ESTRO 2024 - Abstract Book

S2532

Clinical - Urology

ESTRO 2024

Material/Methods:

The study included 26 prostate SBRT patients treated at our institution between June 2019 and May 2021, and the TPUS was used for 130 fractions. The real-time prostate displacement readings were assessed for each treatment minute. TPUS recorded the shifts regarding couch and room relatives in three directions: superior-inferior (SI), left right (LR), and anterior-posterior (AP). The median prescription dose was 40Gy to the High-Dose planning target volume (HD-PTV) in five fractions with 5 mm planning margins. The percentage of the time prostate movements in various directions was calculated for each fraction for all patients. The ultrasound image was adequate for prostate delineation and motion tracking, and the ultrasound probe did not compromise the contouring of targets or critical structures. For comparing the intra-fraction motion, a student t-test was used, and the categorical variables were analyzed using the Chi-square test. A p-value of <0.05 was considered statistically significant.

Results:

The mean setup time for each fraction under ultrasound-guided prostate SBRT was 9.1 ± 4.6 min, and the total treatment time per fraction was 12.3 ± 7.6 min. The ultrasound probe did not compromise the contouring of targets or critical structures. For intra-fraction motion, prostate movement exceeded 2 mm tolerance in 5 of the 26 patients for 19 of 130 fractions, and the mean percentage of time when the prostate moved more than 2 mm in any direction during each fraction was 6% (0-58%). The intrafraction monitoring resulted in a mean prostate displacement of (-0.12 ± 0.31) mm, (0.18 ± 0.57) mm, and (-0.22 ± 0.45) mm in the SI, LR, and AP directions, respectively. The prostate moved less than 2mm in 99.7%, 98.9%, and 98.7% of the treatment time in the SI, LR, and AP directions, respectively. In the real-time tracking for every second recording, the recorded median changes were 0.04, 0.02, and 0.09 mm in SI, LR, and AP directions, respectively. All displacements were within the predefined safety margins, ensuring good target coverage. The average PTV margins were 1.9, 1.3, and 0.9 mm in AP, SI, and LR directions.

Conclusion:

TPUS-guided prostate SBRT is an excellent treatment delivery measure for intra-fraction motion monitoring. The most significant variation of intrafraction prostate displacement was observed along the AP direction. The prostate moved within a few millimeters throughout most of the treatment time; however, the prostate position was within the clinical safety margins. Furthermore, the clinical safety margins for delivering SBRT to the prostate can be further decreased to less than 2mm, especially in the SI and LR directions with intra-fraction motion monitoring.

Keywords: Prostate SBRT, Clarity TPUS, intra fraction motion

2462

Poster Discussion

Is 25Gy in 5 fractions to the pelvis the right dose? Pooled outcomes from the SHARP consortium