A Solitary Soft Tissue Mass in the Thigh as the First Manifestation of Occult Lung Squamous Cell Carcinoma: A Case Report

ABSTRACT

A 69‐year‐old male patient developed a painless, gradually increasing mass in the upper part of his right thigh, with no pulmonary symptoms. Magnetic resonance imaging revealed a nodular lesion in the lateral femoral muscle, which was initially suspected to be a primary soft tissue tumour. After the mass was surgically removed, the histopathological examination confirmed squamous cell carcinoma (SCC), and the immunohistochemical staining of P63 and CK5/6 was positive. Subsequent FDG PET/CT examination revealed a hypermetabolic nodule in the upper lobe of the right lung. Endobronchial ultrasound‐guided biopsy confirmed it as primary lung SCC. This case indicates that isolated skeletal muscle metastasis can be the initial manifestation of occult lung cancer. The research results suggest that seemingly isolated soft tissue masses may be the first sign of latent malignant tumours.

We report the case of a 69‐year‐old male patient with a nodular lesion in the lateral femoral muscle, which was initially suspected to be a primary soft tissue tumour. After the mass was surgically removed, examinations confirmed squamous cell carcinoma (SCC). Subsequent FDG PET/CT examination revealed a hypermetabolic nodule in the upper lobe of the right lung. Endobronchial ultrasound‐guided biopsy confirmed it as primary lung SCC. This case indicates that isolated skeletal muscle metastasis can be the initial manifestation of occult lung cancer.

1. Introduction

Skeletal muscle metastasis (SMM) from solid tumours is extremely rare [1, 2]. The mechanism of its spread to the muscle is not yet fully understood; currently, haematogenous dissemination is considered the most likely route [3].

A retrospective study in 2014 showed significant differences in the sources of SMM in solid tumours. Among them, lung cancer accounted for the highest proportion, followed by gastrointestinal and urological tumours [4].

2. Case Report

A 69‐year‐old male with a 40‐pack‐year smoking history presented to the orthopaedic outpatient clinic with a painless, progressively enlarging mass in the right thigh for 2 months. The patient had no respiratory symptoms including coughing, haemoptysis, dyspnoea or chest pain. There was no history of malignancy, recent trauma, or systemic symptoms such as weight loss or fever.

Physical examination revealed a solitary, hard, non‐tender and immobile mass on the lateral side of the right thigh, approximately 1.7 cm in diameter. The skin overlying the mass was normal, without ulceration or pigmentation changes. The range of motion of the right hip and knee joints was complete, and the distal nerve and vascular status was intact. No lymph node enlargement was palpable.

Contrast‐enhanced magnetic resonance imaging (MRI) of the thigh showed a well‐defined nodular lesion in the lateral thigh muscle, approximately 1.7 cm in diameter (Figure 1A). Initially, this lesion was suspected to be a primary soft tissue tumour, such as undifferentiated pleomorphic sarcoma.

FIGURE 1.

The results of MRI and PET/CT. (A) MRI shows the tumour indicated by arrows in the thigh from different angles, (B) after the operation on the thigh, PET/CT examinations of the entire body and the lungs were conducted and showed a solitary hypermetabolic nodule in the upper lobe of the right lung indicated by arrows.

The patient underwent biopsy of the mass, and the margins were negative. The specimen was observed to be a greyish‐white hard nodule. Histopathological examination confirmed squamous cell carcinoma (SCC). Immunohistochemical staining showed positive CK5/6 and P40 (Figure 2A). Due to the rarity of primary SCC occurring in skeletal muscle, we systematically investigated potential occult primary malignant tumours. A 18F‐fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) scan of the whole body showed a solitary hypermetabolic nodule in the upper lobe of the right lung (Figure 1B), and no other hypermetabolic lesions were found in other areas. Subsequently, a bronchoscopic ultrasound‐guided transbronchial biopsy was performed on the lung nodule, and immunohistochemical staining showed CK5/6+, P40+ and Ki‐67+ (Figure 2B). The patient was referred to the oncology department for specialised treatment. At the three‐month follow‐up, the patient was asymptomatic, and no local recurrence was observed at the surgical site.

FIGURE 2.

The results of immunohistochemical staining of the lesion tissue. (A) P63 and CK5/6 immunohistochemical staining of soft tissue tumours in the thigh; (B) immunohistochemical staining of P40, CK5/6 and Ki67 for the tumour in the upper lobe of the right lung.

3. Discussion

Muscle metastasis is relatively rare. The presence of SMM indicates that the disease is aggressive. The choice of treatment strategy is a key factor in determining the survival rate of patients. Local treatment can bring significant survival benefits to specific patient groups, especially those with isolated heterogeneous metastatic lesions or those meeting the criteria for surgical resection [2]. When SMM occurs, the most common primary cancers include lung cancer, kidney cancer and gastrointestinal cancers [3, 4]. Pulmonary SCC presenting initially as isolated manifestations is extremely rare, and only a few cases have been reported in the literature.

This case demonstrates several unique and clinically significant characteristics. Firstly, the patient had no symptoms from the primary lung lesion, neither respiratory tract discomfort nor systemic symptoms, highlighting the characteristic that lung cancer may remain clinically asymptomatic while causing isolated distant metastasis. Secondly, the initial MRI manifestations were similar to those of primary soft tissue tumours, so a surgical treatment plan was adopted; without routine histopathological examination, the correct diagnosis would have been delayed. Thirdly, the diagnosis of SCC in muscle tissue should be confirmed by systematic examination methods (including contrast‐enhanced CT or PET/CT) to suggest the need to investigate the primary site [5].

The limitation of this report lies in the lack of long‐term follow‐up data to evaluate treatment response and patient survival. Nevertheless, this case enriches the limited literature on isolated SMM caused by pulmonary SCC and reminds us that the initial symptoms of advanced lung cancer may occur in locations far from the chest.

Author Contributions

Jian Gao wrote the initial draft and acquired the patient data. MengWei Zhang analysed the patient data. YuYing Li conceived the study and revised the manuscript. All authors approved the final version.

Funding

The authors have nothing to report.

Disclosure

The authors have nothing to report.

Ethics Statement

All procedures were performed in accordance with the Declaration of Helsinki and relevant institutional guidelines. Patient anonymity has been strictly protected throughout this manuscript.

Consent

The authors declare that written informed consent was obtained for the publication of this manuscript and accompanying images using the consent form provided by the Journal.

Conflicts of Interest

The authors declare no conflicts of interest.

Gao J., Zhang M., and Li Y., “A Solitary Soft Tissue Mass in the Thigh as the First Manifestation of Occult Lung Squamous Cell Carcinoma: A Case Report,” Respirology Case Reports 14, no. 8 (2026): e70713, 10.1002/rcr2.70713.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.