A 47-year-old female was referred to the hospital because of persistent pain at the ventral right side of the chest for several weeks. Except for ankylosing spondylitis, her medical history was insignificant. The patient doesn’t smoke and there is no history of thoracic trauma or surgery. There were no alarm symptoms. Clinical examination revealed no abnormalities and routine hematologic and biochemical evaluation were normal. Breast US was performed and incidentally a sharply delineated heterogeneous intra-thoracic soft tissue mass of 4.5 × 9 cm was observed arising from the right chest wall. Additional X-rays of the chest showed a large intra-thoracic right-sided mass with a broad pleural base, smoothly bordering the lung parenchyma (Fig. A). Contrast-enhanced CT of the chest confirmed the presence of a right-sided sharply delineated heterogeneous intra-thoracic mass with a broad pleural base (Fig. B). Bony alterations were present on the adjacent ribs. There were no enlarged lymph nodes, pleural fluid or intrathoracic nodules. At MRI imaging, the mass had mixed signal intensity on T2WI and was iso-intense to skeletal muscle on axial 3D gradient-echo T1WI images. The lesion was irregularly marginated where it bordered the thoracic wall, suggesting growth in the intercostal spaces. Strong and heterogeneous enhancement occurred on T1WI after gadolinium administration (Fig. C). Finally PET-CT showed only mild, heterogeneous FDG-uptake in the mass, suggesting a benign lesion. Because malignancy could still not be ruled out, a surgical biopsy was performed. Pathologic examination showed a fibromatous spindle cell lesion without signs of high grade malignancy. No mitosis, nor necrosis nor pleomorphism were observed. Histological and immunohistochemical image was strongly suggestive for desmoid tumor with staining negative for keratin, CD34 and S100 and positive for beta-catenin and actin. These results were re-examined and confirmed in a second hospital. Surgery was recommended, but the patient rejected and chose radiotherapy. Follow-up imaging six months later showed slight volume loss of the tumor. The patient remains in further follow-up.