Diaphragmatic fibromatosis is an exceptionally rare condition, with only 2 previously reported cases in the literature. We present the case of a 23-year-old woman with a left hypochondrial swelling measuring 19×18×13 cm on computed tomography. An endoscopic ultrasound-guided biopsy indicated low-grade papillary proliferation. Surgical exploration revealed a large mass originating from the left hemidiaphragm. Complete excision of the mass and reconstruction of the diaphragm with double-layer mesh were performed.Microscopic examination of the mass revealed a bland-looking spindle cell proliferation.Immunohistochemical staining showed a positive nuclear reaction in tumor cells for β-catenin, a focal positive reaction for SMA, and negative reactions for S100, CD34, and desmin.Diaphragmatic fibromatosis is an extremely rare tumor for which complete excision and reconstruction of the diaphragm is the best suggested modality of treatment.

Diaphragmatic fibromatosis is an exceptionally rare condition, with only 2 previously reported cases in the literature. We present the case of a 23-year-old woman with a left hypochondrial swelling measuring 19×18×13 cm on computed tomography. An endoscopic ultrasound-guided biopsy indicated low-grade papillary proliferation. Surgical exploration revealed a large mass originating from the left hemidiaphragm. Complete excision of the mass and reconstruction of the diaphragm with double-layer mesh were performed.Microscopic examination of the mass revealed a bland-looking spindle cell proliferation.Immunohistochemical staining showed a positive nuclear reaction in tumor cells for β-catenin, a focal positive reaction for SMA, and negative reactions for S100, CD34, and desmin.Diaphragmatic fibromatosis is an extremely rare tumor for which complete excision and reconstruction of the diaphragm is the best suggested modality of treatment.

Diaphragmatic fibromatosis is an exceptionally rare condition, with only 2 previously reported cases in the literature. We present the case of a 23-year-old woman with a left hypochondrial swelling measuring 19×18×13 cm on computed tomography. An endoscopic ultrasound-guided biopsy indicated low-grade papillary proliferation. Surgical exploration revealed a large mass originating from the left hemidiaphragm. Complete excision of the mass and reconstruction of the diaphragm with double-layer mesh were performed.Microscopic examination of the mass revealed a bland-looking spindle cell proliferation.Immunohistochemical staining showed a positive nuclear reaction in tumor cells for β-catenin, a focal positive reaction for SMA, and negative reactions for S100, CD34, and desmin.Diaphragmatic fibromatosis is an extremely rare tumor for which complete excision and reconstruction of the diaphragm is the best suggested modality of treatment.