282 - Analysis of Rib Fracture Risk after Breast Radiation with Various Dose Fractionations

J. Howell¹, J. Fenlon², and M. M. Poppe²; ¹Department of Radiation Oncology, Huntsman Cancer Institute, University of Utah, Salt Lake City, UT, ²Huntsman Cancer Institute, University of Utah, Salt Lake City, UT

Purpose/Objective(s): Rib fractures are an infrequent (incidence ~ 1-3%) but distressing adverse effect of radiation therapy for breast cancer. The low incidence hampers the development of clinically meaningful dose constraints for the ribs. We performed a single-institution, retrospective analysis of rib fractures after breast radiation by dose fractionation. As the a/ß of ribs is uncertain, we hypothesized that post-treatment rib fracture rates would be similar between prescribed regimens but correlate positively with rib dose. Materials/

Methods: We identified all patients radiated for breast cancer at our institution from 2012 – 2022 and utilized a natural language processing algorithm to search electronic medical records and imaging reports for the term “fracture.” Flagged reports were evaluated to identify in-field, post-therapy rib fractures. Chart reviews of these patients and a case cohort of patients matched by age, prescription, and treatment year who did not experience rib fractures were then completed to identify relevant oncologic history and treatment details. Finally, we created a specialized workflow in our treatment planning system to assist in contouring the in-field ribs of these patients and calculating their rib and fracture doses.
Results: Our query identified 2,373 breast cancer patients and 52 cases of in-field, post-therapy rib fractures (incidence = 2.19%). When comparing fracture rates between 13 dose prescriptions, using [50 Gray (Gy)/ 25 Fraction (Fx)] + [10 Gy/ 5 Fx] boost as the reference (fracture incidence = 2.50%), [40.05 Gy/ 15 Fx] + [10 Gy/ 4 Fx] boost carried a reduced risk of rib fracture (incidence = 1.08%, OR = 0.424, P = 0.0290). [36.63 Gy/ 11 Fx] + [13.32 Gy/ 4 Fx] boost carried an increased risk (incidence = 7.27%, OR = 3.05, P = 0.029). Weighted regression analysis demonstrated no correlation between fracture incidence and prescription equivalent dose in 2 Gy fractions (EQD2) at multiple a/ß values (0.85, 1.5, 3, 4, 10, 17.5, 25). Univariate analysis revealed neoadjuvant chemotherapy increased the risk for subsequent rib fracture (OR = 4.27, P = 0.002), but adjuvant chemotherapy and hormonal therapy did not. Multiple dose-volume statistics of the rib contours, including D0.03cc (OR = 1.044, P = 0.030), D2cc, D4cc, D8cc, and D10cc (OR = 1.039, P = 0.033), were increased in the fractured patients’ treatment plans.
Conclusion: Our data showed no consistent correlation between rib fracture rate and prescription EQD2 for breast radiation regimens, in agreement with our hypothesis. Dose-volume statistics were significantly increased in fractured patients, highlighting the importance of ribs proximity to target volumes and suggesting the presence of generalizable rib dose constraints that could reduce fracture risk across treatment regimens. Our future work will focus on expanding our data set to determine these constraints using normal tissue complication probability modeling.