A Rare Case of Crystal-Storing Histiocytosis of the Breast in a Patient With Undifferentiated Connective Tissue Disease.

Crystal-storing histiocytosis (CSH) is a rare entity usually presenting microscopically as sheets of histiocytes with intralysosomal accumulation of refractile, crystalline substances. CSH is most often the result of monoclonal immunoglobulin light chain deposition in the setting of a plasma cell neoplasm or lymphoma with plasma cell differentiation. However, this condition may arise in association with autoimmune, inflammatory, infectious, and drug-related etiologies and can harbor polyclonal immunoglobulins or other crystalline materials. It usually presents in a localized fashion, with the bone marrow being the most frequent site, but may also involve multiple anatomical sites. CSH is typically identified incidentally without clinical suspicion and can be challenging to detect histologically. However, in rare cases, it may be the sole presenting sign in an undiagnosed malignancy or immune disorder. Here, we describe an unusual case of CSH, occurring as an isolated breast mass in association with an undifferentiated connective tissue disease (UCTD) with inflammatory arthritis, features of Sjögren's (Sicca) syndrome, and subsequent laboratory findings concerning for antiphospholipid syndrome.