Rectal Spacer Quality and Rectal Dose Sparing In Prostate Sabr: Evidence from a Prospective Clinical Trial
Poster
AAPM2026 Abstract 26298 POTENC SQS.pdf Hosted by AAPM · 0.6 MBAbstract
Purpose
Rectal spacers are widely used in prostate stereotactic ablative radiotherapy (SAbR) to reduce rectal dose; however, their value as a practical, planning-oriented QA tool remains underdefined. Using real-world clinical trial data, this study evaluates whether a clinically assigned spacer quality score (SQS), based on prostate–rectum separation on planning imaging, predicts achievable rectal dosimetric sparing in routine prostate SAbR planning.
Methods
A total of 110 prostate SAbR cases from a multi-institutional prospective clinical trial (POTEN-C) were retrospectively analyzed. Spacer quality was assessed using a clinically assigned spacer quality score (SQS), derived from geometric separation between the prostate/PTV and rectum on planning imaging. Rectal dosimetric endpoints—including rectal hotspot dose (D0.03cc) and high-dose volume metrics—were extracted and correlated with spacer quality to evaluate the planning and QA relevance of SQS. Analyses focused on identifying dosimetric trends attributable to spacer quality rather than spacer presence alone.
Results
Higher SQS were associated with improved rectal dosimetry, with higher SQS achieving lower rectal hotspot dose (D0.03cc) and reduced high-dose rectal exposure. Associations between SQS and early gastrointestinal (GI) quality-of-life (QoL) outcomes were weak overall; however, a clear threshold effect was observed. Patients with very low spacer quality (SQS = 0, e.g 0. Together, these findings indicate that SQS serves both as a planning-oriented quality assurance metric and as a threshold-based indicator of early GI QoL risk.
Conclusion
In a real-world prostate SAbR cohort, a clinically used spacer quality score provides a simple, planning-oriented QA metric that anticipates rectal dosimetric benefit. Beyond confirming spacer presence, spacer quality assessment informs planning decision-making by flagging cases in which achievable rectal sparing is constrained, supporting appropriate planning expectations in routine prostate SAbR practice.