Case 378 -- A young woman with an unusual "cyst"

Contributed by Amy Davis, MD and Drazen Jukic, MD, PhD
Published online February 2004


The patient is a Caucasian female in her 30's who is followed closely by her dermatologist after an in-situ melanoma was diagnosed on her lower extremity approximately 2 years prior. A relatively large scalp nodule was bothersome to the patient despite the fact that it had been present for several years. Clinically, the nodule was firm and indurated and measured 2 cm in greatest dimension. The lesion was thought to be a cyst and was not of serious clinical concern. It was "shelled-out" and submitted for pathologic examination with the pre-operative differential diagnosis of epidermal inclusion cyst or pilar cyst.


The specimen consists of multiple pieces of light brown tissue, measuring 0.8 x 0.5 x 0.2 cm in aggregate. Since the lesion was "shelled-out," no epidermis is included.

H & E:

Low power histologic evaluation reveals irregular fragments of dermis and subcutaneous tissue with no overlying epidermis (images 1 & 2). There is a diffuse proliferation of spindle cells in a fibrotic and collagenous background (image 3). The spindle cells splay around collagen bundles (image 4) and infiltrate adipose tissue (image 5). Focal areas of more marked cellularity are noted (image 6), but no pattern is evident.

On high power, the majority of the spindle cells are relatively bland, and despite close examination, only 1- 2 possible mitoses are identified in the entire lesion. Of concern however are the smaller numbers of slightly larger pleomorphic spindle cells (image 7). At this point, the diffuse infiltration of reticular dermis and subcutis by spindle cells with mild cytologic atypia in a middle-aged adult with a firm scalp nodule strongly suggests a diagnosis of dermatofibrosarcoma protuberans (DFSP), and the immunohistochemical stain CD-34 is ordered to confirm the diagnosis. Also ordered is Factor XIIIa that should be positive in dermatofibroma (benign fibrous histiocytoma). As always, when dealing with a dermal spindle cell neoplasm, melanocytic origin must be ruled out.

Continued high-power examination reveals very few plump cells with pale blue-grey cytoplasm, some with prominent nucleoli and intranuclear pseudo-inclusions (image 8). Fine brown pigment is seen on close inspection. A panel of melanocytic markers including S100, Melan-A, tyrosinase, and HMB-45 is also ordered. Vimentin is added as well as Fontana-Masson and iron stains to determine the identity of the pigment.


At first glance, CD-34 has a brown tint and appears positive, however on closer inspection, it is clear that only vessels are staining along with faint background positivity (image 9). The spindle cells and larger more epithelioid cells are negative. Factor XIIIa is negative. Vimentin is strongly and diffusely positive. Results of the melanocytic panel are surprising. S100 is focally weakly positive with only a few cells demonstrating the characteristic nuclear and cytoplasmic staining (images 10 & 11). Repeat S100 reveals similar results. In contrast, Melan-A (images 12 & 13), tyrosinase (images 14 & 15), and HMB-45 (images 16 & 17) are strongly and diffusely positive. Dusty black staining is seen throughout with Fontana-Masson stain (image 18), confirming the identity of the pigment as melanin. The iron stain is negative.


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