What Is the Prevalence and Fate of Myxoid Soft-Tissue Tumors With an Indeterminate Diagnosis Prior to Resection?

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Introduction: Differentiating benign and malignant myxoid soft-tissue tumors preoperatively can be challenging due to shared clinical, imaging, and histologic features. Biopsy specimens are frequently described as “low-grade myxoid neoplasms” without benign or malignant classification (ie, indeterminate). Our purpose is to evaluate the prevalence and prognosis of myxoid tumors with an indeterminate diagnosis before resection. Methods: A retrospective review identified 439 patients with a musculoskeletal myxoid soft-tissue tumor confirmed on final resection pathology. Biopsy result (benign, malignant, indeterminate) for each biopsy type was compared with the final resection diagnosis. Indeterminate diagnosis before resection was defined as when the preoperative diagnosis was uncertain to be benign or malignant based on all biopsy data. For patients with sarcomas on final resection and 2-year follow-up, the prevalence of positive margins and local recurrence were compared between patients with an indeterminate diagnosis before resection and those with a malignant diagnosis known preoperatively. Results: The prevalence of indeterminate diagnosis before resection was 28% (66/235). In these patients with sarcomas on final resection and 2-year follow-up, the positive margin rate at resection surgery was 37% (10/27) compared with 15% (11/74) in patients with a malignant diagnosis known preoperatively (P = 0.049). Their 5-year local recurrence–free survival was 73% (95% confidence interval, 58 to 92) compared with 92% (95% confidence interval, 86 to 98) in patients with a malignant diagnosis known preoperatively (P = 0.022). Conclusion: Indeterminate diagnosis before resection in myxoid tumors is relatively common, compared with prior reports in nonmyxoid neoplasms. Patients with indeterminate diagnosis before resection had two times higher rate of positive margins at resection surgery and markedly shorter local recurrence–free survival at 5 years compared with patients with malignant diagnosis known preoperatively. Diagnostic uncertainty at the time of resection is associated with worse oncologic outcomes in myxoid tumors.