Recognizing the Benign Behind Worrisome Histology: A Case Report of Proliferative Fasciitis

Background and Clinical Significance: Proliferative fasciitis (PF) is an infrequent benign fibroblastic/myofibroblastic proliferation that may closely resemble a soft tissue sarcoma, creating a diagnostic dilemma out of proportion to its biological behaviour. Because no single clinical, histological or immunohistochemical feature is diagnostic, accurate classification depends on the integration of complementary findings. We describe a challenging case of PF involving the lower leg and present a practical clinicopathological approach to its evaluation. Case Presentation: A 34-year-old man presented with a painless subcutaneous nodule on the lateral aspect of the left lower leg, discovered incidentally. Clinical examination suggested a benign superficial soft-tissue lesion, and because no features raised suspicion for malignancy, complete excision was performed without preoperative imaging. Gross examination revealed a 1.9 × 1.6 × 0.7 cm fascial-based lesion composed of spindle cells and scattered ganglion-like cells within a variably myxoid stroma. Focal nuclear pleomorphism, typical mitotic activity (2 mitoses/10 high-power fields), and limited extension into adjacent adipose tissue broadened the differential diagnosis. Immunohistochemistry demonstrated focal SMA positivity, weak focal desmin and S100 expression, absence of CD31 and CD34 staining, and a low Ki-67 proliferative index (approximately 2–3%). Negative surgical margins, together with integration of the clinical presentation, gross findings, histomorphology, and immunophenotype, supported the diagnosis of proliferative fasciitis. The patient remains free of local recurrence four months after surgery. Conclusions: PF should be considered in the differential diagnosis of superficial spindle-cell proliferations showing deceptively aggressive histological features. Careful clinicopathological correlation remains the cornerstone of diagnosis and helps distinguish this benign entity from its malignant mimics. The clinicopathological framework proposed in this report may assist pathologists in the systematic evaluation of similar diagnostically challenging lesions.