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How morphometric analysis of metastatic load predicts the (un)usefulness of PET scanning: the case of lymph node staging in melanoma

Background: In primary cutaneous melanoma, the sentinel node (SN) biopsy is an accurate method for the staging of the lymph nodes. Positron emission tomography (PET) has been suggested as a useful alternative. However, the sensitivity of PET may be too low to detect SN metastases, which are often small. Aim: To predict the value of PET for initial lymph node staging in melanoma based on morphometric analysis of SN metastatic load, without exposing patients to PET. Material and methods: In 59 SN positive patients with melanoma, the sizes of tumour deposits in the SNs and subsequent dissection specimens were measured by morphometry and correlated with the detection limits of current and future PET scanners. Results: The median tumour volume within the basin was 0.15 mm3 (range, 0.0001–118.86). Seventy per cent of these deposits were smaller than 1 mm3. State of the art PET scanners that have a resolution of about 5 mm would detect only 15–49% of positive basins. Logistic regression analysis revealed no pretest indicators identifying patients expected to have a positive PET. However, the SN tumour load was a significant and single predictor of the presence of PET detectable residual tumour. Conclusion: Morphometric analysis of metastatic load predicts that PET scanning is unable to detect most metastatic deposits in sentinel lymph nodes of patients with melanoma because the metastases are often small. Therefore, the SN biopsy remains the preferred method for initial regional staging.