The utility of PET-CT in predicting malignant potential of GIST

38 Background: The risk category of gastrointestinal stromal tumors (GIST) is usually determined by tumor size and mitotic count. Because of technical complexity and tumor heterogeneity in biopsy, accurate preoperative evaluation of malignant potential of GIST is very difficult. The aim of this study is to evaluate the utility of preoperative PET-CT examination for predicting the malignant potential of GIST. Methods: Ten patients with GIST who underwent preoperative PET-CT examination were enrolled. They were divided into two groups (low/intermediate-risk or high-risk) by risk category (tumor size, mitotic index). The relationships between the standardized uptake value (SUV) and GIST parameters including the size, mitotic index and Ki67 index were examined. Results: There was a significant correlation between the mitotic index and the SUV (p=0.029) but tumor size is not correlated with SUV. The SUV was significantly higher in the high-risk group (11.0 ± 3.04) than in the low/intermediate-risk group (2.1 ± 1.5). The Ki67 labeling index was also significantly higher in the high-risk group (8.63 ± 6.2) than in the low/intermediate-risk group (1.75 ± 0.52). There was a significant correlation between the Ki67 labeling index and the SUV (p=0.028). A mitotic index of 5/50 HPF was equivalent to an SUV of 4.3, and a Ki67 labeling index of 5 was equivalent to SUV of 6.3. SUV of 5 can predict the malignant potential between the high and low/intermediate risk. Conclusions: PET-CT can predict malignant potential. Cases with a high SUV, using a criterion of 5, might have malignant potential.