Undifferentiated Pleomorphic Sarcoma of the Proximal Thigh With Neoplastic Fever and Metachronous Sternal Metastases: A Case Report and Review of the Literature.

Undifferentiated pleomorphic sarcoma (UPS) is a rare subtype of undifferentiated soft tissue sarcoma (USTS). UPS is a fast-growing malignant tumor with a high mitotic rate and a high risk of distant metastasis that increases proportionally to tumor size. Obtaining an early diagnosis confers a more favorable prognosis. Neoplastic fever (NF) is the refractory fever subsumed under the category of classic fever of unknown origin (FUO) caused by malignant neoplasms. NF is a paraneoplastic syndrome that rarely occurs in STS. Among UPS tumors, there is an exceptional subtype that causes refractory fever and signs of systemic inflammation. The coexistence of UPS with NF (UPS-NF) represents an unusual event. We present an exceptional case of a 58-year-old male patient with a two-month history of FUO that revealed an atypical clinical presentation of NF generated by a rare STS of the anterior thigh with subsequent sternal metastases. The patient received neoadjuvant chemotherapy and underwent a compartmentectomy with a microscopic residual tumor (R1), followed by postoperative adjuvant radiotherapy with unfavorable results. The patient also presented overlapping paraneoplastic manifestations (NF and areolar hyperpigmentation) that occur concurrently with the onset of metachronous sternal metastases, highlighting the uninvolved lung parenchyma.