Background and Objectives: The purpose of this study is to determine the tumor volume threshold for successful positron emission tomography (PET) imaging of melanoma nodal metastases. Methods: Review of a clinical series of patients who had FDG-PET imaging of regional lymph node basins followed by lymphadenectomy. Lymph node tumor volumes were calculated from direct measurements of metastatic nodule(s) in formalin fixed specimens. PET scan interpretations were correlated with histology to determine sensitivity. Sensitivity was correlated with the aggregate lymph node tumor volume in the nodal basin and with AJCC stage group. Results: Forty-five patients with 49 pathologically positive regional nodal basins comprised the study group. Median total basin tumor volume was 28.3 mm3 (range 0.004-22,879 mm3). FDG-PET sensitivity for detection of all tumor volumes was 0.49. The observed 90% sensitivity threshold for detection of nodal metastases was ≥78 mm3. PET sensitivity was 0.14 for detection of tumor volumes <78 mm3. PET sensitivity differed by prescan AJCC stage: I - 0.0; II - 0.24; III - 0.81; IV - 1.0 (P < 0.001). Conclusions: FDG-PET reliably detects lymph node tumor deposits greater than approximately 80 mm3 volume, but sensitivity falls rapidly below this. This amount of tumor is most likely to occur in patients with AJCC stage III or IV disease.
- Lymph nodes
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