2025 Volume 11 Issue 1 Article ID: cr.25-0428
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.
alpha smooth muscle actin
FDGfluorodeoxyglucose
FEV1a forced expiratory volume in one second
FVCa forced vital capacity
h-caldesmonheavy caldesmon
RATSrobotic-assisted thoracoscopic surgery
SMHsmooth muscle hyperplasia
SRIFsmoking-related interstitial fibrosis
SUVmaxa maximum standardized uptake value
VCa vital capacity
Pulmonary SMH is a rare benign tumor that is characterized by the hyperplastic proliferation of smooth muscle bundles forming small nodules. We describe a case of pulmonary SMH that was difficult to distinguish from primary lung cancer.
The postoperative follow-up chest CT examination for the gastric cancer of a 76-year-old Japanese man (a current tobacco smoker with a smoking history of >20 pack-years) revealed an abnormal shadow. A chest CT scan then revealed a 2.2 × 1.4-cm nodule in the right lower lobe (S6) with an unclear border and a tendency to grow, plus suspected infiltration into the right upper lobe (S2) (Fig. 1A). In addition, features consistent with malignancy, such as pleural indentation and focal pleural thickening, were observed. PET showed minimal accumulation of 18F- FDG in the nodule, with SUVmax of 1.0 in the early phase (Fig. 1B). Although a transbronchial lung biopsy revealed no evidence of malignancy, the nodule continued to grow over time, leading to the suspicion of malignancy; a decision was made to perform surgery for the nodule. Preoperative pulmonary function testing revealed VC of 2.37 L (66.4% of predicted), FVC of 2.38 L, and FEV1 of 2.38 L (91.9% of predicted), with an FEV1/FVC ratio of 100%, indicating mildly reduced lung volume but preserved airflow. Because it was difficult to distinguish between a benign and malignant tumor based on imaging findings and clinical course, we decided to determine the surgical procedure intraoperatively with the aid of frozen section analysis. If the lesion was found to be malignant, we planned to perform a segmentectomy as a passive limited resection, considering the patient’s pulmonary function and age. If the lesion was benign, a wedge resection would be sufficient.
Intraoperatively, a tumor located in S6 with suspected partial invasion into S2 was observed (Fig. 2A). A wedge resection from S6 to S2 was thus performed (Fig. 2B). A frozen section of the resected specimen revealed irregularly distributed atypical cells forming mildly irregular glandular structures (Fig. 3A), leading to a diagnosis of “suspected adenocarcinoma.” RATS for a right S6 segmentectomy with a combined wedge resection of S2 was performed (Fig. 2C and 2D). The patient was discharged on POD 9 without complications.
Macroscopic findings of the resected specimen revealed a grayish-white lesion with an unclear border (Fig. 3B). The histopathological examination showed spindle-shaped cells with eosinophilic cytoplasm forming thick fascicles. Normal smooth muscle proliferation was scattered, and no malignant features were observed (Fig. 3C). The immunohistochemical analysis revealed positivity for α-SMA, desmin, and h-caldesmon, leading to a diagnosis of pulmonary SMH. Although the tumor appeared to extend from S6 to S2, smooth muscle proliferation was observed separately under the visceral pleura in each segment. Since the lesion was benign and completely resected, the patient was followed without additional treatment. There is no established consensus regarding the optimal follow-up interval for such cases. Given that the lesion was a benign nodule, we plan to monitor the patient every 6 to 12 months for several years, provided that no evidence of lesion progression is observed.
Pulmonary SMH is a rare benign lesion characterized by the hyperplastic proliferation of smooth muscle bundles forming small nodules.1,2) Histologically, SMH is composed of spindle-shaped smooth muscle cells arranged in fascicles, and it typically expresses markers such as α-SMA, desmin, and h-caldesmon.1–4) Other benign pulmonary lesions with spindle cell morphology include hamartoma, leiomyoma, and inflammatory myofibroblastic tumor. In the present case, the lesion is composed predominantly of smooth muscle without cartilage or adipose tissue, which excludes hamartoma.2,4) Pulmonary leiomyoma is typically diagnosed as a metastatic lesion from uterine fibroids and was excluded in this case due to the patient being male.2) Inflammatory myofibroblastic tumor was also excluded, as inflammatory cell infiltration was observed, but no proliferation of myofibroblasts was identified.2,5) Although SMH has been reported in extrapulmonary sites such as the testicular adnexa and in association with Crohn’s disease,6,7) its occurrence in the lung is extremely rare. Unlike the diffuse pulmonary smooth muscle proliferation that is observed in conditions such as asthma and pulmonary hypertension,8) pulmonary SMH forming a solitary pulmonary nodule is extremely rare, and its etiology remains unclear. Possible contributing factors include chronic lung diseases such as SRIF, or scarring from prior interstitial pneumonia.1) In the present case, although no radiological or pathological evidence of fibrosis was found, the patient’s long-term tobacco-smoking history may have induced chronic inflammatory changes contributing to pleural-based smooth muscle proliferation.
