2910 - Minimizing V103 and V102 in Whole-Breast Irradiation: Implications for Hypofractionated and Conventionally Fractionated Regimens

Purpose/Objective(s): Hypofractionated whole-breast irradiation (HF-WBI) has been associated with lower incidence and severity of acute skin toxicity compared to conventional fractionation (CF-WBI). Previous studies attribute this reduction in toxicity to the lower total dose used in HF regimens. Most of those studies focused on treatment plans that minimize volumes > 105% of the prescribed dose (V105) but often leaves significant volumes receiving > 103% (V103). This study assesses whether further reducing the total dose—by minimizing V103 and, when possible, V102 without compromising coverage—can further lower the incidence of acute skin toxicity in HF-WBI and CF-WBI.

Materials/Methods: A total of 93 WBI patients were divided into 3 cohorts based on dose homogeneity:

• Cohort I (HF_V103/CF_V103; 21): minimized V103 & V102.
• Cohort II (HF_V105/CF_V105; 51): minimized V105.
• Cohort III (HF_V105/UltraHF_V105; 21): minimized V105

Minimized V103 & V102 refers to significantly reduced 103% and 102% dose volumes without compromising coverage with103% and 102% isodose lines not extending to the central or posterior regions of the breast. Cohort I underwent prospective analysis, while Cohort II (Institution 1) and Cohort III (Institution 2) were retrospectively analyzed for incidence of acute skin toxicity. All treatment plans were created in a technology company's treatment planning system using 3D tangents in supine position (Cohorts I and II) or prone position (Cohort III). Field-in-field and e-compensator techniques were used. Acute skin toxicity (dermatitis) was assessed by providers (CTCAE scale grade = 1)

Results: Minimizing V103 & V102 significantly reduces the incidence of acute skin toxicity in both HF and CF regimens, with a more pronounced effect observed in HF treatments, where only 40% of patients reported experiencing toxicity. Furthermore, CF with minimized V103/V102 resulted in an acute skin toxicity incidence of 81.1%, which is comparable to the 83.3% observed in HF with V105 minimization. In the prone position, HF_V105/UltraHF_V105 showed acute toxicity rates similar to those of supine HF_V105, suggesting that treatment position does not significantly impact treatment outcomes. Results summarized in Table 1.

Conclusion: This study shows that minimizing V103 & V102 significantly lowers the incidence of acute skin toxicity in WBI, especially in HF regimens, where the incidence of acute skin toxicity is more than halved compared to V105 minimization. Notably, these findings reveal for the first time, V103/V102 minimization has similar toxicity rates to HF_V105 minimization. These findings underscore the significance of treatment planning quality in reducing toxicity and support adopting strategies beyond V105 minimization.

Abstract 2910 - Table 1: Incidence of acute skin toxicity for UltraHF, HF and CF regimens

Fractionation + High dose volume minimization	HF_V103Acute skin toxicity incidence
HF_V103	40.0%
HF_V105	83.3%
CF_V103	81.1%
HF_V105	96.3%
HF_V105/UltraHF_V105	85.7%