KEY POINTS
- This retrospective single-institution study included 208 women with nonmetastatic invasive breast cancer and clinically positive internal mammary nodes on baseline MRI who subsequently completed neoadjuvant therapy, surgery and postoperative radiotherapy. Median follow-up was 36 months.
- Internal mammary nodal clinical complete response was defined as complete disappearance on post-neoadjuvant MRI of all nodes considered metastatic at baseline. All patients received postoperative internal mammary irradiation to 50 Gy in 25 fractions; 130 received a simultaneous integrated boost to 60 Gy at the initially involved nodal site and 78 did not.
- In the overall cohort, adding the boost was not significantly associated with improved disease-free survival after inverse probability weighting: adjusted hazard ratio 0.66 (95% CI 0.32–1.39; P=.277). Importantly, there was significant interaction between boost effect and post-neoadjuvant MRI response (P for interaction=.042).
- Among patients whose internal mammary nodes did not completely disappear on MRI, boost was associated with better disease-free survival: adjusted hazard ratio 0.36 (95% CI 0.14–0.93; P=.034). Distant metastasis-free survival was also higher, with a 3-year rate of 86.8% versus 72.4% and adjusted hazard ratio 0.34 (95% CI 0.12–0.97; P=.044).
- In contrast, patients who achieved an MRI complete response showed no detectable disease-free survival benefit from boost, with an adjusted hazard ratio of 1.13 (95% CI 0.47–2.74; P=.781). MRI nodal complete response itself was associated with breast pathological complete response (OR 2.29) and combined breast/axillary pathological complete response (OR 2.10).
- Failure was predominantly systemic: among 36 disease-free survival events, 28 were isolated distant metastases, whereas only 3 patients experienced internal mammary recurrence. All three nodal recurrences occurred after MRI complete response, and two coincided with distant metastases.
- The association in non-complete responders was therefore driven mainly by distant-metastasis outcomes rather than demonstrably better internal mammary control, an important caution for interpreting a locoregional intervention. Treatment was not randomized, boost recipients had different baseline disease characteristics, and dedicated internal mammary imaging was not uniform at systemic relapse.
CLINICAL TAKEAWAY
Post-neoadjuvant MRI response may eventually help select which patients with initially involved internal mammary nodes actually need dose escalation beyond standard regional nodal irradiation. For now, the result is hypothesis-generating: the retrospective design and predominantly distant-metastasis-driven association do not justify omitting boost solely because an internal mammary node disappears on MRI.
SOURCE
International Journal of Radiation Oncology, Biology, Physics