Evaluation of two-dimensional bolus effect of immobilization/support devices on skin doses: A radiochromic EBT film dosimetry study in phantom

Purpose: In this study, the authors have quantified the two-dimensional (2D) perspective of skindose increase using EBT filmdosimetry in phantom in the presence of patient immobilization devices during conventional and IMRTtreatments. Methods: For 6 MV conventional photon field, the authors evaluated and quantified the 2D bolus effect on skindoses for six different common patient immobilization/support devices, including carbon fiber grid with Mylar sheet, Orfit carbon fiber base plate, balsa wood board, Styrofoam, perforated AquaPlast™ sheet, and alpha-cradle. For 6 and 15 MV IMRT fields, a stack of two film layers positioned above a solid phantom was exposed at the air interface or in the presence of a patient alpha-cradle. All the films were scanned and the pixel values were converted to doses based on an established calibration curve. The authors determined the 2D skindose distributions, isodose curves, and cross-sectional profiles at the surface layers with or without the immobilization/support device. The authors also generated and compared the dose area histograms (DAHs) and dose area products from the 2D skindose distributions. Results: In contrast with 20% relative dose [(RD) dose relative to d max on central axis] at 0.0153 cm in the film layer for 6 MV 10 × 10 cm 2 open field, the average RDs at the same depth in the film layer were 71%, 69%, 55%, and 57% for Orfit, balsa wood, Styrofoam, and alpha-cradle, respectively. At the same depth, the RDs were 54% under a strut and 26% between neighboring struts of a carbon fiber grid with Mylar sheet, and between 34% and 56% for stretched perforated AquaPlast™ sheet. In the presence of the alpha-cradle for the 6 MV (15 MV) IMRT fields, the hot spot doses at the effective measurement depths of 0.0153 and 0.0459 cm were 140% and 150% (83% and 89%), respectively, of the isocenter dose. The enhancement factor was defined as the ratio of a given DAH parameter (minimum dose received in a given area) with and without the support device. For 6 MV conventional 10 × 10 cm 2 field, the enhancement factor was the highest (3.4) for the Orfit carbon fiber plate. As for the IMRT field, the enhancement factors varied with the size of the area of interest and were as high as 3.8 (4.3) at the hot spot of 5 cm 2 area in the top film layer (0.0153 cm) for 6 MV (15 MV) beams. Conclusions: Significant 2D bolus effect on skindose in the presence of patient support and immobilization devices was confirmed and quantified with EBT filmdosimetry. Furthermore, the EBT film has potential application forin vivo monitoring of the 2D skindose distributions during patient treatments.