We performed a biopsy of the lesion with the aid of immediate adequacy by fine needle aspiration cytology. We saw mitotically active malignant cells with large nuclei, high nuclear-to-cytoplasmic ratios, nucleoli, and dense cytoplasm, suggesting a metastatic squamous cell carcinoma. Because infection was part of the differential, it is pertinent to note that there was no significant inflammatory infiltrate. The core biopsy was consistent with metastatic lung cancer (Figure 6).
Discussion
This patient presented an interesting diagnostic challenge, particularly because of his previous malignancy. The differential diagnosis of acute onset thenar pain without history of trauma would include encompassing soft-tissue abscess, osteomyelitis, and infectious myositis. Soft-tissue hematoma is also in the differential for this patient, especially given the malignancy. Bony metastasis should be considered in this patient given the propensity of lung carcinoma to metastasize to bone. The location would certainly be atypical, with metastasis to the bones of the forearm or hand representing only 0.1% of all metastasis of any type of primary carcinoma or sarcoma.4 Primary bone or soft-tissue sarcoma should also be considered. Some authors have also suggested that necrosis, peritumoral edema-like signal, and lobulation are more common with skeletal muscle metastasis than with a primary sarcoma.10 In this case, the degree of surrounding postcontrast enhancement made simple muscle tear with hematoma unlikely, despite the presence of increased T1 signal. The lack of evidence for localized infection and the presence of a firm focal mass on physical examination made tumor more likely than infection.
Acrometastasis
Metastatic disease distal to the elbow and knee is very rare; specifically, metastatic disease of the hands or feet accounts for approximately 0.1% of all metastases.4 Carcinoma of the lung accounts for 44% to 47% of all acrometastasis.11,12 When hand acrometastasis is considered, the right hand accounts for 55% of bony cases, likely because of hand dominance, although approximately 10% of patients had bilateral acral metastatic disease.12 The underlying mechanism of acrometastasis remains unclear; however, some authors have postulated that it may result from an increase in vascularity or a trauma to the affected extremity.12,13 Flynn and colleagues12 reviewed the literature and reported a total of 257 cases of acral metastasis to the hand; they found that the median age at presentation was 58 years. Men were more than twice as likely to be affected when compared with women. Most commonly, the primary malignancies were in the lung (44%), kidney (12%), and breast (10%). The authors also reported less common cases of acral metastasis with primary malignancies located in the stomach, liver, rectum, prostate, and colon. Most commonly, these metastases were found in the distal phalynx, followed by the metacarpals, proximal phalynx, and middle phalynx.12
Soft-Tissue Metastasis
Skeletal muscle metastasis occurs in 0.8% to 17.5% of metastatic neoplasms.14-17 Studies in lung cancer patients have also revealed a low prevalence of muscular metastasis (0% to 0.8%).16 The rarity of muscular metastatic disease has been attributed to local inhibition of tumor survival secondary to muscle contraction, increased diffusing capacity of enzymes and immune cells, and extreme variability in blood flow and pH, lactate, and oxygen concentration. Skeletal muscular metastases most commonly arise from the lung, kidneys, colon, or melanoma.16 In a recent large series of more than 1400 patients imaged for soft-tissue masses, 2.5% were metastatic.18 There are only 2 reports of soft-tissue metastatic disease involving the hand: one from a patient with a thyroid carcinoma and the other from a patient with a lung adenocarcinoma.18 Soft-tissue metastatic disease from squamous cell carcinoma distal to the wrist has never been reported in the literature.
Acral Soft-Tissue Metastasis
A review from 2012 found 264 cases of skeletal muscle metastasis from 151 articles.6 Only 2 (0.75%) of these patients, as reported above, had a soft-tissue metastasis distal to the wrist.6,17
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