(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: To measure the skin dose in total body photon irradiation (TBI) therapy when the treatment is administered using multi-isocenter volumetric arc therapy (VMAT) to a patient positioned on the treatment couch.
Methods: A 20x20 cm2 stack of water-equivalent phantom material, 9.5 cm high was placed on the couch, simulating the geometry of a child previously treated using VMAT. Treatments were planned with Eclipse (Varian, Palo Alto, CA) and designed to deliver 200 cGy at midline with a 6MV beam, while dose to lung and kidney were reduced to 75 and 40% of the midline dose, respectively. Adjacent isocenter groups had a minimum of 3 cm overlap and were optimized together to provide smooth junctions. The skin dose was measured with a pancake surface ion chamber (0.5 mg/cm2 entrance window) embedded in the anterior surface of the phantom.
Results: The surface dose at the central plane of the beam was 71% of the planned midline dose and increased rapidly with depth, attaining 82, 88, 93 and 108% at 0.27, 0.53, 1.05 and 5 mm, respectively.
Conclusion: The tangential angles of incidence and exit dose in VMAT treatment result in a high surface dose. Assuming a skin surface of 0.5 m2, the mass that receives less than 90% of the prescription dose would be less than 0.5 kg. This mass is less than the mass of lung and overlying tissues planned to receive in typical TBI treatment about 50% of the prescription dose, and exceeds the lung dose. Bolus may therefore not be required in VMAT treatments unless there is skin involvement. Nevertheless, these results need to be verified for individual patients using anthropomorphic phantoms and in-vivo dosimetry.
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