INTRODUCTION
Malignant melanoma has the capacity to metastasize widely and quickly to every organ in the body, with pulmonary involvement occurring in almost all cases of generalized disease. The other common sites include liver, brain and bone (1). Metastatic malignant melanoma constitutes about 5% of all secondary malignancies of the lung. Systemic spread can be direct, lymphatic or hematogenous. The radiological patterns described in thoracic melanoma metastases include most commonly either multiple (40%) or solitary (20%) pulmonary nodules. Rare presentations in this group include a miliary pattern, mediastinal lymphadenopathy, pleural effusions, lytic bone lesions and extra-pleural mass (1).
CASE REPORT
A 75-year-old male, non-smoker with history of asbestos exposure, presented with rapidly progressive dyspnea, left pleuritic chest pain, and weight loss. He had undergone excision of a 2 mm Clark IV cutaneous malignant melanoma from the right upper back 15 months previously. Lympho-scintigraphy and subsequent sentinel node lymphadenectomy did not demonstrate any lymph node involvement. Chest radiography at the time demonstrated pleural plaques but no other abnormality and no further staging was performed at the time. He was referred for a CTPA to assess for pulmonary emboli. The CTPA demonstrated bilateral pulmonary emboli, massive left pleural effusion with collapse of the left lung, bilateral calcified pleural plaques, left hilar and subcarinal lymphadenopathy and a left adrenal lesion (Figure 1). Subsequently the patient was referred for a FDG-PET/CT scan to assess for malignancy. Axial and Maximum Intensity Projection (MIP) images from whole-body FDG-PET/CT (64 slice Phillips TF Gemini, 1 hour post injection of 6.6 mCi (245 MBq) of F-18 FDG) demonstrated diffuse intense FDG uptake (SUVmax 14.1) throughout the pleura of the left hemi-thorax, bilateral hilar and mediastinal lymph nodes, bilateral adrenals and left gluteal musculature, highly suspicious of metastatic malignancy (Figure 2). As extensive, intensely FDG avid pleural disease in a patient with history of asbestos exposure and bilateral calcified pleural plaques is highly suggestive of mesothelioma, a provisional diagnosis of mesothelioma was made. Subsequent thoracoscopic pleural biopsy confirmed metastatic melanoma (Figure 3A, [fig:5009]3B[/fig], [fig:5010]3C[/fig]). The patient was referred for consideration of palliative therapy but died 10 days later. This case illustrates that other malignancies including metastatic melanoma can have a similar appearance as mesothelioma.
DISCUSSION
Pleural effusion is an infrequent presentation of metastatic malignant melanoma (1,2,3,4), with 70% of cases associated with solitary or multiple lung nodules. The literature on this topic is scarce. Impaired lymphatic drainage due to infiltrated mediastinal lymph nodes is the primary mechanism for accumulation of pleural fluid. The pleural site may have been reached through the lymphatics. Rarely, tumors can metastasize and spread diffusely within the pleura to form an encasing mass, which clinically and radiologically mimics malignant mesothelioma (5), the most frequent pathology being primary adenocarcinoma of the lung. Other tumors which grow diffusely within the pleura to simulate mesothelioma include malignant vascular tumors, synovial sarcoma, thymoma and malignant lymphoma (5). The prognosis of malignant melanoma is variable with distant metastases sometimes occurring more than 10-years after initial therapy. The best prognostic indicators are tumor thickness and ulceration (5). The prognosis of pseudo-mesotheliomatous tumors is poor and is similar to mesotheliomas with the exception of malignant lymphomas and thymomas which show longer survival with appropriate treatment. The prognosis of “pseudo-mesotheliomatous” malignant melanoma is uncertain (5). Although malignant melanoma is typically FDG avid, the intense FDG uptake is nonspecific. The role of FDG-PET in staging asymptomatic patients with stage 1 or stage 2 melanoma is limited. The vast majority of data in the literature suggests that FDG-PET is the most accurate imaging modality for identifying distant metastases in patients at high risk for harboring malignant lesions. FDG PET/CT is also an excellent tool to detect distant melanoma metastases and also allows quantification of FDG uptake using standardized uptake value, which may have prognostic implications (6,7), as demonstrated in this patient who succumbed soon after the PET scan and only 10 days following the thoracoscopic pleural biopsy.