Resection of a desmoid-type fibromatosis with a CTNNB1 p.S45P mutation using a cervico-thoracic approach: A case report and literature review.

Desmoid-type fibromatosis (DF) is a rare, locally infiltrative, and fibroblastic proliferative disease. DF usually arises from abdominal fascial tissue, but in rare cases, it can occur in extra-abdominal areas. A 73-year-old Japanese male complained of a painless, left anterior neck mass of 3-month duration. Computed tomography revealed the mass measured 9 × 7 × 6 cm and extended to the anterior mediastinum, with invasion of the left clavicle. En bloc resection of the tumor with the left sternoclavicular joint and the medial portion of the left clavicle was performed by cervico-thoracic approach with L-shaped partial sternotomy. Histopathologic examination showed fascicular growth of spindle-shaped cells separated by abundant collagen. Immunohistologic examination revealed nuclear staining of β-catenin and cytoplasmic staining of vimentin. Genetic analysis of 160 cancer-related genes by next-generation sequencing (NGS) demonstrated only a missense mutation in the CTNNB1 gene (c.133T>C, p.S45P). DF extending from the neck to the anterior mediastinum is rare. We report the complete resection of a large-sized DF with the clavicular invasion. A low-frequency CTNNB1 mutation of DF was identified. Genetic analysis with NGS was beneficial for the diagnosis.