[A Case of Combined Small Cell Lung Cancer Presenting as Malignant Psoas Syndrome].

Malignant psoas syndrome(MPS)is characterized by intractable pain in the region innervating the first to fourth lumbar nerves resulting from the invasion of malignant tumors into the psoas muscle. A 57-year-old man underwent a right upper lobectomy with lymph node dissection for pStage ⅠB combined small cell lung cancer(SCLC). Adjuvant chemotherapy was subsequently administered in 2022. At 9 months postoperatively, metastases to the liver and lymph nodes of the hepatic portal region were detected. After multidisciplinary treatment, the recurrent lesions were identified as progressive disease. Eight months after the recurrence, the patient complained of severe pain in the left leg. Contrast-enhanced CT showed swelling of the left psoas muscle, and the patient was diagnosed with MPS. Usually caused by cancer of the abdominal organs, MPS is uncommon in patients with lung cancer. Here we report a case of combined SCLC presenting as MPS.