Osseous metaplasia is an extremely rare histological variation of thymus tumors. Despite intratumoral calcifications are described ossification is exceptional.
A 27-year-old woman with no relevant medical history was referred to our department for the treatment of a full-calcified anterior mediastinal tumor. It was detected on a spinal X-ray performed for chronic back pain and confirmed by a chest X-ray. On the computed tomography, a bulky anterior mediastinal tumor containing heterogeneous calcification was confirmed. The patient underwent a posterolateral thoracotomy. Intraoperatively there was a large hard stony mass in the anterior mediastinum extending to the right hemi thorax. Complete excision of the mass including thymectomy was done. The patient underwent uneventful recovery. Histopathology reported a thymoma type B1 with osseous metaplasia according to the World Health Organization classification and Masaoka I stage according to intraoperative findings.
To our knowledge it is the third case published in all literature of a thymus tumor with complete osseous metaplasia in an adult and the first case that tumor is almost entirely ossified.
The mechanism of osseous metaplasia is unknown. Several theories have been given. Certain cell mediators would stimulate and influence connective tissue to be replaced by heterotopic bone. Further cases of osseous metaplasia will be necessary to figure out pathogenesis, prognosis and treatments outcomes of these particular types of thymomas.