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Objective: To explore the efficacy of adjusting the dose of imatinib dose in the context of therapeutic drug monitoring (TDM) in patients with gastrointestinal stromal tumors (GISTs) who are receiving adjuvant therapy after complete resection of their tumors. Methods: This was a descriptive study. Inclusion criteria were (1) complete surgical resection with a pathological diagnosis of GIST, (2) postoperative adjuvant therapy with imatinib and dosage adjustment, (3) multiple TDM of imatinib, and (4) complete clinical, pathological, and follow-up data. The data of 70 patients with GISTs treated at Union Hospital, Tongji Medical College, Huazhong University of Science and Technology between January 2015 and December 2023 were collected retrospectively. The study cohort comprised 15 (21.4%) men and 55 (78.6%) women of median age 60 years (range: 25-82). Of the eligible patients, 49 (70.0%) were at high-risk, 14 (20.0%) at intermediate-risk, six (8.6%) at low-risk, and one (1.4%) at very low risk. Patients were followed up by the gastrointestinal stromal tumor clinic every 2-3 months and their plasma concentrations of imatinib were checked. The dose was adjusted to 300 mg/d or 200 mg/d depending on whether they had had ≥ grade III adverse reactions, and whether the first plasma concentration of imatinib was ≥ 1,500 μg/L or between the expected range of 760 μg/L-1,100 μg/L. Studied indicators included adverse reactions, quality of life before and after dose adjustment, and overall survival and recurrence-free survival (RFS) after dose adjustment. Results: Before dose adjustment, all 70 patients received 400 mg of imatinib daily, with initial TDM values of 1,900 ± 568 μg/L, for a median duration of 8.3 months. After dose adjustment, 60 patients received 300 mg daily, with a TDM of 1,216 ± 350 μg/L, whereas 10 received 200 mg daily, with a TDM of 1,023 ± 269 μg/L. The median duration of treatment after dose adjustment was 23.4 months. Compared with those whose dosages were not adjusted, the incidence of bone marrow suppression was significantly lower (74.3% [52/70] vs. 51.4% [36/70], χ 2 =9.202, P =0.010); as were the incidences of edema (95.7% [67/70] vs. 50.0% [35/70], χ 2 =40.526, P <0.001); skin reactions (70.0% [49/70] vs. 32.9% [23/70), χ 2 =22.495, P <0.001); and gastrointestinal reactions (38.6% [27/70] vs. 10.0% [7/70], χ 2 =15.899, P <0.001) in those whose dosages were adjusted. The average total scores for physical health before and after dose adjustment were 76 ± 5 and 88 ± 4, respectively; whereas the mental health scores were 75 ± 6 and 89 ± 4, respectively. The median follow-up period was 36 months (range 6-126). During the first 3 years of follow-up, five high-risk patients with non-gastric GISTs developed recurrences. The 3-year overall survival rate was 100%, and the 3-year RFS rate was 92.8%, high-risk patients having a 3-year RFS rate of 89.8%. Conclusion: The adverse reactions and quality of life of GIST patients with severe adverse reactions to adjuvant imatinib therapy after complete resection can be mitigated by appropriately reducing the dosage of imatinib under the guidance of TDM.