Melanoma (and other Cutaneous Malignancies, below)

Fast Facts:

• 1:40 Caucasians; 1:1000 Blacks
• Common among both young adults and older populations.
• Aggressive: Both hematogenous and lymphatic spread.
• Stage I: 5-yr survival ≈ 90%
• Stage IV: 5-yr survival 10-20%

Whole Body Imaging: 

• Because of the high incidence of distant (often very distant) metastases, most recommend scanning the entire body in melanoma cases (top of head to bottom of feet).  

Indications for PET/CT Scan: 

Diagnosis: 

• As melanoma lesions are visible and easily biopsied, there is no role for PET/CT in the diagnosis of melanoma.

Initial Staging: 

• Generally not used for Stage I & Stage II disease. Sentinel node biopsy is the standard of care for such cases.
• Valuable for patients with high risk of metastatic disease at time of diagnosis (changes management in > 30% of such patients).
• While PET/CT can be useful for staging regional disease, its true utility lies in its assessment of distant metastases.

Distant Disease: Most commonly to the skin and subcutaneous fat, lung, liver, brain and lymph nodes.

Assessing Response to Therapy & Prognosis: 

• Late Response: Assess success or failure of therapy, and ultimate outcome.
• Early Response: As alternative therapies are being developed, some clinicians are now utilizing PET/CT to identify non-responders, so that alternative treatments can be offered.

Recurrence & Restaging: 

• Valuable in restaging suspected recurrence.
• Distinguishing recurrence from post-therapeutic inflammation.
• Pre-treatment/surgical assessment of known recurrence.

PET/CT Appearance of Melanoma Lesions:

• Because primary melanoma skin lesions are visible and readily biopsied, nearly every melanoma patient will have already undergone surgical resection prior to PET/CT scanning.  As such, it is extremely rare to see the primary skin lesion. 
• Melanoma lesions (primary & metastatic) are amongst the most intensely avid cancers, and would be identifiable by your half-blind 94-year old grandmother.
• Because of their aggressive nature, metastatic lesions are often extensive and spread throughout the body.
• Multiple FDG-avid nodules within the subcutaneous fat are a common and fairly unique presentation of metastatic melanoma.

False Negatives:

• Brain, Pulmonary & Liver Metastases:  Because metastasis to these three organs are common, and because such lesions are often less than 8.0 mm in size (considered “beneath the resolution of PET”), some advocate MRI for any suspicion of liver or brain metastases, and CT for any suspicion of pulmonary involvement.

OTHER CUTANEOUS MALIGANCIES:

• Basal Cell Carcinoma
• Squamous Cell Carcinoma
• Merkel Cell Carcinoma
• Primary Cutaneous Lymphoma
• Cutaneous Metastases

Each of these malignant skin lesions is approached in the same manner as melanoma cases.

The primary skin lesion may or may not have been resected at the time of PET/CT imaging.  

FDG uptake is generally considered positive if its metabolic activity is greater than liver uptake (some radiologists instead use the lower threshold of mediastinal blood pool as a comparative reference).

As post-surgery/biopsy inflammation may cause significant FDG-uptake, PET/CT scanning is best delayed at least 8-weeks after surgery.