To date, only four instances of nodular pulmonary SMH have been documented, including our patient’s case (Table 1).3,4) All four cases occurred in elderly individuals and involved peripherally located nodules with ill-defined margins, and the nodules were initially suspected to be malignant. These findings highlight the diagnostic challenges posed by this entity, especially in patients with risk factors such as advanced age, smoking history, or prior malignancy.
| First author |
Age | Sex | Smoking history |
Interstitial pneumonia |
CT scan findings |
Tumor size, mm |
Tumor location |
Surgical procedure | α-SMA | Desmin | h-caldesmon |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Shikata3) | 81 | M | Unknown | Unknown | Nodule with an unclear boundary | 10 | Peripheral, LUL | Wedge resection | + | n.a. | n.a. |
| Ikeda4) | 81 | M | Unknown | Unknown | Nodule with an unclear boundary | 10 | Peripheral, RLL | Wedge resection | + | + | n.a. |
| 72 | F | Unknown | Unknown | Nodule with an unclear boundary | 5 | Peripheral, LLL | Wedge resection | + | + | n.a. | |
| Present case | 76 | M | + | − | Nodule with an unclear boundary | 22 | Peripheral, right S6 to S2 | Wedge resection→ segmentectomy |
+ | + | + |
α-SMA, alpha smooth muscle actin; F, female; h-caldesmon, heavy caldesmon; LLL, left lower lobe; LUL, left lower lobe; M, male; n.a., not available; RLL, right lower lobe
Radiologically, SMH can closely mimic primary lung cancer. In our patient’s case, the lesion demonstrated interval growth and a poorly defined border on CT, and both of these findings were suggestive of malignancy. Although PET revealed minimal FDG uptake (SUVmax 1.0), the lesion’s progressive enlargement necessitated surgical resection. This underscores the limited sensitivity of PET imaging in distinguishing rare benign entities such as SMH from early-stage malignancies.
Unlike inflammatory nodules, which are typically stable or regress over time,9) SMH may demonstrate gradual growth. This characteristic can contribute to misdiagnoses and unnecessary invasive intervention. Clinicians should consider SMH in the differential diagnosis of slowly enlarging peripheral nodules, particularly in patients with chronic lung disease or a history of smoking.
The present case demonstrates the complexity of diagnosing pulmonary SMH both radiologically and pathologically. Intraoperatively, the tumor was initially suspected to be adenocarcinoma based on a frozen-section analysis, which revealed irregularly distributed atypical cells forming mildly irregular glandular structures. However, a retrospective review suggested that these glandular structures were likely attributable to the proliferation of type II pneumocytes in response to inflammation rather than true adenocarcinoma. The diagnostic challenge was further compounded by the extreme rarity of pulmonary SMH and the inability to perform immunohistochemical staining intraoperatively, which limited the accurate assessment of smooth muscle proliferation during surgery.
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. Given the diagnostic uncertainty, surgical resection is often necessary; however, limited approaches such as wedge resection or segmentectomy should be prioritized to minimize the possibility of overtreatment. Increased awareness and accumulation of SMH case reports are essential to improve the diagnostic accuracy and guide the optimal management of this rare condition.
We thank Dr. Yoshihisa Umekita (Department of Pathology, Faculty of Medicine, Tottori University, Tottori, Japan) and all of the other individuals who contributed to this work.
No funding was received for this study.
Authors’ contributionsMY administered the clinical treatments.
MY drafted the manuscript.
YK and YT performed a dedicated review and contributed to the discussion.
WF and SM helped make the pathological diagnosis.
All authors read and approved the final manuscript and have agreed to be responsible for the manuscript.
Availability of data and materialsAll of the data in this article are available within the article.
Ethics approval and consent to participateThis work does not require ethical considerations or approval. Informed consent to participate in this study was obtained from the patient.
Consent for publicationWe obtained the patient’s fully informed written consent for the publication of his case and images. His identity has been protected.
Competing interestsThe authors declare that they have no competing interests.
