Imaging of ocular melanoma metastasis

Ocular melanoma is the most common adult primary intraocular tumour. Although <1% of patients have metastatic disease at the time of initial diagnosis, most will develop metastasis at varying lengths of time. Metastasis surveillance is therefore critical in the follow-up of patients with ocular melanoma. Liver is the most common site of metastasis and prognosis is based on the treatment of liver metastasis. Hence, imaging of liver metastasis is vital. MRI is the most specific modality for imaging liver metastasis and is at least as sensitive as CT. Extrahepatic metastasis such as retroperitoneal nodules and bone metastases are also better evaluated on MRI. Gadolinium-based contrast agents are extremely helpful for detecting liver lesions. In particular, newer hepatobiliary contrast agents which offer an additional hepatobiliary phase of excretion help in the detection of even tiny liver metastases. Diffusion-weighted imaging is helpful when an i.v. contrast cannot be administered. Treated lesions are also better evaluated with MRI. CT is useful for evaluating lung nodules, large liver metastasis or in patients in whom MRI is medically contraindicated. The disadvantage lies in its inability to detect small liver metastasis and the radiation dose involved. The lesions treated with iodized oil as part of chemoembolization procedures can be followed on CT. Ultrasound can be used only for detecting hepatic metastases. However, it is heavily operator dependent, technically challenging and time consuming especially in patients who are large. Extrahepatic metastasis cannot be seen on ultrasound. Its utility is primarily for the biopsy of liver lesions. Positron emission tomography (PET)-CT can detect lung nodules and large liver lesions but is insensitive to small liver lesions. Moreover, the high radiation dose is a major disadvantage.