The receiver operating characteristic (ROC) curve for classification of 18F-NaF uptake on PET/CT

Objectives To assess the cutoff values established by ROC curves to classify 18F-NaF uptake as normal or malignant.

Methods PET/CT images were performed 1 hour after the administration of 185 MBq of 18F-NaF. Volumes of interest (VOIs) were drawn on 3 bone regions: right umeral proximal diaphysis (UD), right femoral proximal diaphysis (FD) and first vertebral body (VB1) in a total of 254 patients, totalizing 712 VOIs. The uptake in the VOIs was classified as normal or malignant based on the pattern of the radiopharmaceutical distribution and the CT images. A total of 675 volumes were classified as normal and 52 were classified as malignant. Thirty five VOIs classified as undetermined or non malignant lesions were excluded from analysis. The standardized uptake value (SUV) measured in the VOIs were plotted in an ROC curve for each one of the three regions. It was calculated the Area Under the ROC (AUC) as well as the best cutoff SUVs to classify the VOIs. The best cutoff values were established by two methods: the SUV value with higher result of the sum of sensitivity and specificity, and the one with higher specificity value with 100% sensitivity.

Results The AUCs were 0,933, 0,889 and 0,975 for UD, FD and VB1, respectively. The best SUV cutoffs based on the higher result of the sum of sensitivity and specificity were 9,0 (sens.: 73%; spec.: 99%), 8,4 (sens.: 79%; spec.: 94%) and 21 (sens.: 93%; spec.: 95%) for UD, FD and VB1, respectively. The best SUV cutoffs based on the higher specificity value with 100% sensitivity were 3,7 (sens.: 100%; spec.: 55%), 4,1 (sens.: 100%; spec.: 29%) and 14,4 (sens.: 100%; spec.: 59%) for UD, FD and VB1, respectively.

Conclusions The best cutoff value varies from bone region of analysis and it is not possible to establish one value for the whole body. The area under the ROC curve also varies for each region. A further analysis is necessary comparing the uptake value in malignancy and non malignant bone lesions