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A Radiochromic Film Based Quantitative Assessment of Thermoplastic Mask Bolus Effect in Head and Neck IMRT/VMAT

C Kalavagunta

C Kalavagunta*, M Lin , J Snider , H Xu , A Schrum , P Vadnais , K Marter , M Suntharalingam , K Prado , University of Maryland School of Medicine, Baltimore, MD


SU-E-T-71 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To quantify the factors leading to thermoplastic mask bolus-associated-increased skin dose in head and neck IMRT/VMAT using EBT2 film.

Methods: EBT2 film placed beneath a dual layer 3-point ORFIT head, neck and shoulder mask was used to test the effect of mask thickness, beam modulation, air gap, and beam obliquity on bolus effect. Mask thickness was varied based on the distribution of 1.6mm Orfilight layer on top of 2 mm Efficast layer. Beam modulation was varied by irradiating the film with an open field (no beam modulation) and a step and shoot field (beam modulation). Air gap between mask and film was varied from 0 to 5mm. Beam obliquity was varied by irradiating the film at gantry angles of 0°, 35°, and 70°.Finally, film strips placed on a Rando phantom under an Orfit mask, in regions of expected high dose, were irradiated using 5 IMRT and 5 VMAT plans with various modulation levels (modulation factor 2 to 5) and the results were compared with those obtained placing OSLDs at the same locations.

Results: An 18-34% increase in mask bolus effect was observed for three factors where the effect of beam obliquity ≥ beam modulation > mask thickness. No increase in mask bolus effect was observed for change in air gap. A 6-13% increase in dose due to mask bolus effect was observed on film strips.

Conclusion: This work underlines the role of beam obliquity and beam modulation combined with thermoplastic mask thickness in increasing mask bolus-associated skin dose in head and neck IMRT/VMAT. One possible method of dose reduction, based on knowledge gained from this work, is inclusion of skin as an avoidance structure in treatment planning. Another approach is to design a mask with the least amount of thermoplastic material necessary for immobilization.

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