Metabolic tumor volume and total lesion glycolysis on FDG PET are strong predictors for survival of patients with differentiated thyroid carcinoma - a comparison with SUVmax

Objectives Volume based parameters on FDG PET, such as metabolic tumor volume (MTV) and total lesion glycolysis (TLG), are useful indicators for predicting prognosis of patients with differentiated thyroid carcinoma. However, studies with large population have not been reported. In addition, although volume based parameters may be theoretically superior to maximum standardized uptake value (SUVmax) in terms of representing the whole-body tumor burden, comparative studies for volume based parameters vs. SUVmax are lacking. Therefore, in this retrospective and single-center study, we aimed to investigate a large number of patients to test the prognostic values of SUVmax, MTV, and TLG.

Methods We reviewed the hospital information system to extract patients who underwent FDG PET or PET/CT before receiving I-131 radioactive iodine therapy for differentiated thyroid carcinoma between January 2003 and December 2014. Of 819 patients found, we investigated 299 patients who showed visually positive FDG uptake in any metastatic lesions. The patients fasted for at least 6 hours before the injection of FDG (4.5MBq/kg) and underwent whole-body scanning 1 hour after the injection. For image analysis, any uptake masses with SUV蠅3.0 were extracted. Then, an experienced nuclear medicine categorized each uptake mass into tumor or non-tumor uptake. In case the tumor and non-tumor masses were connected, the non-tumor parts were carefully removed using a manual region-of-interest tool. MTV was the sum of the tumor volumes all through the body. TLG was the sum of [each tumor volume × each SUVmean] all through the body. To determine optimum cut-offs of SUVmax, MTV, and TLG, any considerable cut-off values were exhaustively tested using Kaplan-Meier method with log-rank test, and the best cut-off producing the maximum chi-square value was adopted. The univariate analysis was followed by the multivariate analysis using a Cox regression model.

Results Seventy-one of 299 (23.7%) patients died during the follow-up period, resulting in 5-year and 10-year survival being 82.55% and 54.43%, respectively. The best cut-offs of SUVmax, MTV, and TLG were 14.3, 15.5 ml, and 126.5 ml, respectively. In univariate analysis, age (P<0.01), metastatic site (P<0.0001), SUVmax (P<0.0001), MTV (P<0.0001), and TLG (P<0.0001) were strongly correlated with prognosis. Sex (P=0.04) and histopathological type (P= 0.04) were significant but relatively weak predictors. MTV and TLG were not strongly correlated with SUVmax (R= 0.29 and 0.33, respectively), whereas the correlation between MTV and TLG were very strong (R=0.97). The multivariate analysis including age, metastatic site, SUVmax, and MTV demonstrated that MTV was the strongest independent prognostic factor (P<0.0001), followed by age (P=0.0022) and SUVmax (P=0.0140). When TLG was included in the multivariate analysis instead of MTV, TLG was also the strongest independent prognostic factor (P<0.0001).

Conclusions MTV and TLG are strong prognostic factors for patients with differentiated thyroid carcinoma before receiving I-131 therapy. Thus, MTV and TLG should be taken into consideration for risk stratification in patients with differentiated thyroid carcinoma.

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