Left laterality is an independent prognostic factor for metastasis in N3 stage breast cancer

Purpose: Development of metastasis in patients with breast cancer (BC) is the most important negative prognostic factor and this process mainly begins with lymphatic involvement. Therefore, axillary, subclavicular, internal mammary or supraclavicular nodal involvement is a crucial step before metastasis. Anatomical differences between the right and left lymphatic drainages of the breasts may significantly affect the rate, site and time to development of distant metastasis. The purpose of this study was to investigate if laterality is an independent prognostic factor for metastasis in N3 breast cancer patients. Methods: From a total of 4215 BC patients diagnosed between 1994 and 2015 in our center, 305 non-metastatic women with pathological N3 (pN3) nodal status at presentation were enrolled in this study. Patients were divided into two groups: left and right BC. Analysis of overall survival (OS) and time to first metastasis (TTM) was performed according to Kaplan-Meier method with log-rank test. Results: The median number of lymph node involvement and lymph node ratio (number of positive lymph nodes / total number of excised lymph nodes) between the two groups was equal (14 and 0,66 respectively). Recurrence was observed in 123 patients [53 (35%) right vs 70 (44%) left group]. Patients with left BC had significantly higher rate of axial bone metastases compared with the right BC group (55.7 vs 35.8%, p<0.02, respectively). TTM was significantly shorter in the left BC group [49.1 months (95% CI 36.5-61.8) vs 103.6 months (95% CI 47.0-160); p=0.03, respectively]. Median OS did not differ between the groups, however, there was a trend towards lower OS in patients with left BC (p=0.68). Conclusion: Left laterality in patients with pN3 non-metastatic BC is an independent prognostic factor associated with shorter TTM, increased risk of distant metastases and axial bone involvement compared with right laterality.