: This study investigated the dosimetric impact when the flattening filter-free (FFF) photon beams are used with the topical agent or dressing on the patient’s skin in radiotherapy. Different topical agents (creams and lotions) and dressings (wet and dry) were evaluated for their skin dose enhancements using the FFF and FF photon beams.
Methods: Skin doses were measured using the 6 MV FFF and FF (Field size = 10×10cm²) photon beams produced by a Varian TrueBeam Linac. A MOSFET detector was located in a bolus under a PMMA slab of 1 mm thickness to measure the skin dose. The distance between the photon source to the PMMA surface was equal to 100 cm. The topical agent was filled in a specific sample holder of 1.5 mm thickness on the phantom surface. Twelve topical agents and five dressings were used. Skin dose ratios (skin dose of FFF photon beams/skin dose of FF beams) were calculated for all measurements.
Results: Skin doses were found higher for the FFF photon beams than FF for all topical agents. The dose ratios were in the range of 1.044 – 1.134. When no agent was used in the measurement, the skin dose ratio was equal to 1.102. The skin dose enhancement using the FFF photon beams is due to the increased weight of low-energy photon when the flattening filter is removed from the central beam axis. For the dressings, the skin dose ratio was found higher for the wet than dry. The range of skin dose ratio was 1.047 – 1.140.
Conclusion: Variations of skin dose enhancement of topical agents and dressings were found when the flattening filter was taken out from the photon beams. Skin doses with topical agents or dressings were found higher for the FFF photon beams compared to the FF in radiotherapy.

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