Background: ¹⁸F-FDG PET/CT is useful for localising metastatic pheochromocytoma and paragangliomas (PPGLs). In other cancers ¹⁸F-FDG PET/CT is used for therapy monitoring. Actual tumour shrinkage on CT according to Response Criteria in Solid Tumors (RECIST) can be preceded by a decline in uptake of ¹⁸F-FDG. This study aimed to evaluate the predictive value of ¹⁸F-FDG PET/CT compared to RECIST for assessing response to systemic therapy in patients with metastatic PPGL.

Methods: 78 patients with progressive metastatic or non-resectable PPGL were prospectively included in FIRSTMAPPP (NCT 01371201), a randomised phase II study of sunitinib. Both high-dose CT and ¹⁸F-FDG PET/CT were performed at baseline and every three months until progressive disease (PD) was identified with RECIST 1.1. Response evaluation with ¹⁸F-FDG PET/CT was retrospectively performed according to PET Response Criteria in Solid Tumors (PERCIST) 1.0 and European Organization for Research and Treatment of Cancer (EORTC) criteria. Response was classiffied as: stable metabolic disease (SMD) or progressive metabolic disease (PMD). The best overall metabolic response (BoMR) was registered with both sets of criteria and compared.

Results: A total of 60 patients and 264 ¹⁸F-FDG PET scans were eligible for response evaluation. Agreement on BomR according to PERCIST and EORTC was observed in 83% of patients with corresponding κ-coefficient of 0.74. Response according to RECIST and PERCIST were concordant in 72% and disconcordant in 28% of patients. PMD coincided with PD in 30%, PMD preceded PD/SD in 59% and PD preceded PMD/SMD in 11% of patients. The median time to PD was 6 months versus 3 months to PMD (P<0.001).  

Conclusion: Metabolic response monitoring by ¹⁸F-FDG PET/CT adds essential information to treatment response monitoring in patients with metastatic PPGL, applying either PERCIST or EORTC criteria. In the majority of cases with progressive disease, this was detected earlier with ¹⁸F-FDG PET/CT than based CT-based RECIST criteria, in particular due to the detection of new bone metastases.