[Nodular fasciitis in children: a clinicopathological analysis of twenty-four cases].

Objective: To investigate the clinicopathological characteristics of nodular fasciitis (NF) in children. Methods: Twenty-four cases of pediatric NF (including four cases of cranial fasciitis and one case of intravascular fasciitis) diagnosed at the Department of Pathology of Henan Children's Hospital from January 2013 to February 2025 were collected. Their clinical features and histological characteristics were retrospectively analyzed. Immunophenotyping was performed using the EnVision method, USP6 gene rearrangement was detected by fluorescence in situ hybridization (FISH), and follow-up information was obtained. Results: There were 17 male and 7 female patients. The median age was 5.0 (2.0, 8.5) years (interquartile range, 2.0-8.5). Sixteen cases of NF were identified in the head and neck. Superficial masses presented as gradually enlarged or persistence lesions, while a mesenteric mass presented with abdominal pain. No definite history of trauma was reported. Microscopically, the lesions showed diverse morphologies. Most lesions were ill-defined. The tumor cells were spindle-shaped fibroblasts/myofibroblasts within a myxoid stroma, arranged in a loosely woven pattern, and accompanied by extravasated red blood cells. Nearly half of the cases showed prominent collagen proliferation. Other uncommon morphological features included densely packed tumor cells, arranged in a woven pattern, and stroma with abundant foamy cells, dense inflammatory infiltrate, or multinucleated giant cells. Immunohistochemical staining was positive for SMA (24/24), Caldesmon (23/24), and Calponin (22/24). FISH analysis for USP6 rearrangement was positive in 22 of 24 cases; one case of decalcified specimen showed no signals, and the single case of intravascular fasciitis was negative. Follow-up data were available for 20 cases, and none of the patients experienced recurrence after complete surgical excision. Conclusions: Nodular fasciitis in children predominantly affects the head and neck of preschool-aged children. The lesion exhibits diverse histological features, and demonstrates a high positive rate for USP6 rearrangement by FISH. The prognosis is excellent following complete surgical excision.