disease is more frequent among whites that nonwhites and the incidence rises with increasing age. Diagnosis is established by biopsy, either excisional or by a core-punch technique. Important histopathologic factors include the lesion thickness (Breslow’s microstaging) and level of invasion (Clark’s microstaging).
Staging is according to the tumor microstaging as adopted by the American Joint Committee on Cancer (AJCC). Melanoma can metastasize to almost any organ site. However, metastatic spread to the regional lymph nodes, lung, liver, bone, and brain is more common. Wide surgical excision is standard treatment for early-stage disease. Surgical excision of the regional nodes, as directed by mapping the sentinel lymph node, and accessible isolated distant metastases are also indicated in selected patients. Radiation therapy is considered for bone, brain, skin, and soft tissue metastases. There is a lack of effective chemothera- py, but immunotherapy with a variety of immunological and biological agents has been promising for metastatic disease. The five-year survival rates are 92.5%, 49%, and 17.9% for early disease (stages I and II), stage III, and stage IV disease, respectively.
The optimal management of melanoma depends on the accurate deter- mination of the extent of disease. Whole-body FDG PET is cost-effective in the imaging evaluation of patients at high risk for metastatic disease (Breslow thickness greater than 1.5 cm). FDG PET impacts the clinical management of patients with melanoma by detecting unknown metastases prior to planned surgical intervention. PET can also be useful in evaluating treatment response.
In fact, FDG PET is evolving as a standard diagnostic imaging tool in patients with melanoma and is an approved indication by the Centers for Medicare and Medicaid Services.
FDG PET provides more accurate assessment of the extent of disease in comparison to conventional imaging techniques, leading to significant alter- ations in treatment planning (Fig. 1). From the referring physician’s perspec- tive, FDG PET has a major impact on the management of patients with melanoma by either down-staging or up-staging the disease leading to changes in treatment strategy in up to 53% of patients. However, false-positive results may occur with unrelated benign and malignant lesions, infection, and inflammation. PET may also miss small-volume disease in the lungs and the brain due to spatial resolution limitations. PET may be falsely negative in detecting micrometastatic lymph nodal disease. Sensitivities of 22% and 100%
have been reported for PET and sentinel node biopsy, respectively, for detecting lymph node metastatic involvement in ≥1-mm Breslow thickness melanoma.
One study determined the tumor volume threshold for successful PET imaging
of melanoma nodal metastases. The observed 90% sensitivity threshold for
detection of nodal metastases was ≥78 mm
3. The sensitivity of PET was only
14% for detection of tumor volumes <78 mm
3. In another study, PET detected
100% of metastases ≥10 mm, 83% of metastases 6–10 mm, and only 23% of
metastases ≥5 mm. Therefore, FDG PET can reliably detect lymph node tumor deposits greater than approximately 80 mm
3in volume, which is most likely to occur in patients with AJCC stage III and IV disease, and cannot substitute for sentinel lymph node mapping and tissue sampling.
Figure 2: A 77-year-old man with a history of resection of left scalp melanoma who now presents with exophytic local recurrence and metastatic left cervical lymph nodes. There is some misregitration of the brain activity in the fused PET-CT image.
Figure 1: Melanoma Satellite Lesions. Sequential CT/FDG-PET slices in a 58-year-old woman with a 16-mm ulcerated melanoma with microsatellite lesions on pathology.
Adjacent to the sit e of wide surgical excision, three FDG-avid subcutaneous nodules are identified. On the left are the CT slices; in the center are the corresponding attenuation- corrected FDG-PET slices; on the right are the corresponding uncorrected FDG-PET slices.
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Figure 3: A 74-year-old man with a history of an excised melanoma of the upper back.
He now presents with advanced metastatic disease involving the left supraclavicular nodes, both hila, lungs, numerous bones, and subtotal replacement of the liver.
Figure 4: Necrotic Lymph Node Metastasis. On the left is a non-contrast CT and in the center is an FDG-PET from a PET/CT scan. On the right is a contrast CT from six weeks earlier. The necrotic left inguinal lymph node shows peripheral FDG uptake.
In a study that reviewed the published literature between 1980 and 2000, an overall sensitivity of 74–100% and specificity of 67–100% were reported.
PET was considered particularly valuable when surgical intervention was being considered and for clarification of the abnormal radiological findings at follow- up. In another, more rigorous systematic review and meta-analysis of the diag- nostic accuracy of FDG PET in cutaneous melanoma that included 11 studies, the pooled sensitivity and specificity of PET were reported to be 79% (95% CI:
66–99%) and 86% (95% CI: 78–95%). Subgroup analysis revealed that PET was more accurate for systemic staging than for regional staging.
In summary, FDG PET provides a single imaging tool for surveying the whole body in patients with melanoma and is superior to CT for the detection of metastatic disease. Although PET cannot detect micrometastases, this is a common limitation with any imaging modality. In patients at high risk for metastases (Breslow thickness more than 1.5 mm), PET may be performed first
A
B
Figure 5: Melanoma Bone Metastases. A. focal FDG uptake is seen in the sacrum corresponding to a lytic lesion on CT. (Intense activity is also seen in a normal left ureter.) B. Focal FDG uptake is seen in the L1 vertebral body in association with a sclerotic lesion on CT.
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Figure 6: Mesenteric Melanoma Metastases. FDG-PET/CT abdominal slices showing three mesenteric melanoma metastases.
Figure 7: Melanoma Metastasis in Muscle. FDG-PET/CT in a patient with a prior history of recurrent melanoma shows intense focal uptake in the posterior neck corresponding to an asymmetric muscle mass. On the left is the CT; in the center is the attenuation-corrected PET; on the right is the non-attenuation-corrected PET.
to evaluate for distant metastatic disease. In patients with negative PET studies, sentinel lymph node mapping and tissue sampling may be performed to assess for micrometastatic nodal disease. In patients with PET demonstration of extensive disease, therapeutic decisions may be made in a cost-effective manner without additional diagnostic work-up.
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