Lung Dose and Pneumonitis Risk Following Five-Fraction Breast Radiotherapy

To evaluate associations between lung dose and symptomatic pneumonitis in five-fraction breast radiotherapy and identify strategies to limit lung exposure.

We retrospectively analysed 224 consecutive patients with early-stage breast cancer treated between 2020 and 2024 at a single institution using five-fraction adjuvant radiotherapy, including whole-breast radiotherapy (WBRT [N = 157]: 26 Gy [5.2 Gy daily] or 28.5 Gy [5.7 Gy weekly]) and accelerated partial-breast irradiation (APBI [N = 67]: 30 Gy [6 Gy every other day]). Associations between lung dose-volume parameters and symptomatic pneumonitis were examined, along with potential dose thresholds linked to increased risk.

At a median follow-up of 13 months, 8 patients (3.6%) developed symptomatic (grade ≥ 2) pneumonitis. Symptoms were mostly mild and self-limited, with only one patient experiencing grade 3 pneumonitis. Pneumonitis risk showed a dose-response pattern: Each 1% increase in ipsilateral lung volume receiving 8 Gy, 10 Gy, and 20 Gy was associated with relative risk increases of 13%, 23%, and 33%, respectively (all p < 0.05). An exploratory threshold of V20Gy ≥ 7.5% identified a subgroup of patients at higher risk, with pneumonitis rates of 9.0% (7/78) above this threshold compared to 0.7% (1/146) for those below (AUC 0.79). In patients receiving WBRT, prone positioning reduced lung V20Gy. APBI was also associated with lower lung V20Gy (both p < 0.05).

Although mild and self-limited, the risk of symptomatic pneumonitis progressively increased with higher fractional lung doses during five-fraction breast radiotherapy. APBI or prone positioning for WBRT may help reduce intermediate-to-high lung dose volumes (such as V20Gy) and pneumonitis risk in eligible patients.