Low‐grade fibromyxoid sarcoma: A report of the Cooperative Weichteilsarkom Studiengruppe (CWS)

Background Low‐grade fibromyxoid sarcoma (LGFMS) is a rare soft‐tissue tumor with benign histologic appearance, though fully malignant behavior is possible. Methods Patients with LGFMS <21 years registered in Cooperative Weichteilsarkom Studiengruppe trials until 2017 were analyzed. Firstline treatment consisted of complete surgical resection whenever possible. Results Median age of 31 patients was 10.9 years (first month to 17.1 years). Twenty‐six tumors were confirmed to the tissue of origin (T1), four invaded contiguous structures (T2), one was TX. Eight were >5 cm. The best surgical result was resection with free margins (R0) in 24 and microscopic residuals (R1) in seven. Five‐year event‐free (EFS), 5‐year local‐relapse‐free (LRFS), and 5‐year overall‐survival were 71 ± 18.6% confidence interval (CI) 95%, 76 ± 17.6% CI 95%, and 100%, respectively. Six patients suffered local relapse in a median of 1 year, one combined within 1.3 year and one metastatic relapse with lesions in the lung, back muscles, and thigh discovered in whole‐body imaging 6 years after the first diagnosis. In univariate analysis, T status correlated with EFS (T1 79.6 ± 18.6%, T2 50.0 ± 49.0%, P = .038). Resection with free margins tends to be associated with better LRFS (R0 82.4 ± 18.6%, R1 53.6 ± 39.4%, P = .053). Among 24 patients with R0 resection, five (21%) suffered relapse, thereof three local, one metastatic, and one combined. Among seven patients with R1‐resection, three (43%) suffered local relapse. Conclusion Special caution is advisable in T2 tumors. The metastatic potential with lesions in unusual sites indicates that affected patients need to be informed. If long‐term follow‐up with whole‐body imaging is beneficial, it may be addressed in larger intergroup analyses. Further research in disease biology is essential for optimal treatment and follow‐up care.