Purpose: To assess accuracy of dose calculations in the near-surface region for different treatment planning systems (TPSs), plan complexities, and energies to improve clinical decisions for patients receiving whole breast irradiation (WBI). This work is part of a multi-institutional effort investigating the correlation between mean dose calculated in the near-surface region and acute skin toxicities considering different WBI treatment techniques.
Methods: A portable custom breast phantom was designed for measurements at eight institutions. Clinically-relevant treatment plans (open, field-in-field (FiF), FiF mixed-energy, and IMRT) were created in four TPSs on a patient dataset (50Gy/25fx) and peer reviewed by participants. After transfer to the phantom dataset, plans were re-calculated with fixed monitor units. For each irradiation, which included linear accelerators from different vendors, the phantom was aligned with predetermined moves or CBCT. Dose was measured with radiochromic film (1.5cmx13.0cm) placed at two depths (0.5 and 1.0cm) and three locations per depth within the phantom. Film was scanned (> 24h post-irradiation) and analyzed using FilmQA Pro. Dose differences were evaluated relative to the calculation.
Results: Measured and calculated doses agreed well for all TPSs, complexities, and beam energies with 86.7%, 77.8%, 91.9%, and 80.6% of measurements within ±5% agreement for open field, FiF, FiF mixed-energy, and IMRT, respectively. Based on a Gaussian fit of the difference distribution, the mean percent difference was -0.2±2.9%, with 95% of measurements within 6% agreement. Furthermore, Pearson correlation scores (# of measurements) of 0.8(60), 0.9(54), 0.7(36), and 0.8(72) showed good agreement for open fields, FiF, FiF mixed-energy, and IMRT, respectively.
Conclusion: To the best of our knowledge, these results demonstrate that dose calculations from clinically relevant WBI plans for various complexities and beam energies are accurate in the near-surface region. This lays the foundation for future work to investigate the correlation between near-surface dose and patient skin toxicities.
Funding Support, Disclosures, and Conflict of Interest: The Michigan Radiation Oncology Quality consortium is financially supported by Blue Cross Blue Shield of Michigan and the Blue Care Network of Michigan as part of the BCBSM Value Partnerships Program.