Muscle Metastasis as an Atypical Presentation of Non-small Cell Lung Cancer: A Case Report and Literature Review.

Skeletal muscle metastases from non-small cell lung cancer (NSCLC) are rare and often clinically silent, representing an uncommon site of disease dissemination and accounting for approximately 2.3% of cases. Diagnosis is frequently delayed due to nonspecific symptoms and imaging findings that can mimic benign conditions. We report the case of a 64-year-old man with a history of chronic smoking who presented with right gluteal pain and swelling. Imaging revealed a soft tissue mass within the gluteus maximus muscle. Histopathological analysis of a biopsy specimen demonstrated a poorly differentiated adenocarcinoma. Immunohistochemical staining was positive for CK7 and TTF-1 and negative for CK20, PSA, and CDX2, supporting a pulmonary origin. Molecular testing was negative for ALK, ROS1, RET, MET, and EGFR mutations. The patient received first-line chemotherapy with carboplatin and paclitaxel, achieving stable disease. He subsequently developed brain metastases, which were treated with whole-brain radiotherapy, and is currently undergoing second-line treatment with docetaxel. This case highlights that gluteal muscle metastasis can present as the initial manifestation of NSCLC, preceding the detection of the primary tumor. It underscores the diagnostic value of imaging and immunohistochemistry in characterizing atypical soft tissue lesions. Multidisciplinary discussion is crucial for accurate diagnosis and optimal management of such rare presentations.