Online Adaptive Radiotherapy for Left-Sided Breast Cancer with Comprehensive Regional Nodal Coverage Including the Internal Mammary Chain: Case Report and Narrative Review

Online adaptive radiotherapy mitigates errors in absorbed dose delivery due to daily anatomical changes during hypofractionated breast treatment, particularly when comprehensive nodal therapy includes the internal mammary chain. To illustrate this, we present a case of a 65-year-old woman with left-sided luminal B invasive carcinoma, who underwent segmentectomy and level 1–2 dissection. Pathology revealed an 18 × 15 × 13 mm primary tumor with lymphovascular invasion, two of eleven axillary nodes positive, and intramammary metastasis, staged pT1cN1a. She received adjuvant docetaxel–cyclophosphamide followed by letrozole. Hypofractionated radiotherapy (40 Gy in 15 fractions) was administered in an inspiration breath-hold setting using a CBCT-guided online-adaptive platform. Adaptive planning improved V95% coverage over the planned treatment for all targets: on average, whole breast coverage increased from 88.4% to 96.3%, supraclavicular from 93.0% to 97.1%, axilla from 90.6% to 96.7%, and internal mammary from 91.8% to 95.9%. Organ-at-risk metrics remained within limits: the mean heart dose increased slightly (from an average of 0.12 Gy in scheduled to 0.15 Gy in adaptive plans). At the same time, the LAD D0.03 cm3 decreased, and the heart V4 Gy fell modestly (from 13.3% in the scheduled plan to 8.2% in the adaptive plan), reflecting low-dose redistribution without exceeding constraints. Lung and thyroid mean doses remained comparable. The patient tolerated treatment well, with no acute toxicity or local recurrence. This case highlights the importance of daily adaptation for complex left-sided radiation treatment involving internal mammary nodes, demonstrating target recovery without exceeding absorbed dose constraints and supporting future studies on control, toxicity, and quality of life.