Age / Sex : 51 / M

Age / Sex : 51 year old, male

Chief complaint :  An abnormal hot uptake on the PET/ CT for routine health evaluation

metastatic malignant melanoma

Discussion

Findings:  MRI shows an ovoid soft tissue mass in the left axilla. T1WI show internal high signal intensity area in relation to muscle. T2WI and STIR Images show internal amorphous low signal intensity. Fat saturated contrastenhanced T1WI show heterogeneous internal enhancement.

Diagnosis:  malignant  melanoma, metastatic  

Discussion:

Metastatic melanoma could be categorized into melanotic and amelanotic groups on the basis of specific MR imaging pattern. Melanoma metastases exhibit a varaiable MR pattern that is dependent on several factors. Stable free radicals (indole semiquinones and semiquinones) within the melanin pigment are paramagnetic and effect a shortening of T1 and T2 relaxation times. Unpaired eletrons in free radicals or, more likely, chelated metal ions present in melanin may be a secondary cause of this proton relaxation enhancement after dipole-dipole interactions. Therefore, the expected signal pattern for melanotic melanoma is hyperintensity in relation to muscle on T1WI, hypointensity in relations to fat on T2WI. The expected signal pattern for amelanotic melanoma is similar to that for other soft tissue tumor: hypointensity or isointensity in relation to muscle on T1WI and hyperintensity or isointensity in relation to fat on T2WI.  In addition, the presence of hemorrhage within many melanoma metastses may contribute blood products that alter signal patterns. The appearance of hemorrhagic lesions on MR images depends on the age of the hematoma and the type of pulse sequences used.

Malignant melanoma can metastasize to any part in the body, including the brain, the gastrointestinal tract, and the myocardium. It is well known that malignant melanoma is one of the most avidly FDG-accumulating tumors.  With exception of the brain, whole-body FDG PET is a very sensitive and effective imaging modality for staging in patients with a high likelihood of metastases (Breslow thickness 2 mm, known metastases). Surgical resection is the treatment of choice for regional lymph node metastases or a single distant metastasis. If multiple metastases are present, only palliative symptomatic therapy is indicated.  In patients in whom surgery is planned, whole-body PET should be performed to exclude occult metastases. At many institutions, combined PET/CT is important for planning minimally invasive surgery for small lesions.

References:

Isiklar I, Leeds NE, Fuller GN, Kumar AJ. Intracranial metastatic melanoma: correlation between MR imaging characteristics and melanin content. AJR AM J Roentgenol 1995;165:1503-12