CTNNB1 mutations play important roles in the development of soft tissue tumors, such as desmoid fibromatosis (DF), sinonasal tract angiofibroma, sinonasal glomangiopericytoma, intranodal palisaded myofibroblastoma, neuromuscular choristoma (NMC), and the recently reported pseudoendocrine sarcoma. Here, we report a unique hybrid soft tissue tumor with classic DF, unusual epithelioid component, and NMC in a 23-year-old female. The classic DF and NMC and the unusual epithelioid component and NMC were locally intermixed and closely related to each other. Immunohistochemically, the DF, unusual epithelioid component, and NMC exhibited nuclear positivity for β-catenin to varying degrees. More critically, all of the above components harbored identical CTNNB1 p.Ser45Pro missense mutations. To the best of our knowledge, this is the only reported CTNNB1 mutation-driven hybrid tumor with DF, unusual epithelioid component, and NMC. The present case further confirmed that CTNNB1-mutational soft tissue tumors are highly heterogeneous, but the morphological spectrum is wide and consecutive.

BACKGROUND: Neuromuscular choristomas (NMCs), are extremely rare developmental lesions that, have been previously established associated with recurrent fibromatosis after surgery, leading to several operations or even amputation. However, reports on the ultrasound imaging features and clinical conditions of NMCs are rare. The purpose of this study is to describe the ultrasound features and clinical analysis of NMCs to provide suggestions to identify the optimal management strategy.
METHODS: From September 2020 to September 2021, 7 patients with a confirmed diagnosis of NMC who underwent ultrasound examination in our department were enrolled in our study. Physical examinations were performed to detect motor deficits, sensory deficits, neuropathic pain, limb undergrowth, muscular atrophy, cavus foot and bone dysplasia. Ultrasound imaging was performed and investigated both in affected nerves and neuromuscular choristomas associated desmoid-type fibromatosis (NMC-DTF). All patients had a definite history and regular follow-up. The clinical course, physical examinations, ultrasound features and pathologic results of NMC patients were analyzed.
RESULTS: Seven patients with an average age of 7.0 ± 7.2 years (range: 2-22 years) were enrolled in our study. The affected nerves included the sciatic nerve (6 cases) and the brachial plexus (1 case). Six patients (85.7%) presented with limb undergrowth, 6 (85.7%) with muscular atrophy, and 5 (71.4%) with cavus foot deformity. Based on ultrasound findings, all the visibly affected nerve segments presented with hypoechoic and fusiform enlargement with intraneural skeletal muscle elements. Five patients (71.4%) had NMC-DTFs at the site of the affected nerve. All NMC-DTFs were shown as hypoechoic solid lesions adjacent to the nerve and were well circumscribed. In the subset of the surgery group, all 5 patients presented with progression to NMC-DTFs at the site of the NMCs. No fibromatosis was detected in the other two nonsurgical patients.
CONCLUSIONS: Understanding the typical ultrasound features and clinically associated conditions would support the early diagnosis of this rare disease. When a potential diagnosis is determined, an invasive procedure such as biopsy or resection might not be a good choice given the frequent occurrence of complications such as aggressive recurrence.