A diagnostic dilemma between epithelioid leiomyoma and PEComa in the right retroperitoneum: A case report of risk-adapted management with long-term recurrence-free follow-up

RATIONALE: The accurate classification of retroperitoneal neoplasms remains challenging, particularly when distinguishing between epithelioid leiomyoma and perivascular epitheloid cell tumor (PEComa) due to significant histological and immunohistochemical overlap. This distinction carries profound therapeutic implications, as leiomyomas are benign while PEComas possess malignant potential. The diagnostic dilemma is compounded by the documented phenomenon of aberrant HMB45 expression in otherwise conventional leiomyomas. PATIENT CONCERNS: A 47-year-old woman presented with an incidentally discovered right retroperitoneal mass. DIAGNOSES: Following complete surgical resection, pathological examination revealed a tumor composed of uniform epithelioid cells with minimal atypia, exceptionally low mitotic activity (1/entire specimen), and low Ki-67 index (2%). Immunohistochemistry showed strong diffuse smooth muscle marker expression (Desmin, h-Caldesmon) but focal HMB45 positivity. Despite this atypical immunophenotype, the diagnosis favored epithelioid leiomyoma based on predominant benign features. INTERVENTIONS: A multidisciplinary team recommended against adjuvant therapy, implementing instead a structured surveillance protocol. OUTCOMES: The patient remains disease-free after > 4 years of follow-up. LESSONS: This case suggests that, for carefully selected patients with retroperitoneal tumors exhibiting focal HMB45 expression yet predominant benign histological features, a risk-adapted management strategy of complete surgical resection followed by active surveillance may represent a feasible and safe approach. The favorable long-term outcome provides supportive evidence that prioritizing biological behavior over diagnostic nomenclature alone can avoid overtreatment while maintaining oncologic safety. It should be acknowledged that, in the absence of confirmatory molecular studies, the favored diagnosis of epithelioid leiomyoma cannot be considered definitive and PEComa remains a differential consideration. This strategy may serve as a reference for managing similar lesions in the diagnostic gray zone, though larger cohort studies are required to further validate this approach.