A Case Report and Literature Review on Malignant Behavior in a "Benign" Deep Fibrous Histiocytoma of the Lower Limb: Pulmonary and Cutaneous (Scalp) Metastases Despite R0 Amputation

Abstract

Background: Deep fibrous histiocytoma (deep FH), sometimes still referred to as "deep benign fibrous histiocytoma", is a rare fibrohistiocytic neoplasm of the subcutaneous and deep soft tissues that is formally regarded as histologically benign. Unlike its common cutaneous counterpart, dermatofibroma, deep FH can clinically and radiologically mimic a sarcoma, and a small subset of reported cases have demonstrated local recurrence or, exceptionally, frankly malignant behaviour with distant metastasis. Case Presentation: We report the case of a 43-year-old woman who presented with a one-year history of exertional pain and a rapidly enlarging, hypervascular mass in the right popliteal fossa/distal thigh. An external core biopsy suggested deep FH; however, the lesion's clinical and radiological aggressiveness (rapid growth, large size, and hypervascularity adjacent to the popliteal vessels) raised multidisciplinary concern for sarcoma and led to a right transfemoral amputation that achieved an R0 resection, with tumour-free bone and soft-tissue margins. A comprehensive exclusionary immunohistochemical panel confirmed deep FH. Despite the R0 resection and adjuvant doxorubicin-based chemotherapy, the patient developed pulmonary and cutaneous (scalp) metastases approximately 12 months later, with an immunophenotype identical to that of the primary tumour, illustrating frankly malignant clinical behaviour arising from a histologically benign tumour. Conclusion: This case illustrates that deep FH, although formally classified as benign, can behave in a locally aggressive and, rarely, frankly metastatic manner. Clinical and radiological aggressiveness should not be discounted on the basis of a reassuring histological label, and long-term oncological surveillance is warranted even after apparently curative resection.