Mucinous carcinoma of the skin, primary, and secondary: a clinicopathologic study
of 63 cases with emphasis on the morphologic spectrum of primary cutaneous forms:
homologies with mucinous lesions in the breast.Am J Surg Pathol. 2005 Jun;29(6):764-82.

We present the largest series of mucinous carcinoma involving the skin,
describing the histopathologic, immunohistochemical, electron microscopic, and
cytogenetic findings. Our aim was fully to characterize the clinicopathologic
spectrum and compare it with that seen in the breast. In addition, we wished to
reevaluate the differential diagnostic criteria for distinguishing primary
mucinous carcinomas from histologically similar neoplasms involving the skin
secondarily, and study some aspects of their pathogenesis. We demonstrate that
primary cutaneous mucinous carcinomas span a morphologic spectrum compatible to
their mammary counterparts. Both pure and mixed types can be delineated
morphologically, and some lesions have mucocele-like configurations. Most lesions
seem to originate from in situ lesions that may represent, using mammary
pathology terminology, ductal hyperplasia, atypical ductal hyperplasia, or ductal
carcinoma in situ or a combination of the three. Inverse cell polarity appears to
facilitate the progression of the changes similar to lesions in the breast. The
presence of an in situ component defines the neoplasm as primary cutaneous, but
its absence does not exclude the diagnosis; although for such neoplasms, full
clinical assessment is essential. Mammary mucinous carcinoma involving the skin: 
all patients presented with lesions on chest wall, breast, axilla, and these
locations can serve as clue to the breast origin. Microscopically, cutaneous
lesions were of both pure and mixed type, and this correlated with the primary in
the breast. Dirty necrosis was a constant histologic finding in intestine
mucinous carcinomas involving the skin, and this feature may serve as a clue to
an intestinal origin.

Primary cutaneous mucinous carcinoma: presence of myoepithelial cells as a clue
to the cutaneous origin.Am J Dermatopathol. 2004 Oct;26(5):353-8.

BACKGROUND: Primary cutaneous mucinous carcinoma (PCMC) is a rare malignancy with probable apocrine differentiation. It is important to differentiate it from metastatic mucinous carcinoma (MMC), especially from the breast. The histologic and immunohistochemical features overlap between PCMC and breast mucinous carcinomas. In this study, we introduce the presence of myoepithelial component in PCMC as a new morphologic parameter to distinguish it from MMC from either breast or sites elsewhere in the body. MATERIALS AND METHODS: We studied 7 cases of PCMC. The possible in situ component in the tumor was assessed by the presence of a peripheral myoepithelial cell layer. Myoepithelial cell differentiation was confirmed with immunohistochemical stains for p63, CK 5/6, calponin, smooth muscle actin (SMA), HHF-35, and CD10. Estrogen and progesterone receptor (ER/PR), gross cystic disease fluid protein (GCDFP 15), CK7, CK20, and S-100 immunostains were also performed. RESULTS: Histologically, multiple small monomorphic epithelial islands floating in multilocular pools of mucin characterized the tumor. Focally, epithelial islands were bordered by dermal connective tissue at the periphery of mucin pools. Secretory snouts were apparent in all cases providing evidence for apocrine differentiation. In 5 of the 7 cases, an in situ component was identified as epithelial islands being bounded by a myoepithelial layer, which was highlighted by p63, CK 5/6, calponin, SMA, and HHF-35. ER/PR and CK7 were positive in all the cases. GCDFP-15 and CD10 were focally positive in the tumor cells and myoepithelial cells, respectively. All 7 cases were negative for S-100 and CK 20. CONCLUSION: We conclude that an in situ component is frequently present in PCMC (5/7) and may help in distinguishing this entity from MMC, especially of breast origin. Furthermore, it may provide insight into the pathogenetic mechanism of mucinous carcinoma evolving from in situ carcinoma with luminal mucinous distention to cellular tumor with a little surrounding mucin.

Secondary mucinous carcinoma of the skin: metastatic breast cancer. Dermatol Surg. 2004 Feb;30(2 Pt 1):234-5

BACKGROUND: Breast cancer is the most common cancer in women. Its involvement of skin is the most frequent of visceral cancers in women. In cutaneous metastatic disease, including breast cancer, the clinical and histologic pattern may be specific or nonspecific. Specific clinical patterns of cutaneous metastatic disease are linked with breast cancer but occur less often with other cancers metastatic to skin. Likewise, specific histologic patterns of cutaneous metastatic disease are linked with breast cancer but occur less often with other cancers metastatic to skin. OBJECTIVE: To present a case of a mucinous breast cancer metastatic to skin where the histologic pattern is similar to the primary tumor. METHODS: This is a case report and a literature review. RESULTS: Metastatic breast cancer may rarely resemble primary skin cancer, in this case primary mucinous carcinoma of the skin. We describe a 60-year-old woman with breast cancer with the incidental finding of a nonspecific, soft, solitary nodule on her back. It was found to contain mucinous material and on close examination was found to be a metastatic mucinous carcinoma of the skin from a primary adenocarcinoma of the breast. CONCLUSION: One usually considers that hard, firm nodules are more suggestive of cutaneous metastatic disease than soft, nondescript ones, but one should be careful to consider secondary mucinous carcinoma of the skin and a histologically similar solitary cutaneous metastasis.

Immunohistochemical analysis of cytokeratin and human milk fat globulin expression in mucinous carcinoma of the skin. J Cutan Pathol.  2002 Jan;29(1):38-43.

BACKGROUND: Mucinous carcinoma of the skin (MCS) is a rare epithelial tumor which arises primarily in the skin. Metastatic MC from extracutaneous sites, especially breast or colon, mimics MCS and cannot be differentiated from MCS by routine histology alone. METHODS: Nine cases of MCS were analyzed immunohistochemically using monoclonal antibodies against cytokeratins (CKs) and human milk fat globulin 1 (HMFG) in order to clarify their nature and compare the immunophenotypes with those of other MCs studied in the literature. RESULTS: Expression of simple epithelial CKs in most of the tumor cells of all cases studied and co-expression of simple and stratified epithelial CKs in some tumor cells of two cases were recognized. CK 20 expression could not detected in any tumor cells. Focal HMFG expression in the luminal or outer surface of the nests was observed in three cases. CONCLUSION: From CKs expression, MCS was speculated to differentiated mainly toward the secretory cells of the sweat glands, and some tumor cells toward the transient portion between the dermal duct and the secretory portion. Focal HMFG expression suggested either a consequence of malignant transformation or apocrine differentiation. No expression of CK 20 in MCS suggests that we may exclude the diagnosis of metastatic colorectal MC which expressed CK 20.

Primary mucinous carcinoma of the skin. A case report:  Ann Pathol. 2006 Jun;26(3):211-4.

Primary mucinous carcinoma of the skin is a rare sweat-gland neoplasm with a high recurrence rate. We report a new case of a primary recurrent mucinous carcinoma of the face in a 59-year-old man. Histopathologic examination of the neoplasm showed epithelial islands floating in mucoid material compartmentalized by fibrous septa. Cytokeratin 7, protein S100, estrogen and progesterone receptors were detected at immunohistochemical study, while cytokeratin 20 and actin were undetectable. Histologically, mucinous carcinoma of the skin can be mistaken for a metastasis from extracutaneous sites, particularly the breast or the gastrointestinal tract. Mucinous carcinoma of the skin has a relatively good prognosis with rare distant metastases, but high recurrence rate.