Metastatic squamous cell carcinoma of the lung masquerading as a soft tissue tumor.

Carcinoma of lung can metastasize to any organ system; however, metastasis to skeletal muscles is extremely rare. A 63-year-old man, known case of pulmonary tuberculosis on treatment, presented with a painful swelling in his left leg. Examination revealed a 5.0 cm × 3.0 cm calf swelling, which on imaging was suggestive of a soft tissue tumor. Fine-needle aspiration cytology of the swelling revealed it to be squamous cell carcinoma. Further investigations revealed a mass in the left lower lobe of the lung. Biopsies from both the lung lesion and calf swelling confirmed the diagnosis of squamous cell carcinoma of lung with metastasis to the calf muscle. The case is being presented because of its unusual presentation and rarity.

Introduction

Although skeletal muscles comprise nearly 50% of the total body mass, it is an extremely rare site for metastatic tumors. The prevalence of skeletal muscle metastasis in various autopsy series of patients with any type of cancer ranged from 0.8% to 17.5%, whereby, the most common tumors found to metastasize to the skeletal muscles were from the genitourinary and gastrointestinal tract.[123] The prevalence of lung cancer metastasis to skeletal muscles is very low and ranges between 0.0% and 0.8%[123] Sometimes, these metastatic lesions are the first clinical signs of underlying malignancy. We report one such case of squamous cell carcinoma of the lung who presented with a metastatic swelling in the calf muscle, mimicking a soft tissue tumor.

Case Report

The case we present here is about a 63-year-old man who was referred to our out-patient clinic with a gradually increasing swelling in his left calf since 3 months and associated pain since the last 1 month. He gave a history of loss of appetite and weight. He had been diagnosed to be having pulmonary tuberculosis 5 months ago for which he was receiving treatment regularly. The patient was a nonsmoker.

General physical examination revealed mild pallor and clubbing of his fingers. The left calf swelling was ill-defined and measured 5.0 cm × 3.0 cm approximately. It was firm to hard in consistency, tender and showed mobility in the transverse axis. Systemic examination revealed no abnormality except for decreased breath sounds and crepitations in both the lung fields.

Magnetic resonance imaging of the leg swelling showed a 3.2 cm × 2.4 cm × 2.0 cm mass in the lateral border of the gastrocnemius muscle encasing the peroneal nerve — suggestive of a peripheral nerve sheath tumor. Computed tomography scan of the chest revealed a fibro-cavitatory lesion in the right upper lobe of the lung which was consistent with tuberculosis. In addition, an ill-defined mass was visualized in the left lower lobe, measuring 3.5 cm × 2.7 cm, with associated collapse. This lesion was suspected to be neoplastic. Abdominal ultrasound was normal.

Hemogram showed mild iron deficiency anemia. All the biochemical parameters were within normal histological limits.

Based on the above investigations, a presumptive diagnosis of soft tissue tumor of the leg with possible metastasis to the lung and coexistent pulmonary tuberculosis was made.

The calf swelling was first subjected to fine-needle aspiration cytology (FNAC). Cytology revealed loose groups as well as singly placed polygonal and round cells with squamoid appearance [Figure 1a]. The cells had high nuclear-cytoplasmic ratio, hyperchromatic irregular nuclei, small nucleoli and a moderate amount of dense eosinophilic cytoplasm. Few bizarre looking cells were also seen. Possibility of a metastatic squamous cell carcinoma was suggested.

Figure 1

(a) Fine-needle aspiration cytology of the calf swelling showing malignant squamous cells (H and E, ×400); (b and c) Core biopsies from the lung lesion and calf swelling respectively, showing squamous cell carcinoma (H and E, ×400)

This was followed by core biopsies from the left lung mass as well as the calf swelling. Histopathology from both the sites revealed moderately differentiated squamous cell carcinoma [Figure 1b and c]. A final diagnosis of squamous cell carcinoma of the lung with skeletal muscle metastasis was made.

Discussion

Lung carcinomas can metastasize to various organ systems. Local intra-thoracic spread can occur to mediastinal lymph nodes, pleura, diaphragm, chest wall and pericardium. The most common extra thoracic sites are the liver, adrenal glands, brain, bone and kidney.[1] Rarely, metastasis can occur to the skeletal muscles, producing a soft tissue swelling, which can clinically be confused with a soft tissue sarcoma.[4]

In spite of its rich vascularity, skeletal muscles are resistant to hematogenous metastasis from epithelial neoplasms; the reported incidence being less than 1% in various clinical case series.[56] Di Giorgio et al. in their study of 3000 patients treated for lung cancer, described only three cases showing skeletal muscle metastasis.[5]

Various hypotheses viz. mechanical, metabolic, and immunological have been proposed to explain the rarity of metastasis to skeletal muscles. Mechanical hypothesis attributes muscle contraction, increased tissue pressure and variable blood flow to creating an inhospitable tissue environment for seeding of tumor.[7] According to the metabolic hypothesis, presence of lactic acid and protease inhibitors in the muscle inhibit the growth of tumor cells and resist invasion.[8] Immunological hypothesis suggests a role of cellular and humoral immunity.[9] None of these can however, provide a complete explanation.

The present case is interesting due to the fact that the presenting feature of an underlying lung carcinoma was a metastatic lesion in the skeletal muscle, masquerading as a soft tissue tumor. It also highlights the role of FNAC as a convenient tool to correctly diagnose such lesions.