Early Response Evaluation By 18f-Fdg-Pet Influences Management In Gastrointestinal Stromal Tumor Patients Treated With Neo-Adjuvant Intent

Background: Early response evaluation by 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) is effective in gastrointestinal stromal tumors (GISTs) treated with imatinib and recommended in GISTs treated with neo-adjuvant intent. Yet, it is unclear whether this effects treatment decisions. Methods: All patients in the Dutch GIST Registry treated with imatinib with neo-adjuvant intent were identified. Only FDG-PETs made within 8 weeks after initiation or change (in dose or switch) of imatinib were included. Responses were derived from radiological reports and defined in 3 categories: 1) complete response; 2) partial response; 3) no response. Change in management was defined as a difference between pre-PET and post-PET treatment plans. Four categories were defined: change in 1) surgical management; 2) systemic treatment; 3) treatment objective (from curative to palliative); 4) management regarding a second tumor. Results: Seventy FDG-PETs for early response evaluation in 63 patients treated with neo-adjuvant intent were identified. Forty-one patients (65.1%) had a KIT exon 11 and 22 (34.9%) had a non-KIT exon 11 mutation (15 other and 7 unknown mutations). Of the 70 scans 64 (87.1%) had a baseline, 50 (71.5%) showed metabolic response (partial and complete), and 18 (25.7%) led to change in management. Change in management was strongly correlated with a lack of response (p < 0.001) and a non-KIT exon 11 mutation (p < 0.001). Mutational status and response were strongly correlated (p < 0.001). Out of 29 FDG-PETs conducted in non-KIT exon 11 GISTs, 15 (51.7%) led to change in management: 1 (3.4%) in surgical management, 6 (20.7%) in systemic treatment, 7 (24.1%) in both and 1 (3.4%) regarding a second tumor. Out of 51 FDG-PETs conducted in KIT exon 11 GISTs, change in management was seen 3 times (5.9%): twice in systemic treatment (dose increase after partial response was seen) and once regarding a second tumor. No change in treatment objective was seen. Conclusions: In contrast to GIST patients harboring a KIT exon 11 mutation, in non-KIT exon 11 mutated GISTs treated with neoadjuvant intent early response evaluation by FDG-PET often leads to change in management. Legal entity responsible for the study: Neeltje Steeghs Funding: Novartis, Pfizer and Bayer Disclosure: N. Steeghs: Research grant for the Dutch GIST Registry from Novartis, Pfizer and Bayer. All other authors have declared no conflicts of interest.