Benign Nodular Smooth Muscle Proliferation of the Lung Mimicking Malignancy.

We present an extremely rare and diagnostically challenging case of benign nodular smooth muscle proliferation (NSMP) in the lungs. A 41-year-old female patient presented with a cough and a right upper lung lobe lesion. Multislice computed tomography and positron emission tomography/computed tomography suggested malignancy despite relatively low metabolic activity. Fiberoptic bronchoscopy and transthoracic needle biopsy were inconclusive, so a right upper lobectomy with mediastinal lymphadenectomy was performed. Histopathology unveiled benign NSMP. This case underscores the diagnostic challenges of rare benign lung lesions mimicking malignancy and emphasizes a structured diagnostic and therapeutic approach. Clinical suspicion may arise in patients such as older adults presenting with well-defined pulmonary nodules, an inconclusive biopsy demonstrating smooth muscle proliferation, and low to moderate fluorodeoxyglucose uptake on positron emission tomography/computed tomography. When other diagnostic methods fail to provide a definitive diagnosis, surgical resection should be considered both as a diagnostic and therapeutic option. Histopathology remains the gold standard for differentiating benign from malignant lung lesions and enables definitive treatment in benign conditions, such as benign NSMP.