Primary Mucinous Adenocarcinoma: The Critical Role of Morphology in a Rare Neoplasm

Objectives Primary mucinous adenocarcinoma of the skin is a rare clinical entity derived primarily from sweat glands with reported sites mainly arising from the head and neck region. Difficulty then arises with distinguishing between these primary neoplasms and those with similar morphology in the breast and/or gastrointestinal tract. Presentation of Case We report a case of an 85-year-old female with a past medical history of endometrial cancer presenting to clinic with a slow-growing right temporal lesion that was excised and diagnosed as primary mucinous adenocarcinoma. Further clinical workup and imaging revealed an oral lesion diagnosed as squamous cell carcinoma after excision along with an enhancing right breast mass with biopsy and excision showing an invasive ductal carcinoma with papillary and micropapillary features. This unusual presentation of more than two malignancies provides a challenge to differentiate between primary versus metastatic disease, especially with a rare malignancy that is present. Discussion A review of the literature on primary mucinous adenocarcinoma of the skin is presented, discussing epidemiology, clinical presentation, morphology, and immunohistochemistry sets historically studied to aid in diagnosis, with our case relying on morphology to finalize our diagnosis. Conclusion This unusual case highlights the difficulty in differentiating between primary versus metastatic mucinous adenocarcinoma yet highlights the value of morphology to aid in arriving at a diagnosis to help guide correct clinical treatment and follow-up.