ESTRO 2025 - Abstract Book

S3130

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Results: Of patients treated, four had reduced GTVmb1/mb2 DIL-microboost volumes (ART platform: n=3 MR-linac, n=1 CBCT-based), and two with complete response had DIL-microboost omitted. For the partial responders, offline GTVmb delineation based on interim response PSMA-PET was integrated into the ART workflow, enabling pre contouring/registration with reference image-set without time pressure and improving adaptive planning efficiency. Online adaptation using updated targets and OARs ensured precise treatment customization. DIL GTV volumes substantially decreased from an initial mean of 11.4 cc (GTVinitial) to 4.1 cc (GTVmb1) and 3.0 cc (GTVmb2), generally correlating to degrees of biochemical (PSA) response. Dosimetric comparisons ( Figure 2 ) showed meaningful reductions in DIL-microboost GTV volumes and in bladder/rectum OAR doses, while max urethra doses remained relatively stable. The online ART platform effectively facilitated simulation-free plan re-optimization, ensuring efficient integration into clinical workflows while addressing daily anatomical variations.

Conclusion: Initial prospective evaluation of PSMA-PET response-guided SAbR DIL-microboost in HRPCa is associated with substantial reductions in OAR doses and DIL volume targeted, using existing commercially available MR- and CBCT based ART systems. Correlations to toxicity, impact of increasing time between SAbR fractions, and confirmation of retained tumor control will be assessed in long term follow-up. Keywords: PSMA PET, Adaptive Radiotherapy, SABR