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The optimal staging and treatment of the internal mammary nodes (IMNs) among patients with invasive breast cancer (IBC) is controversial. Although medial tumors have been reported to more commonly drain to IMNs, other variables predictive for IMN drainage may help identify those patients who may benefit from further IMN assessment. Factors associated with IMN drainage were analyzed among 141 patients who underwent lymphatic mapping and selective sentinel lymphadenectomy using intradermal injection (ID) or peritumoral (PT) injection. Fourteen of 83 patients (17%) receiving PT injections had IMN drainage, compared to none among the 58 patients who underwent ID injection alone (p = 0.0004). There were no differences in patient or tumor variables detected between the two groups. Among patients receiving PT injections, no factors examined were significantly associated with IMN drainage on univariate analysis. Using the multivariate logistic regression model, palpable disease was the most important factor associated with IMN drainage (risk ratio [RR] = 6.02; 95% confidence interval [CI] 0.64-56.34; p = 0.05). In addition, lymphatic/vascular invasion (LVI) and age less than 50 years were associated with IMN drainage (RR = 6.17; 95% CI 1.02-37.50; p = 0.09 and RR = 2.94; 95% CI 0.82-10.49; p = 0.09, respectively). IMN drainage occurred in a significant proportion of patients after PT injection, but not ID injection. In the final model, palpable disease was the most important factor associated with IMN drainage; LVI and age less than 50 years were of borderline significance. These factors may aid in the selection of patients who might benefit from further staging or treatment of the IMNs. [ABSTRACT FROM AUTHOR]