Case 3B -

Invasive micropapillary carcinoma

History: This is a 2.3 cm breast mass from a 61 year old woman. Nine axillary lymph nodes were positive for metastatic carcinoma. How would you classify this lesion? Case 3B - Figure 1 Case 3B - Figure 2 Diagnosis: Invasive micropapillary carcinoma. Discussion: Tavassoli et al. in 1993, brought attention to breast carcinomas having a distinctive growth pattern characterized by small individual clusters of cohesive tumor cells within distinctive well demarcated, rounded clear spaces. These spaces are quite numerous, imparting a "spongy" appearance at low magnification that distinguishes these lesions from retraction artifacts within collagenous stroma. The cytology of these tumors resembles serous ovarian carcinomas. Their unusual growth pattern combined with the characteristic architecture of the tumor cells prompted use of the term "invasive micropapillary". They are uncommon, but not rare lesions, accounting for 2-3% of primary breast carcinomas. The clear spaces bear a strong resemblance to foci of angioinvasion, however the spindle cells that line these spaces are not endothelial. Peterson has shown that the clusters of tumor cells are characterized by reversed polarity, such that the apical (secretory) pole of the cells is directed toward the outside of the cell groups (that is, not into a lumen), thus accounting for the microscopic findings. Although the relationship to histologic growth pattern is unknown, invasive micropapillary carcinomas are highly lymphotropic; over 80% are node positive, most with multiple metastases (average: 3-5 lymph nodes). It is noteworthy that the tumor cells in foci of angioinvasion (i.e. in conventional ductal carcinomas) also have reversed polarity; perhaps this feature is a requirement for vascular invasion and the dominance of this phenotype explains the peculiar lymphotropism of IMPCa. Due to the relatively small number of published series, it has been difficult to define the clinical behavior of these lesions. Studies published to date suggest that patients with invasive micropapillary carcinomas have an outcome similar to those with ductal carcinomas of similar stage. They do not, in other words, behave like inflammatory carcinomas, which are characterized by a very high frequency of systemic metastasis. Unlike inflammatory carcinoma, moreover, they do not present with cutaneous erythema/induration and do not have dermal angiolymphatic emboli. Hence, the importance of recognizing IMPCa lies in recognition of their unusually lymphotropic character. References: Nassar H, Wallis T, Andea A, et al. Clinicopathologic analysis of invasive micropapillary differentiation in breast carcinoma. Mod Pathol 2001; 14(9):836-841. Siriaunkgul S, Tavassoli FA. Invasive micropapillary carcinoma of the breast. Mod Pathol 1993; 6:660-662. Luna-More S, Gonzalez B, Acedo C, et al. Invasive micropapillary carcinoma of the breast. A new special type of invasive mammary carcinoma. Pathol Res Pract 1994; 190:668-674. Peterson JL. Breast carcinomas with unexpected inside out growth pattern. Rotation of polarization associated with angioinvasion. Pathol Res Pract 1993; 189:780A.