ESTRO 2025 - Abstract Book

S3223

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

Figure 2. Surface-tumor position correlation summary.

Conclusion: Our study found a constant drift in the vertical direction over the 60 minute acquisition time with SGRT due to stress and relaxation. While SGRT is effective for monitoring targets related to the surface like breast and lung tumors, it does not fully account for internal prostate position due to variable patient anatomies, requiring additional PTV margins for accurate targeting without CBCT.

Keywords: surface guided RT, intra-fractional motion, ART

1316

Proffered Paper Evaluating the dosimetric impact of intrafractional motion for prostate cancer patients treated with MR guided online adaptive radiotherapy Shu Xing 1 , Nima Hassan Rezaeian 1 , ChengCheng Gui 2 , Victoria Brennan 2 , Ergys Subashi 3 , Neelam Tyagi 1 1 Medical Physics, Memorial Sloan Kettering Cancer Center, New York, USA. 2 Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, USA. 3 Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, USA Purpose/Objective: To evaluate the impact of intrafractional motion on the dose distribution and clinical outcome in prostate cancer patients, treated with MR-guided adaptive radiotherapy on a 1.5T MR-linac system. Material/Methods: Fifty-five prostate cancer patients treated with 1.5 T MR linac, received five-fraction stereotactic radiotherapy of 40 Gy to the prostate and 45 Gy to the dominant intraprostatic lesion. Multiplanar 2D cine MR images were acquired at 1.6 frames/s during treatment. The first 90 frames were used to create a template. A motion trace capturing center of-mass shifts in anterior-posterior (AP), superior-inferior (SI), and left-right (LR) directions was generated by registering the subsequence images to the template. The average and maximum shifts along all directions were analyzed for each treatment fraction. The daily planned dose distribution was shifted based on the motion traces. The cumulative shifted dose distributions were averaged over all frames to produce a motion-blurred dose distribution. The difference between the planned and motion-blurred dose distributions was calculated. The 3D motion displacement, accounting for motion in all three directions, was analyzed to evaluate its dosimetric impact on prostate CTV, bladder, and urethra dose metrics. In addition, urinary toxicities including frequency, retention, incontinence, and hematuria were evaluated for these patients using the Common terminology criteria for adverse events (CTCAE), with Grade > 2 classified as clinically significant. CTCAE was collected within 6 months (acute toxicity) and beyond 6 months (long-term toxicity) post-treatment.