Pulmonary Smooth Muscle Hyperplasia Difficult to Differentiate from Primary Lung Cancer: A Case Report.

Introduction: Pulmonary smooth muscle hyperplasia (SMH) is a rare benign tumor that presents CT imaging findings that require differentiation from those of primary lung cancer.

Case Presentation: The postoperative follow-up chest CT for gastric cancer in a 76-year-old Japanese man revealed an abnormal shadow. A 2.2-cm nodule with an unclear border and showing a tendency to grow was detected in the right lower lobe (S6), and suspected infiltration into the right upper lobe (S2). PET showed minimal accumulation of 18F-fluorodeoxyglucose in the nodule, with a maximum standardized uptake value of 1.0. A transbronchial lung biopsy showed no malignant findings. Due to the tumor's progressive growth, surgical resection was performed. Intraoperatively, a tumor located in S6 with suspected partial invasion into S2 was observed, and a wedge resection from S6 to S2 was thus performed. A frozen section of the resected specimen revealed irregularly distributed atypical cells forming mildly irregular glandular structures, leading to a diagnosis of "suspected adenocarcinoma." Robotic-assisted thoracoscopic surgery for a right S6 segmentectomy with combined wedge resection of S2 was performed. However, the final histopathological examination revealed spindle-shaped smooth muscle cells' proliferation. The immunohistochemical analysis revealed positivity for α-SMA, desmin, and h-caldesmon, leading to a diagnosis of pulmonary SMH.

Conclusions: SMH is an extremely rare benign disease that can mimic lung cancer and may be considered among the possible differential diagnoses of solitary pulmonary nodules. A careful treatment strategy, including the choice of surgical procedure, is recommended to minimize the possibility of overtreatment.