Subcutaneous Dermatofibroma: A Rare Presentation of a Common Lesion

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Dermatofibroma (DF) is a prevalent mesenchymal skin growth that typically involves the dermis layer, with the overlying epidermis often exhibiting hyperplasia. While instances of DF extending into the subcutaneous tissue are not uncommon, pure subcutaneous DF is a rarity. This review presents the case of a 41-year-old male with a painless, firm subcutaneous papule on his left thigh, which was diagnosed as a subcutaneous DF upon histopathological examination after complete removal.

Characteristics and Prevalence

DF is the most frequently encountered mesenchymal skin lesion, particularly among young women in their early twenties with a predisposition for the lower legs. The benign form of DF is usually limited to the dermis, but pure subcutaneous occurrences are seldom reported, with the WHO noting that less than 1% of all benign fibrous histiocytomas are found in a deep location.

Immunohistochemical Findings

The case discussed demonstrated strong immunohistochemical positivity for factor XIIIa and negativity for CD34, desmin, and CD68 in the spindle cells. Recognizing the pure subcutaneous variant of DF is crucial due to its potential confusion with other subcutaneous tumors, both benign and malignant.

Differentiating from Other Tumors

Deep penetrating dermatofibroma (DPDF), a variant extending into the subcutaneous tissue, was differentiated from dermatofibrosarcoma protuberans (DFSP) based on histologic criteria. DPDF typically shows a regular, smooth demarcation pattern, unlike the irregular patterns of DFSP. Factor XIIIa is usually positive in DPDF and negative in DFSP, aiding in differentiation.

Malignant fibrohistiocytic neoplasms, which require careful distinction from benign neoplasms, are characterized by pleomorphic sarcomatous cells and frequent mitotic activity. However, histological features are less predictive of biological behavior in these cases.

Diagnosis and Treatment

Diagnosis often relies on electron microscopy and immunohistochemistry. DFSP, which occurs in the dermis and subcutis, is distinguished by a more uniform cellular population and a storiform pattern, in contrast to the well-defined margins and short curlicue fascicles of DF. Immunostaining can reveal the cellular composition differences, with DFSP expressing CD34 in a significant portion of neoplastic cells.

Treatment for DF is generally not required unless symptomatic. Most subcutaneous DF cases are managed by local excision, as was the case with the patient in this review, who underwent removal during a biopsy incision.

Prognosis and Recurrence

DF is typically benign, with a low recurrence rate of less than 2%. However, certain variants and locations are associated with higher recurrence rates, suggesting that complete local excision is advisable for subcutaneous DF.

In conclusion, subcutaneous DF, while rare, is an important variant to recognize and differentiate from other subcutaneous lesions. Accurate diagnosis and appropriate management are essential to ensure the best patient outcomes.