A Dosimetric Study of Accidental Internal Mammary Lymph Node Irradiation during Locoregional Breast Irradiation

Background: One remaining grey zone in locoregional breast irradiation is whether to irradiate internal mammary lymph nodes (IMN) in the lack of gross involvement as well as optimum dosimetric recommendations for IMN irradiation. Our study aimed to assess whether IMNs are incidentally irradiated during locoregional breast irradiation, and whether this non-intended irradiation could contribute to lower IMN recurrence risk.Method: This retrospective study was conducted on 137 adult female patients treated at Alexandria Main University Hospital who had pathologically proven breast cancer. Patients should have undergone surgery and were proven to be: T3 (tumor > 5 cm), had positive axillary lymph nodes (ALNs; ≥ 1 ALN involvement) or had positive surgical margins, from January 1st, 2015, to December 31st, 2017. IMNs were delineated, and the IMN mean dose was recalculated. Data were analyzed using SPSS Ver. 25 and data was considered statistically significant at a P-value of ≤ 0.05.Results: Only the IMN mean dose percent, at a cutoff dose of 28.5% mean dose percent, was associated with significant reduction in the risk of IMN recurrence (P = 0.05). Other factors including tumor site, size, grade, or nodal status were not associated with higher risk of recurrence.Conclusion: Although the indication of IMN irradiation is still debatable, our results suggest that a minimum prescribed dose percent of 28.5% is needed to reduce the risk of IMN recurrence in patients with no IMN gross involvement. Yet, studies with larger sample size are needed to better characterize optimum IMN dosimetric criteria.