ObjectiveTo investigate the impact of postoperative application of Pseudomonas aeruginosa injection on recurrence free survival (RFS) and overall survival (OS) in patients with abnormal serum calcitonin levels following surgery for medullary thyroid carcinoma (MTC). MethodsA retrospective collection of data was conducted for 214 patients with abnormal serum calcitonin levels following MTC surgery at West China Hospital of Sichuan University from January 2015 to April 2024. Propensity score matching (1∶2) was utilized to match patients’ data to reduce confounding bias, comparing RFS and OS between patients who used (Pseudomonas group) and did not use (control group) Pseudomonas aeruginosa injection. ResultsAfter propensity score matching, 72 patients with abnormal postoperative calcitonin levels were included, with 24 in the Pseudomonas group and 48 in the control group. The median follow-up time for the 72 patients was 66 months (11–168 months). The 1-year RFS rates for the Pseudomonas group and the control group were 100% and 75.0%, respectively, and the 2-year RFS rates were 87.5% and 56.3%, respectively. The RFS in the Pseudomonas group was superior to that in the control group (χ2=4.791, P=0.029). The 5-year OS rates for the Pseudomonas group and the control group were 90.9% and 93.5%, respectively, with no significant difference between the two groups (χ2=0.469, P=0.491). The Cox proportional hazards regression model indicated that the median RFS was extended in the Pseudomonas group [25 months vs. 21 months, RR=0.350, 95%CI (0.135, 0.900), P=0.029], but there was no significant impact on OS [66 months vs. 69 months, RR=2.22, 95%CI (0.229, 21.444), P=0.503]. ConclusionPostoperative use of Pseudomonas aeruginosa injection in MTC patients with abnormal serum calcitonin level shows significant improvement in RFS, but no significant change in OS.