(P 31) NEOADJUVANT IMATINIB IN LOCALLY ADVANCED GASTROINTESTINAL STROMAL TUMORS (GIST) IS EFFECTIVE AND SAFE: RESULTS FROM A PROSPECTIVE SINGLE CENTER STUDY WITH 108 PATIENTS

Objective: Neoadjuvant imatinib is considered for gastrointestinal stromal tumors (GIST) when decreased tumor size provides less extensive surgery and higher R0 resection rates. This study evaluates the effectivity and safety of neoadjuvant imatinib for large or locally advanced GIST.

Methods: From the prospective database of the Dutch GIST Consortium, all patients who underwent surgery after neoadjuvant imatinib at our center between 2009 and 2022 were selected. Independent and blinded assessment of surgical strategy was performed by two surgeons, based on anonymized CT-scans before and after neoadjuvant imatinib.

Results: Of 113 patients that received neoadjuvant imatinib, 108 (95%) (mean age 61.6 (SD 11.5), 54% male) underwent a GIST resection. Of all GIST, 67% was localized in the stomach and in 25% in the duodenum or small intestine. In 74% of the GIST, a KIT exon 11 mutation was found. Decreased tumor size was seen in 95 (88%). Having a KIT exon 11 mutation (OR=5.64 [CI 95% 1.67–19.1], p< 0.01) or not having a mutation (OR=0.19 [CI95% 0.04–0.89], p=0.04) were positive and negative predictive values for partial response, respectively. In 55 (51%) patients, there was de-escalation of surgical strategy after neoadjuvant imatinib. Surgical complications were documented in 16 (15%) patients (n=8 grade II, n=5 grade IIIa, n=3 grade IIIb) and R0 resection was accomplished in 95 (89%) patients. The 5-year disease free- and overall survival were 80% and 91%, respectively.

Conclusion: This study shows that neoadjuvant imatinib is effective and safe for patients with large or locally advanced GIST.