A hybrid IGRT workflow using SGRT and CBCT for prostate SBRT: Feasibility, efficiency, and safety

Safe delivery of prostate stereotactic body radiotherapy (SBRT) relies on precise target localization. Without access to real-time intrafraction motion management, careful optimization of IGRT protocols is necessary to safeguard treatment accuracy and patient outcomes.

An IGRT workflow is proposed that incorporates surface-monitoring (SGRT) to complement cone-beam CT (CBCT) imaging. The study evaluates 23 consecutive SBRT prostate patients who were treated on a prospective registry study. Each patient received pre- and mid-treatment and a subset received post-treatment CBCTs. The frequency and magnitude of SGRT triggered beam interruptions as well as treatment times were recorded.

The median number of CBCTs acquired per fraction was four and the median treatment time was 23 min (IQR 19-27). SGRT detected intra-fraction surface-based motion beyond a combined 4 mm vector isocenter tolerance in 62% of all fractions treated, with a maximum motion of 15 mm. On average < 2 beam interruptions were triggered by SGRT per treatment fraction. There was no statistically significant correlation between overall treatment time and SGRT-triggered beam interruptions (r = 0.048, p = 0.645). There was a weak but statistically relevant correlation of overall treatment time with the maximum detected motion (r = 0.23, p = 0.026). SGRT detected five fractions where the patients had persistently moved outside the SGRT tolerance, and for three of these (60%), a CBCT verified that the target was out of tolerance.

SGRT is a valuable tool that complements CBCT-based IGRT. An SGRT motion vector tolerance of 4 mm provides a pragmatic compromise between detecting patient motion and treatment efficiency. Overall, persistent patient motion during treatment was infrequent in this cohort, however, SGRT was able to detect several cases where the internal target was outside of the tolerance highlighting that patient monitoring with SGRT can contribute to improved quality and safety for prostate SBRT.