Diagnostically challenging spindle cell lipomas: a report of 34 "low-fat" and "fat-free" variants.

Spindle cell lipomas (SCL) classically occur as subcutaneous masses in the upper trunk/neck of older men and are composed of mature fat, CD34-positive spindled cells, ropey collagen, myxoid matrix, and blood vessels. A number of variants have been reported, including SCL with pseudoangiomatous change, composite SCL hibernoma, and composite SCL/pleomorphic lipoma. A review of over 300 consultation cases diagnosed as SCL revealed 34 cases in which fat was noted to be present in <5% of the tumor (n = 30) or absent (n = 4). These cases posed diagnostic difficulties because of the dearth of fat; we propose the terms "low-fat" and "fat-free" SCL for these variants. The tumors presented in older men (mean, 56 years; ratio of males to females, 11:1) and presented as small (mean, 2.0 cm) circumscribed dermal or subcutaneous masses of the head/neck (n = 18), back (n = 7), shoulder (n = 5), leg (n = 2), arm (n = 1), or unknown location (n = 1). In the majority, referring pathologists considered benign diagnoses, usually benign nerve sheath tumors, but in four cases low-grade sarcoma was considered. In only three cases was SCL considered. The tumors were composed of aggregates of CD34-positive, bland spindled cells arranged in characteristic parallel arrays, admixed with ropey collagen and myxoid matrix. Isolated clusters or single adipocytes were present in 30 cases; four were devoid of fat. CD34 was diffusely positive (10/11). A high index of suspicion based on clinical context and identification of other typical features of SCL are key features to the diagnosis of low-fat and fat-free SCL.