Calculated Vs In-Vivo Film Surface Dose-Area Comparison for Breast Radiotherapy
A Malhotra1,2*, E Carpentier1,2, C Duzenli1,2,3, (1) BC Cancer Department of Medical Physics, Vancouver, British Columbia, Canada, (2) University of British Columbia Department of Physics and Astronomy, Vancouver, British Columbia, Canada, (3) University of British Columbia Department of Surgery, Vancouver, British Columbia, Canada
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Purpose: High surface dose can cause potentially severe radiotherapy induced skin reactions. This occurs in breast treatments, particularly on the inferior surface. Support devices can reduce epidermal dose by alleviating skin folds, and it is important to understand and predict the dose to the skin under the device. High quality epidermal dose prediction prior to treatment would allow for customizable skin sparing treatment plans and the application of appropriate breast positioning techniques. Dose across a 2D region of interest was compared between the treatment planning system (TPS) and in-vivo film dosimetry measurements.
Methods: EBT3 radiochromic film dosimetry was used to measure surface dose during radiotherapy for 18 patients. A skin dose evaluation structure in the Eclipse TPS was constructed by creating a 2 cm margin on the body contour to assist the TPS to correctly assign dose on the skin surface. The region of contact between the breast support device and the skin on the inferior breast surface is analyzed. A 2D 5%, 5mm gamma analysis and a dose area analysis were conducted to compare the measured and predicted dose.
Results: A 5%, 5mm gamma analysis comparing Eclipse and film yielded an average passing rate of 71% across the inferior breast surface, over all patients. The measured and predicted maximum doses delivered to ≥ 1cm2, agree within ± 7% in 89% of cases. Eclipse dose systematically tracks film dose at an average of 6% higher than the film.
Conclusion: This small case study demonstrates consistency between TPS and film dose in the inferior region of the breast where skin reactions are most prevalent. This opens the door to the implementation of dose-area skin toxicity prediction metrics in the treatment planning stage in order to prevent adverse reactions.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Canadian Cancer Society Research Institute.
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