Multifocal Invasive Ductal Cancer: Distinguishing Independent Tumor Foci From Multiple Satellites.

Patients with multifocal breast cancers (MBCs) have a poorer prognosis than patients with unifocal breast cancers. Studies have attributed this to tumor size underestimation in MBC. An alternative hypothesis is that some MBCs behave in a fashion analogous to the "satellite" and "in-transit metastasis" observed in melanoma and, thereby, are more clinically aggressive. We identified 79 cases of MBC, which we classified into 2 groups: study cases defined as ≥2 morphologically similar tumor foci with ≥1 focus without in situ carcinoma (n = 21); and a control group defined as ≥2 morphologically similar or dissimilar foci with associated in situ carcinoma in all foci (n = 58). The odds of being a study case is 1.86 (95% confidence interval [CI] 1.26-2.74) times greater per unit increase in number of tumor foci (median of 4 tumor foci; P = .002). Study cases were 73.33 (95% CI = 8.91-603.16) times more likely to have lymphovascular invasion (LVI) and 14.72 (95% CI = 4.37-49.61) times more likely to have nodal metastases. Grade I/II tumors were 0.20 (95% CI = 0.07-0.59) times less likely to be study cases. There was a significant positive interaction ( P < 0.001) indicated by the relationship of LVI status and nodal status with the study case and control group. We conclude that there is a subset of MBC that presents with more numerous tumor foci and a higher rate of nodal metastasis. The aggressive behavior of these cases may be attributed to their proclivity for LVI.