e23524 Background: The real-world data on outcomes of adjuvant therapy in high-risk gastrointestinal stromal tumors (GIST) are very limited. Methods: We have analyzed the data of 107 consecutive patients (52 male, 56 female) with GIST after resection treated with adjuvant imatinib (for planned 3 years with initial dose 400 mg daily, started not later than 4 months after operation) in 6 oncological centers in 2013-2018. All patients were required to have high risk of recurrence (at least 50% according to NCCN/AFIP criteria), known mutational status to exclude PDGFRA D842V mutants and KIT/PDGFRA-wild type cases from therapy. Median follow-up time was 24 months. Results: The most common primary localization of GIST was small bowel (63 patients; 59%), followed by the stomach (40 patients; 37%). The majority of GIST cases harbored exon 11 KIT mutations (88 cases, 82%), 11 cases had exon 9 KIT mutations (10%), 8 had other KIT/PDGFRA mutations potentially sensitive to imatinib. Thirty three patients (31%) finished 3-year adjuvant imatinib therapy as planned, 59 (55%) still continue therapy, 4 (4%) patients had finished adjuvant therapy prematurely due to toxicity, 6 (6%) due to disease progression on treatment and 5 (4%) due to other reasons. The disease relapse was detected in 16 patients, of them in 4 cases in exon 9 KIT mutants (36%), and 10 cases in patients with exon 11 KIT mutations (11%) [p < 0.05]. Estimated 4-year relapse-free survival (RFS) rate is 78%. Conclusions: The early results of adjuvant therapy with imatinib in routine practice outside clinical trials in high-risk mutation-drive GIST patients only confirm high efficacy of this therapy with better tolerability than in clinical trials. Moreover, overrepresentation of exon 9 KIT mutants in a group of patients with disease relapse may indicate that standard 400 mg dose in adjuvant treatment is not sufficient for prevention of disease relapse.
