Early response evaluation using 18F-FDG-PET/CT does not influence management of patients with metastatic gastrointestinal stromal tumors (GIST) treated with palliative intent

Aim The aim of this study was to investigate the impact of 18 F-FDG-PET/CT on treatment decision making in metastatic gastrointestinal stromal tumor (GIST) patients. Methods This study retrospectively evaluated 18 F-FDG-PET/CT scans to monitor response of metastatic GIST patients treated with palliative intent. Data from the Dutch GIST Registry was used. Early scans (<10 weeks after start of treatment) and late scans (>10 weeks after start of treatment) were scored on the impact in change of treatment. Results Sixty-one PET/CT scans were performed for treatment evaluation in 39 patients with metastatic GIST of which 36 were early scans and 25 were late scans. Early PET/CT scans led to a change in management in 5.6% of patients and late PET/CT scans led to a change in management in 56% of patients. Change in management was more often seen after scans with lack of metabolic response (48% vs. 11% in scans with metabolic response, p=0.002). Neither metabolic response nor change in treatment were more often seen in patients with KIT mutations compared to patients with non- KIT mutations (metabolic response 65% KIT vs. 46% non- KIT, p=0.33, and change in management 28% KIT vs. 21% non- KIT, p=0.74). Conclusion 18 F-FDG-PET/CT is not recommended for early response evaluation in an unselected patient population with metastatic GIST, since it does not influence treatment decisions. 18 F-FDG-PET/CT, however, can be useful for late response assessment, especially in case of indeterminate CT results.