[Comparison of setup errors between two immobilization methods in prostate cancer radiotherapy based on cone-beam computed tomography].

Objective: To analyze and compare the interfractional setup errors between two body positioning fixation methods (lithotomy position with carbon fiber full-body fixation frame vs. conventional carbon fiber body fixation frame combined with thermoplastic membrane) in radical radiotherapy for prostate cancer, and to calculate the clinical target volume (CTV) to planning target volume (PTV) margin (MPTV) for both methods to optimize immobilization techniques and radiotherapy workflows.

Methods: A retrospective analysis was conducted on 37 consecutive patients who underwent radical prostate radiotherapy at Peking University First Hospital between August 2021 and March 2023. The patients were divided into two groups based on the immobilization method: Group A (18 patients, 450 CBCT image sets) used a carbon fiber whole-body fixator in the lithotomy position, while Group B (19 patients, 461 CBCT image sets) used a conventional carbon fiber fixator combined with a thermoplastic mask. All the patients underwent daily cone-beam computed tomography (CBCT) image guidance. Bone registration combined with manual registration was used to obtain the setup error data in the left-right (X), cranio-caudal (Y) and anterior-posterior (Z) directions. The positioning errors of the two groups were compared by using the independent sample t-test, the Mann-Whitney U test and the chi-square test. The average positioning error, systematic positioning error (Σ) and random positioning error (δ) were calculated, and the CTV-PTV extension distance was calculated by using the (MPTV=2.5Σ+0.7δ).

Results: The analysis of the setup errors in the three-dimensional direction showed significant differences between the two groups (all P < 0.01). Specifically, the median (quartile) absolute values of the errors in the X, Y, and Z directions of group A were [0.40 (0.20, 0.70) cm, 0.50 (0.30, 0.80) cm, and 0.35 (0.20, 0.60) cm], respectively. In group B, the corresponding values were significantly reduced to [0.20 (0.10, 0.40) cm, 0.40 (0.20, 0.70) cm and 0.20 (0.10, 0.40) cm]. The results of Mann-Whitney U test showed that the differences in each direction were highly statistically significant (X: z=-6.86; Y: z=-2.76; Z: z=-5.71). The cumulative distribution ratio of the setup error displacement within 0.5 cm in the X, Y, and Z directions in group A and group B were 297 (66.0%) and 408 (88.5%) (P < 0.01), 250 (55.6%) and 285 (61.8%) (P=0.055), 308 (68.4%) and 391 (84.8%) (P < 0.01), respectively. The CTV-PTV margins in three directions were X 0.66 cm in group A and 0.35 cm in group B; Y 0.67 cm and 0.45 cm; Z 0.54 cm and 0.42 cm.

Conclusion: Conventional carbon fiber human body fixator combined with thermoplastic film can significantly reduce the setup error. However, the carbon fiber whole-body fixator in the lithotomy position demonstrated larger setup errors in the X, Y, and Z directions compared with the conventional fixator combined with a thermoplastic mask, indicating the need for further workflow optimization. Given that the lithotomy position is essential for transperineal ultrasound-guided techniques, further research and improvements are required to achieve higher positioning accuracy.