Skeletal muscle metastases as the initial manifestation of an unknown primary lung cancer detected on F-18 fluorodeoxyglucose positron emission tomography/computed tomography

Abstract

Skeletal muscle metastasis as the initial presentation of the unknown primary lung cancer is unusual. A 65-year-old male patient presented with pain and swelling of the right forearm. Fine needle aspiration of the swelling revealed metastatic squamous cell carcinoma. The patient underwent whole body F-18 fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) to identify the site of the primary malignancy. The authors present PET/CT images showing FDG-avid metastases to the skeletal muscles along with a previously unknown primary tumor in the right lung, in a patient presenting with initial muscular symptoms without any pulmonary manifestations.

INTRODUCTION

Skeletal muscle metastasis as the initial presentation of an unknown primary lung cancer is unusual. F-18 fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) imaging is useful in the identification of primary in carcinoma of unknown origin. We describe a patient showing FDG-avid metastases to the skeletal muscles along with a previously unknown primary tumor in the right lung, in a patient presenting with initial muscular symptoms without any pulmonary manifestations.

CASE REPORT

A 65-year-old male presented with pain and swelling of the right forearm. Fine needle aspiration cytology of the swelling showed metastatic squamous cell carcinoma. The patient underwent a whole body F-18 FDG PET/CT to identify the site of the primary malignancy. Increased FDG avidity (standardized uptake value [SUV_max] 9.0) was detected in an irregular heterogeneously enhancing soft-tissue mass in the right paravertebral region in the upper lobe of the right lung with a focus of calcification within the mass [Figure 1b and d, white arrow]. Abnormal FDG uptake was also noted in a presacral mass [Figure 1c and e], the bulky left adrenal gland, several dorsal vertebrae and multiple lesions in the trapezius [Figure 1b and d, red arrow], right brachioradialis [Figure 2], deltoid, and right external oblique muscles [Figure 3], suggestive of metastatic involvement. A diagnosis of primary squamous cell carcinoma of the lung was pathologically confirmed. The patient was treated with 4 cycles of chemotherapy, after which significant decrease in FDG uptake (SUV_max = 5.1) was seen in the primary as well as the right brachioradialis muscle lesion (not shown here).

Figure 1.

Fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) Maximum intensity projection (MIP) image (a) showing multiple foci of abnormal tracer uptake. Transaxial thoracic CT (b) and fused PET/CT image (d) show increased FDG uptake in an irregular, heterogeneously enhancing soft-tissue mass (white arrow) in the paravertebral region in the upper lobe of the right lung with calcification within the mass. Increased FDG uptake is also seen in a peripherally enhancing ring like lesion in the trapezius (red arrow). Axial CT (c) and fused PET/CT (e) images at the level of the rectum show increased FDG uptake in a heterogeneously enhancing pre-sacral soft-tissue deposit

Figure 2.

Coronal and transaxial computed tomography (CT) (a and c) and fused positron emission tomography/CT (b and d) images of the right hand showing increased fluorodeoxyglucose uptake in the right brachioradialis muscle with no increase in attenuation (probably because the images were acquired after completion of the whole body PET scan)

Figure 3.

Axial computed tomography (a) and fused positron emission tomography/computed tomography (b) images showing increased tracer uptake in a ring-like hyperenhancing lesion in the right external oblique muscle suggestive of muscle metastasis

DISCUSSION

Skeletal muscles are uncommon site of hematogenous metastases from epithelial neoplasms. Solitary muscle metastasis has been previously reported in lung cancer.[1] Tuoheti et al. found that only 4 patients (0.16%) among 2,557 patients with lung cancer developed metastasis to the skeletal muscle.[2] Most frequent muscle involvement is seen in the thigh, iliopsoas and paraspinous muscles.[3] Whole-body FDG PET/CT imaging is useful in detection of muscle metastases in lung cancer patients.[4] Multiple muscle metastases from lung cancer are rare, and FDG PET/CT imaging is useful in the identification of unsuspected metastatic sites.[5] Primary presentation of a skeletal muscle metastasis, such as in our case, remains an unusual occurrence.[3,6,7,8,9] The present case, where the initial presentation was of metastatic muscular involvement, highlights the role of FDG PET/CT in tracing the location of primary lung malignancy and unsuspected sites of multiple muscle metastases in a patient with muscle metastases of unknown primary.