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Metastatic choroidal melanoma typically presents in the liver. Therefore, liver enzyme assays are the most common haematological evaluation performed after treatment.1
In 1985, The Collaborative Ocular Melanoma Study required periodic medical evaluations including a physical examination, liver functions studies, a complete blood count, and a chest x ray. If liver enzymes exceeded 1.5 times normal, computed tomography (CT) of the abdomen was required. If low attenuation hepatic nodules suggested metastatic disease, fine needle aspiration biopsy of the liver tumours provided cytopathological confirmation.1
Positron emission tomography (PET) is a molecular imaging technique that uses radiolabelled molecules to image metabolic activity in vivo.2,3 When whole body PET was combined with computed radiographic tomography (CT), PET/CT put anatomy and function on the same page making practical a new era of physiological imaging.2–6
This study examines the ability of positron emission tomography combined with computed tomography (PET/CT) to allow for detection of previously occult metastatic melanoma.
A 77 year old woman presented with a 15.4×15 mm width and 13.2 mm high collar button shaped choroidal melanoma with a large secondary retinal detachment in her right eye. Her preoperative medical evaluation (including CT imaging of the abdomen) proved negative. She was treated by enucleation.
Two years later a follow up medical evaluation revealed elevated liver function studies (table 1) and a chest x ray showed a pleural effusion. CT of the abdomen with contrast revealed multiple low attenuation hepatic foci consistent with metastatic melanoma.
A PET/CT was requested. Fifty minutes after intravenous administration of 15.2 mCi of fluordeoxyglucose, whole body PET/CT imaging revealed enlarged para-aortic lymph nodes and a subcutaneous nodule in the abdominal wall (fig 1). The CT portion of the PET/CT also revealed two 3 mm nodules in the upper lobe of the right lung (too small to be visualised by PET imaging). PET imaging was able to reveal multiple bony metastasis that were not seen on the CT portion (fig 1). Both CT and PET showed a large liver metastasis, but CT was better at defining tumour size. Since it is a physiological assay, PET also demonstrated the metabolic activity of the metastatic tumours (fig 1).
In this case, whole body PET/CT was found to be capable of uncovering metastases not seen with abdominal CT alone. This led us away from considering regional perfusion of the liver, hepatic resection, and towards systemic treatment.7,8 Therefore, when PET/CT identifies extrahepatic involvement, it can have a significant impact on the management of patients with metastatic choroidal melanoma.
PET/CT could also be used for initial staging. Early detection of occult metastases offers the potential to avoid ineffective and expensive enucleations, radioactive plaques, proton irradiation, eye wall resections, or laser treatment.9,10 Local therapies would be abandoned in favour of systemic treatments.
Another issue related to PET/CT is cost. Up to five times more than CT of the abdomen, PET/CT is only covered (Medicare) for melanoma staging/restaging when the stage of the cancer remains in doubt after completion of conventional imaging (or if the clinical management would differ depending on the PET findings). Since PET/CT revealed extrahepatic foci in this case, it changed our clinical approach. There is little doubt about the improved ability of PET/CT to detect lesions; the real issue is cost and if the results will change outcomes.
This study goes one step further than CT, MRI, or PET alone. By combining whole body PET and CT, this examination joins anatomy and function in one examination (fig 1). The relative efficacy of PET/CT to locate metastases should be evaluated within the framework of a prospective study.
This work was supported by The EyeCare Foundation and Research to Prevent Blindness, New York, USA.
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