Extra-Skeletal Osteosarcoma of the Prostate After Treated Prostatic Acinar Adenocarcinoma: A Case Report and Review of the Literature.

Extra-skeletal osteosarcoma is a rare form of malignant soft tissue sarcoma. Its occurrence in the prostate gland is particularly uncommon. In this case report, we present a patient diagnosed with osteosarcoma arising within the prostatic gland. A 58-year-old man was initially diagnosed with Gleason 8 prostate acinar adenocarcinoma following a transurethral resection (TUR) of the prostate. This diagnosis was accompanied by locoregional involvement and multiple bone metastases. The patient underwent a treatment regimen including complete androgen blockade, chemotherapy, greenlight laser prostate vaporization, and palliative radiotherapy. After treatment, he achieved a complete biochemical response, and his bone metastases remained stable. However, at 16 months post-diagnosis, clinical follow-up by means of radiological examinations revealed an increase in the size of the prostatic lesion, along with additional infiltration of the tumor into the rectum and bladder walls. Remarkably, a mesenchymal tumor proliferation with intratumor calcifications was observed. A subsequent TUR biopsy of the prostate showed a malignant tumor spindle and ovoid cell proliferation with high-grade nuclear atypia, necrosis, and islets of osteoid formation, leading to a final diagnosis of high-grade prostatic extra-skeletal osteosarcoma. Despite undergoing chemotherapy, the patient's condition progressed with the development of pulmonary and liver metastases, culminating in his demise.