Interest of tumor to liver ratio 18F-FDG uptake in positron emission tomography for detection of neurofibrosarcoma in neurofibromatosis 1 patients.

Abstract

10049 Background: Malignant peripheral nerve sheath tumors (MPNSTs) are difficult to detect in neurofibromatosis 1 (NF1) individuals. The 18F FDG positron emission tomoscintigraphy (FDG PET) is a useful tool. The Standard Uptake Value (SUV) is the gold standard but it is subject to individual variation among patients and medical centers. So we assessed the interest of FDG PET based on a specific tumor to liver uptake ratio (T/L) instead of SUV to find a reproducible and constant cut-off for detecting malignant tumors. Methods: From 2000 to 2010, we conduct a multi centric study: all patients with NF1 and presenting clinical signs of MPNST suspicion (increased tumor size, induration or pain) underwent a FDG PET. Preliminary study estimated a T/L ratio of 1.5 as cut-off for malignity. Based on semi-quantitative analysis of FDG PET images, patients were compared with this cut-off value. Results: 98 patients with 125 tumors were included. FDG PET evaluation identified 42/125 suspected lesions with T/L > 1.5; among these 30 MPNST and 12 benign lesions were found. 83/125 tumors were classified as non-suspicious and 82 were actually benign according to histology or long term follow up. The 1.5 T/L cut off corresponds to 99 % Negative Predictive Value (NPV) and 71% Positive (PPV). The positive likelihood ratio (LR) was thus equal to 7, 69 and the Negative LR to 0, with 97% sensitivity and 87% specificity. When T/L ratio is < 1.5, MPNST is eliminated with 99% NPV, thus avoiding useless surgery. When T/L ratio is above 1.5, there is a strong suspicion of malignancy. But there is a risk of false positivity, which requires discussion before any therapeutic decision. On the over hand we found no significant correlation between SUVmax and malignancy. Conclusions: This study confirms the FDG PET in the detection of NF1-associated MPNST. The tumor/liver ratio with a cut off 1.5 has a very sensitive and specific value to sort out malignant from benign tumors and thus provides adequate surgery. More this semi-quantitative analysis method is just as easy as SUV, more sensitive and more reproducible.