A 25-year-old woman presented with a palpable breast mass that was excised. Hematoxylin and eosin and immunohistochemical-stained slides of the mass are shown in the figures below.
The correct answer is ...
The histologic sections in this case confirm a monomorphic spindle cell proliferation with a storiform architecture. While submitted as a “breast biopsy,” the lesion is centered in the subcutis, and no breast epithelium proper is present. The nuclei are uniform with minimal cytologic atypia, and mitotic activity is low. Infiltration and entrapment of adipose tissue in a honeycombing pattern is well-demonstrated.
By immunohistochemistry (IHC), the neoplastic cells are diffusely positive with CD34, while negative with STAT6. The overall morphology and immunophenotype are most consistent with dermatofibrosarcoma protuberans (DFSP).
DFSP is an uncommon and locally aggressive neoplasm that is most commonly encountered in the dermis and subcutaneous tissues of the skin. Rarely, DFSP may present as a deeper-seated lesion. It is characterized by diffuse infiltration with a honeycombing pattern of entrapment of adipose tissue. The neoplastic cells are uniform with plump to elongated nuclei and with a whorled storiform histoarchitecture.1,2 Fibrosarcomatous differentiation can infrequently occur, with a more nodular and fascicular appearance of cells with cytologic atypia and increased mitotic activity.3
By IHC, DFSP is strongly and diffusely positive for CD34, a reliably sensitive but not specific marker.2 It is most frequently characterized by a t(17;22)(q22;q13) COL1A1-PDGFB fusion. Rare cases may be negative for PDGFB rearrangements by fluorescence in situ hybridization (FISH), as in this case. They may instead harbor fusions involving PDGFD, which has been observed in some cases of DFSP occurring in the breast.4
Primary sarcomas, especially other than phyllodes tumors, of the breast are exceedingly rare, comprising <0.1% of all breast malignancies. Nonetheless, sarcomas may be encountered in both breast biopsies and excisions. Ancillary testing may be necessary to rule out the differential diagnosis of metaplastic (spindle cell) carcinoma. The subcutaneous/dermal location of the lesion in this case, combined with histologic features and IHC results, allowed for a specific diagnosis of DFSP. While locally aggressive and prone to local recurrence (up to 60%), distant metastases from DFSP are rare.2,3
Kaitlyn Nielson, M.D.
Fellow, Surgical Pathology
Charles Sturgis, M.D.
Consultant, Anatomic Pathology
Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science