ObjectiveTo investigate the clinical feasibility of optical surface imaging (OSI) system in guiding thermoplastic body film to fix radiotherapy of pelvic tumors.MethodsFrom January to May in 2019, 20 pelvic tumor patients fixed with thermoplastic films were selected. After the first treatment, cone beam CT (CBCT) was performed to verify and correct the setup errors. OSI was used to collect the patient’s image information as the reference image for the later treatment setup. The later treatment was performed by traditional three-point setup, and OSI was used to record the setup data and monitor the treatment. CBCT scan results were taken as the standard, to calculate the system error (Σ), random error (σ) and margin of three-point and OSI setup. The result of CBCT scan was defined as three-point setup error.ResultsIn lateral-medial, superior-inferior, and anterior-posterior directions, the OSI system errors were 0.14, 0.37, and 0.14 cm, respectively, the random errors were 0.20, 0.57, and 0.23 cm, respectively, and the corresponding margins were 0.49, 1.32, and 0.51 cm, respectively; the three-point setup system errors were 0.13, 0.39, and 0.12 cm, respectively, the random errors were 0.15, 0.43, and 0.13 cm, respectively, and the corresponding margins were 0.43, 1.28, and 0.39 cm, respectively. If the target coverage rate was counted at the 0.8 cm outward margin, the proportions of the three-point setup in lateral-medial, superior-inferior and anterior-posterior were 100.00%, 80.65%, and 100.00%, respectively, and those of the OSI setup were 100.00%, 95.48%, and 99.35%, respectively. Pearson analysis showed that the weakly correlation of lateral-medial, superior-inferior directions had statistical significance (P<0.05), and there was no statistical significance in the other directions (P>0.05).ConclusionsCompared with the traditional three-point setup, OSI cannot improve the setup precision of thermoplastic body film fixation in pelvic tumor radiotherapy. OSI acquired the image of thermoplastic phantom, which is quite different from the actual target location of the patient. OSI application method should be improved in clinical application.