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Dosimetric Effect of Unwanted Bolus-Air Gap On Surface Dose in Head and Neck and Chest Wall VMAT Treatments

Z Xu

Z Xu1*, M Yao1,2 , T Biswas1,2 , Y Zheng1,2 , (1) Case Western Reserve University, Cleveland, OH, (2) University Hospitals Cleveland Medical Center, Cleveland, OH


SU-I-GPD-T-309 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: This study is to evaluate the dosimetric impact of unwanted bolus-air gaps on surface dose in head and neck and chest wall VMAT treatment delivery.

Methods: Seven clinical VMAT plans (6MV), including four head and neck and three chest wall cases, were delivered on a cylindrical shape phantom (cheese phantom) on the same day. Styrofoam blocks were placed between the phantom surface and 1 cm superflab bolus to create uniform air gaps of 0.5 cm, 1.0 cm, and 1.5 cm thicknesses. Gaf-chromic EBT3 films were placed consistently on top of the phantom for surface dose measurements. A 17 cm by 11 cm region of interest (ROI) was applied for all irradiated films. For each plan, measurements with different bolus-air gaps were compared to the one without air gap and normalized to the global maximal dose to calculated the percentage point dose difference in the ROI. Two-dimensional (2D) dosimetric evaluation between measurements with and without air gaps was performed through absolute dose gamma comparison using 5%/3mm and 3%/3mm criteria.

Results: Percentage point dose difference larger than 5% were 10.3%±3.2% (0.5 cm gap), 14.4%±5.1% (1.0 cm gap), and 20.7%±6.7% (1.5 cm gap). The passing rates for measurements with 0.5 cm, 1.0 cm, and 1.5 cm air gaps were 98.7%±0.7%, 94.7%±1.1%, and 89.5%±3.3%, respectively, using 5%/3mm criteria and 91.5±1.5%, 87.3±2.0%, and 82.7±3.6%, respectively, using 3%/3mm criteria.

Conclusion: Besides dosimetric effect caused by bolus-air gaps, percentage point dose difference was affected by film registration quality, which makes gamma analysis a better method for 2D surface dose evaluation. For head and neck and chest wall VMAT using 1.0 cm bolus, uniform air gaps up to 1.0 cm demonstrated over than 90% passing rate using 5%/3mm criteria. In addition, as bolus-air gap increased, surface dose would decrease and the variation of surface dose would increase.

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