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11535 Background: Life-long tyrosine kinase inhibitor (TKI) treatment is recommended for patients (pts) with metastatic GIST. We investigated whether highly selected pts with metastatic GIST may remain in durable complete remission after imatinib discontinuation. Methods: In this 1-group, prospective, multicenter phase II trial (NTC02924714) selected pts with overtly metastatic GIST discontinued imatinib treatment. Eligible pts had been treated with imatinib > 5 yrs for oligo-metastatic (≤3 metastases) disease, had no GIST progression and had undergone either R0/R1 resection or radiofrequency ablation (RFA) of all metastases, and had no longer detectable metastases on CT/MRI imaging, The primary endpoint was 3-yr progression-free survival (PFS), overall survival was a secondary endpoint. We estimated to achieve an improvement from 15% to 35% in 3-yr PFS compared to pts treated in the BRF-14 trial with a sample size of 31 pts. Results: The trial closed early due to slow accrual. Between January 5, 2017, and June 5, 2019, 12 pts (males, 7) with a median age of 67 yrs (range, 50-85) were enrolled. All pts had their primary tumor surgically removed (stomach, 4; small bowel, 6; large bowel/rectum, 2). Seven pts had KIT exon 11 mutation, 2 KIT exon 9 mutation, 1 PDGFRA exon 12 mutation, 1 had no mutation in KIT/PDGFRA, and 1 pt had unknown GIST mutational status. Eight pts had liver metastases, 3 peritoneal metastases, and 1 had both. Prior to enrollment all pts were in macroscopic complete remission after surgical metastasectomy (n = 7), RFA (n = 2), or both (n = 1), and 1 pt achieved complete radiological response with imatinib. At enrollment, 8 pts had imatinib 400 mg/d and 4 had < 400 mg/d. The median time of imatinib treatment before study entry was 8 yrs (range, 5-17). After imatinib discontinuation 7 (58%) pts progressed; the median time to progression was 10 mos (range, 2-31 mos). Five (42%) patients were progression-free after a median follow-up time of 42 mos (range, 30-59 mos). Median PFS was 23 mos and the estimated 3-year PFS 39%. Patients who progressed all achieved a second remission after restarting imatinib and were alive after a median follow-up time of 48 mos (range, 32 to 61 mos). Conclusions: The findings suggest that discontinuation of imatinib is safe in highly selected patients with oligo-metastatic GIST and that such patients may survive without detectable GIST progression for several years. The findings need to be viewed with caution due to the small pt numbers. Further study on imatinib discontinuation seem warranted. Clinical trial information: NCT02924714.