1064 - Dosimetric Correlates of Acute Toxicities for Hypofractionated Whole-Breast Irradiation, A Single Institution Retrospective Investigation: Is There a Need to Update the Guidelines?

Purpose/Objective(s): Evidence-based consensus guidelines for treatment planning and dose homogeneity were updated by ASTRO in 2018, yet dosimetric correlates of acute toxicities with these guidelines are underreported. We retrospectively examined rates of pain, erythema and edema when adhering to these guidelines while identifying predictors of acute toxicities.

Materials/Methods: We reviewed charts of patients receiving moderate hypofractionated whole breast irradiation (HF-WBI) at our institution. Treatment plans adhered to WBI V105%(cc) < 200 cc and V107%(cc) < 2 cc limits. The highest grade of physician-reported acute pain, erythema, and edema were recorded during on treatment visit (OTV) and at short-term follow-up (ST F/U) 1-month post treatment. Dosimetric, patient body habitus and clinical parameters were analyzed for correlation and predictive potential using MEDCALC V23.0.2 (MedCalc Software Ltd, Belgium) and logistic regression with R 4.4.2.

Results: From 2018 to 2023, 600 patients were treated with HF-WBI. 82.3% received 42.56 Gy in 16 fractions, while 17.7% received 40.05 Gy in 15 fractions. 3D field-in-field planning was used in all patients and 73.7% received a boost. OTV toxicity was most commonly grade = 2 pain (29.7%) with grade 3 pain occurring in 8.5% of patients. Both resolved to = 1% in ST F/U. Multivariate analysis (Table 1) identified V105%(cc) = 50 cc and V105%(%) = 5% as the strongest predictors of grade = 2 pain on OTV. Incidence of grade = 2 pain was 49.4% with V105%(cc) = 50 cc versus 25.8% when < 50 cc, and 45.9% with V105%(%) = 5% vs. 27.8% when < 5%. In ST F/U, grade 2 erythema occurred in 26% of patients with V105%(cc) = 50 cc, compared to only 0.2% when V105%(cc) < 50cc. Similarly, grade = 2 edema was seen in 27.4% vs. 0.4% respectively. When considering V105%(%) = 5%, grade 2 erythema as well as grade = 2 edema occurred at 22.2% vs.1.5% rate. Due to low number of events, statistical analysis was limited. Importantly, no severe (grade 4 or 5) toxicities were observed.

Conclusion: Despite adhering to ASTRO 2018 guidelines, moderate to severe acute pain remained high (nearly 30%) at our institution, even though moderate erythema and edema were limited to under 5% during and after treatment. Since V105%(cc) = 50 cc and V105%(%) = 5% significantly predict pain, optimizing these to < 50 cc and < 5%, respectively could drastically reduce moderate to severe pain during treatment. With ASTRO revising whole breast irradiation guidelines, these findings offer timely and critical data to inform those updates.