Patient-based assessment with total lesion glycolysis may be associated with therapeutic effects of I-131 meta-iodobenzylguanidine (MIBG) radiotherapy in patients with metastatic neuroendocrine tumors

Background: 131I-labeled meta-iodobenzylguanidine (131I-MIBG) radiotherapy has tumor-progression preventive effects in patients with metastatic neuroendocrine tumors (NECT) such as metastatic pheochromocytoma and paraganglioma. 18F-fluorodeoxyglucose (18F-FDG) PET/CT has been used for therapeutic evaluation after 131I-MIBG radiotherapy on tumor metabolic activity. From a patient management point of view, treatment based on a whole-body parameter is important in evaluating therapeutic effects. Recently, 18F-FDG PET/CT-derived whole-body total metabolic tumor volume (WBMTV) and total lesion glycolysis (WBTLG) have shown prognostic value for various malignant tumors. However, the usefulness of these markers has not been evaluated in terms of the therapeutic effects of 131I-MIBG in patients with metastatic NECT. Therefore, this study aimed to evaluate whether WBMTV and WBTLG were useful to show the therapeutic effects of 131I-MIBG radiotherapy in patients with metastatic NECT.Method: Twenty patients (48.9 ± 14.7 years old, 9 male) with metastatic NECT prospectively had 150 mCi of 131I-MIBG radiotherapy. Based on post-therapeutic 131I-MIBG scintigraphy, patients were divided into 2 groups, namely those with positive 131I-MIBG uptake by metastatic lesions and those with negative uptake. 18F-FDG PET/CT was performed before and 3 months after the first 131I-MIBG radiotherapy. Assessment of 18F-FDG PET/CT-based parameters was performed using dedicated software. Maximum standardized uptake value (SUVmax) was estimated for metastatic lesions. The volume of interest (VOI) placed in the liver was used to determine a threshold value for volume-based analysis as follows: threshold = SUVmean + 3 × standard deviation (SD). WBMTV and WBTLG were calculated accordingly. Results: Among 20 patients, 15 showed positive 131I-MIBG uptake in at least one metastatic lesion. 131I-MIBG uptake was not observed in the remaining 5 patients. There was no significant difference in SUVmax, WBMTV and WBTLG between the two groups before 131I-MIBG therapy. With a lesion-based approach, the difference in %change in SUVmax was not significant between the 131I-MIBG positive and negative groups (-4.1 ± 38.4 % vs. 41.2 ± 168.9 %, P=0.089). With a patient-based approach, there was no significant difference in %change of WBMTV between the 2 groups (20.1 ± 24.3 % vs. 198.0 ± 271.1 %, P=0.060). In contrast, the 131I-MIBG positive group showed significantly lower %change of WBTLG than did the 131I-MIBG negative group (39.3 ± 198.1% vs. 222.5 ± 278.7 %, P = 0.036). Conclusion: In lesion analysis, SUVmax did not indicate significant disease progression in the 131I-MIBG negative group after 131I-MIBG radiotherapy. In contrast, with patient-based analysis, following 131I-MIBG radiotherapy, %change in WBTLG was lower in patients in the 131I-MIBG positive group than in those in the 131I-MIBG negative group, thus indicating the disease progression prevention effects of 131I-MIBG radiotherapy. Therefore, a patient-based approach may more accurately reflect the treatment effects than does a lesion-based approach using SUVmax. Therefore, this parameter may be useful for patient management in metastatic NECT.SNMMI 2019 Annual Meetin