Desmoid fibromatosis tumors (DFs) are rare myofibroblastic proliferations notorious for local recurrence potential but lack of metastatic potential. Historically, literature has suggested that desmin expression is negative/infrequent in DFs. Prompted by recent observations, we conducted a study of desmin immunoreactivity in a cohort of molecularly confirmed cases of sporadic DFs to determine the frequency of desmin expression and any potential associations with the tumor's anatomical location and CTNNB1 mutation status.

52 cases of DFs retrieved from surgical pathology archives, including 22 extra-abdominal DFs, 18 intra-abdominal/mesenteric DFs, and 11 abdominal DFs. M:F ratio was 1:1. Immunohistochemical staining, and the status of beta-catenin (CTNNB1) mutations were reviewed. A blinded assessment of desmin expression was conducted, with semi-quantitative scoring based on intensity (0, negative; 1, mild; 2, moderate; 3, strong) and percentage of staining (<25%, 25-<50%, 50-<75%, >75%). Correlative statistical analysis was performed.

Desmin expression was seen in 41 out of 52 cases (78.8%). The expression ranged from moderate to strong positivity. 9 cases (17%), strong; 24 cases (46%), moderate; 8 cases (15%), moderate. 11 cases (21%) were negative across different types of DFs, including abdominal (n=3), intra-abdominal/mesenteric (n=2), and extra-abdominal (n=6).

CTNNB1 mutation status revealed T41A (n=26) and S45F (n=26). There were no statistically significant differences in desmin reactivity based on CTNNB1 mutation or anatomical site of the tumor.

We report an unexpectedly frequent expression of desmin in DFs, not previously reported in literature in different types of desmoid fibromatosis regardless of CTNNB1 mutation and tumor location. The prognostic implications remain to be determined and warrant further investigation.

Dr. O. Hans Iwenofu is an anatomic pathologist with subspecialty interest in soft tissue and bone pathology and currently serves as the chief of the soft tissue and bone pathology at the Ohio State University Medical Center. He has authored/coauthored more than 100 peer reviewed papers, mostly focused on diseases of bone and soft tissue. His clinical and research interests are focused on clinical diagnostics of soft tissue and bone tumors, biomarker discovery, molecular underpinnings of sarcomagenesis and translational studies in soft tissue and bone tumors.