Whole body PET/CT imaging for detection of metastatic choroidal melanoma

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.

Case report

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.

Table 1.

 Patient characteristics

Physical	Examination	Blood examination	X ray	CT scan abdomen with contrast	Whole body PET/CT
					CT	PET
General	Icterus
Subcutaneous tissue	2 nodules in the anterior abdominal wall			No lesions noted	1.4 cm subcutaneous nodule in the anterior abdominal wall	Hypermetabolic focus in the subcutaneous tissue of the anterior abdominal wall
Nodes	None			No lesions noted	Enlarged para aortic lymph nodes on the left side	Hypermetabolic focus in the upper abdomen
Lungs	No abnormalities noted		Pleural effusion of right lung base. No nodules	Pleural effusion right lung base and calcified hilar nodes	Two small 3 mm nodules in the right upper lobe. Right pleural effusion	No foci noted
Liver	Enlarged	High bilirubin, AST, ALT, alkaline phosphatase, GGT		Enlarged. 2 areas of low attenuation in the anterior aspect of the right lobe	Low attenuation lesion >20 cm in greatest diameter in the right lobe with calcification seen posteriorly. Numerous low attenuation lesions throughout both lobes of the liver	Enlarged. Large hypermetabolic focus in the right lobe with mass effect. Numerous hypermetabolic foci throughout remainder of liver.
Kidney				8 mm cyst midpole and 2 cm cyst upper pole of left kidney	Large right renal cyst. Additional smaller cysts	Photopenic defect due to large right renal cyst
Bone				No lesions noted	No lesions noted	Hypermetabolic foci in the right skull base, left scapula, left humerus, sternum, multiple bilateral ribs, thoracic and lumbosacral spine, pelvis, and bilateral femurs

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).

Figure 1.

 On the left, the CT demonstrates the anatomy; on the right the PET shows areas of hypermetabolism (glucose uptake); in the middle the two images are fused. PET/CT revealed enlarged para-aortic lymph nodes and a subcutaneous nodule in the anterior abdominal wall. The PET imaging portion of the PET/CT was able to reveal multiple bony metastases that were not seen on the CT portion of the examination. Both CT and PET showed a large liver metastasis.

Comment

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.