Unlike histological counterparts of the ovary and pancreas, MCA of the breast is extremely rare and was first described in 1998 by Koenig and Tavassoli [2].
All the cases of MCA breast have been reported in postmenopausal women with a median age of 61 years (range 41–96 years) and our patient is also a postmenopausal 65 years old [2,3,4,5,6,7,8,9,10,11,12]. Tumor size in our case was very large being 18cm while rest reported size up to 10cm with only a single case of 19cm [2]. All cases described histomorphology similar to that seen in our case. Three cases have reported focal squamous cell carcinoma differentiation and one case high-grade sarcomatoid components [2,3,4,5,6,7,8,9,10,11,12]. MCA of the breast needs to be differentiated from other mucin-producing tumors of the breast. Mucinous carcinoma of the breast does not form cystic structures rather only clusters of epithelial cells suspended in extracellular mucin. Nuclear grade in mucinous carcinoma is low to intermediate while high-grade nuclei have been reported in MCA breast. Mucinous carcinoma of the breast is strongly and diffusely positive for ER and PR positive while MCA is negative [1]. Non-neoplastic mucocele-like lesions show distended mucin-filled ducts but the presence of myopeithelial cells and lack of nuclear atypia helps in the diagnosis [1]. MCA also needs to be distinguished from another rare cystic hypersecretory carcinoma of the breast which also shows multiple variable-sized cystic spaces but contains colloid-like eosinophilic material that often retracts from the epithelium and no intracellular mucin is seen.
MCA of the breast is rare and shares same morphology with ovarian, pancreatic, and/or appendiceal counterparts. Hence, for the diagnosis of primary breast MCA, the ovary, pancreas, and gastrointestinal tract should be excluded first by combing clinical, radiological, and IHC features. The breast is not a common and early metastatic site of MCA in the ovary or GIT, and the patient should have clinical manifestations of the primary lesion and other metastatic sites before the metastatic lesion appears in the breast in most cases. Incidence of metastatic MCA in the breast as the first sign of presentation is extremely rare. The presence of DCIS favors primary over metastasis.
IHC is a powerful tool to distinguish, although a combination of CK7 and CK20 is useful as both primary pancreatic and ovarian MCA are positive for both while breast MCA is negative for CK20. But our case showed focal CK20 positivity and Chen et al. also reported the same [8]. So it is essential to add lineage markers like CDX2, PAX8, GATA3, and GCDFP in the panel.
Most MCA of the breast are triple negative being negative for ER, PR, and HER2. Only 4 cases including the present case were positive for HER2 and FISH was also positive for HER2 [9,10,11]. Rakici et al. have reported ER positivity [12]. Proliferative index Ki67 has been reported in a wide range of 20.5 to 90% [2,3,4,5,6,7,8,9,10,11,12,13]. We also reported a high Ki67 index of 90%. Kim et al. reported MUC 5 positivity in intracellular mucin while tumor cells were positive for MUC1 and MUC5 while negative for MUC2 and MUC6 [13]. In our case, tumor cells were only positive for MUC1 while negative for MUC2 and MUC5. Only occasional cells in our case showed intracellular mucin.
The risk of lymph node metastasis is low with apart from the current case, only 4 cases have reported ipsilateral axillary lymph node metastasis and a single case of isolated tumor cells (ITC) [2, 4,5,6,7]. No metastasis have been reported in the literature but our case had radiological evidence of lung metastasis at the time of presentation.
Due to the limited number of cases, treatment strategies are still being devised. Most of the previously reported cases underwent partial or radical mastectomy, followed by chemotherapy and radiotherapy. Our patient is also on platinum-based chemotherapy now. Follow-up results ranging from 3 to 46 months indicate that the good prognosis in patients with MCA [3].
