M. P. Dykstra1, K. Griffith2, A. Moncion1, M. Grubb1, R. Marsh1, M. Mietzel1, F. A. Vicini3, and L. J. Pierce1; 1Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, 2Department of Biostatistics, University of Michigan, Ann Arbor, MI, 3MHP Radiation Oncology Institute/GenesisCare, Farmington Hills, MI
Purpose/Objective(s): Tumor bed boost improves local control following whole breast radiotherapy (WBRT) for women with invasive cancer or ductal carcinoma in situ (DCIS) with high-risk features. However, tumor bed boost is also associated with more toxicity. We sought to evaluate acute toxicity associated with boost based on conventional (CF) vs moderate hypofractionation (MH) among women receiving WBRT using real world data. Materials/
Methods: This analysis includes women who underwent definitive WBRT without regional nodal irradiation for invasive carcinoma or DCIS, prospectively enrolled from 2012 to November 2023 at 27 radiation oncology centers in a state-wide quality consortium. Standardized patient, physician, and physicist forms were used to collect baseline and follow-up information. Acute toxicity evaluation included patient- and physician-reported outcomes at treatment end. A severe toxicity composite variable included patient- or physician-reported breast pain or moist desquamation. Multivariable models were used to find associations between boost and toxicity outcomes, accounting for relevant clinical characteristics and treatment planning techniques including use of a photon vs electron boost, stratified by fractionation scheme. Results: Full clinical and treatment data were available for 15,711 women. MH was more common than CF (74.9% vs 25.1%). Boost use was less common with MH compared to CF, 71.1% vs 94.0% (p<0.001). Of those receiving boost, photon (PB) and electron boost (EB) were used in 69.0% and 31.0% of women receiving MH and 58.7% and 41.3% of those receiving CF, respectively. Both PB and EB were associated with increased composite severe toxicity following MH, 26.3% and 19.5% vs 12.2% for no boost, respectively (OR 2.11 and 1.79, p < 0.001), but not following CF (44.0% and 34.1% vs 47.4% for no boost, OR 0.92 and 0.78, p = 0.16). PB and EB worsened patient-reported moderate to severe breast pain following MH, 24.5% and 19.4% vs 13.6% (OR 1.62 and 1.36, p < 0.001), but not CF (32.2% and 25.5% vs 35.9%, OR 0.90 and 0.72, p = 0.03). Physician-reported breast pain showed a similar trend, with 8.0% and 6.4% vs 3.2% (OR 1.91 and 1.95, p < 0.001) following MH, and 15.0% and 10.7% vs 13.1% (OR 1.29 and 1.02, p = 0.11) after CF. Neither PB nor EB worsened severe fatigue in MH (24.0% and 21.5% vs 19.9%, OR 1.17 and 1.14, p = 0.12) or in CF (32.2% and 28.2% vs 34.7%, OR 0.79 and 0.79, p = 0.47). Physician-reported dermatitis was worse with PB and EB for both fractionation schemes: 28.9% and 24.3% vs 13.0% (OR 2.39 and 2.14, p < 0.001) after MH and 48.9% and 44.4% vs 40.0% (OR 1.54 and 1.56, p < 0.001) with CF. Conclusion: Following MH, boost is associated with increased acute toxicity. CF led to more pronounced acute toxicity than MH, but boost is associated with less added toxicity. These data may help shared decision making when discussing the relative benefits of adding a boost following MH or CF.
