ESTRO 2023 - Abstract Book

S89

Saturday 13 May

ESTRO 2023

and 8 months, respectively. Five (36%) patient reached exceptional long tumor control (22-41 months (iCT), 18-32 months (SABR), two of them are still alive without any sign of progression. The dominant pattern of failure was distant metastasis (12/14). There was no Gr. ≥ 2 acute/late RT-related toxicity. Conclusion Linac-based pancreatic SABR with TI+ABH is feasible. Peritumoral fiducials improves utility, while DIBH-CBCT -preferably with half arc- provides the best image quality at a reasonable treatment time. The clinical efficacy is comparable with the literature, showing durable tumor control and favorable survival besides negligible RT side effect. OC-0127 The Role of a Radiopaque Hydrogel Spacer in Image-Guidance for Prostate Stereotactic Radiotherapy O. Icht 1 , S. Schlosser 1 , Y. Landman 1 , M. Rephael 1 , A. Moore 2 , D. Silvern 1 , D. Bragilovski 2 , D. Limon 1 , E. Fredman 1 1 Davidoff Comprehensive Cancer Center, Rabin Medical Center, Radiation Oncology, Petah Tikva, Israel; 2 Davidoff Comprehensive Cancer Center, Rabin Medical Center, Radiation Oncology, Petah Tikva, Israel Purpose or Objective Precise patient positioning with image guidance (IGRT) is essential for safe prostate stereotactic radiotherapy (SABR) delivery. We evaluated the potential of a CT-visible hydrogel spacer to serve as a surrogate fiducial marker for prostate IGRT. Materials and Methods Prior to planning prostate SABR, patients underwent placement of three intraprostatic gold markers for alignment and a SpaceOAR VueTM (SOv) to decrease rectal radiation dose, per standard practice. At treatment, table shifts based on cone- beam CT (CBCT) to the planning CT were initially performed by 3-dimensional alignment of the SOv (experimental arm), then again based on the fiducial markers (standard arm). The six directional shifts (three linear and three rotational) were recorded and the differences compared. Results 140 fractions across 41 consecutive patients were evaluated. Mean/median differences between SOv-based and fiducial- based alignment in linear (vertical, longitudinal, lateral) and rotational (rotation, pitch, roll) shifts were 0.9/0.6mm, 0.8/0.5mm, and 0.6/0.4mm, and 0.38/0, 0.62/0, and 0.35/0 degrees, respectively (table 1). Gland volume did not correlate with shift differences. A learning curve was observed, with significantly less variability between alignment methods in the second half of fractions studied (n=70) compared to the first half in five of the six axes – vertical (p=0.0032), longitudinal (p=0.0464), pitch (p<0.0001), roll (p<0.0001), and rotation (p<0.0001) (Table 1). Further, in the latter 70 fractions, no differences between alignment methods were recorded in 94.3%, 92.9%, and 92.7% of the three rotational shifts.

Conclusion For precise IGRT in prostate SABR, daily alignment with a readily visible SpaceOAR VueTM was highly comparable to intraprostatic gold fiducial markers, with an observed rapid learning curve. Utilizing a single device for a dual purpose may increase procedural efficiency, decrease risk, and lower cost, warranting further prospective study.

OC-0128 Quantifying the benefits of online daily adaptation for MR-guided RT in cervical cancer A. Alshamrani 1 , A. Choudhury 2 , C. Eccles 3 , M. Aznar 1 , R. Chuter 3 , P. Hoskin 4

1 The University of Manchester, Faculty of Biology, Medicine and Health, Manchester, United Kingdom; 2 The Christie NHS Foundation Trust, Clinical Oncology, Manchester , United Kingdom; 3 The Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom; 4 The Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom Purpose or Objective Online MR-adaptive RT enables daily replanning to adapt the changes seen in cervical RT, such as uterus-cervix position and rectum and bladder filling variability affording more accurate targeting of disease and reducing toxicity. This workflow requires considerable effort from the interdisciplinary team. The frequency of required adaptations on a daily basis has been reviewed. Materials and Methods Four women with stage FIGO stage IIB node-negative cervical cancer were included who received 45Gy/25 fractions. Treatment was according to the EMBRACE II guidelines planning on Monaco (v5.40.01) (Elekta, Stockholm) and using an adapt-to-shape (ATS) “MR-adapted” protocol. We compared the cumulative daily re-plans MR-adapt plan versus MR-guided plan. Doses to target structures (D99% to CTV_T_LR and CTV_E), and organs at risk (D0.1% to the bladder, rectum, bowel and sigmoid) were extracted for every fraction. MR-adapted plans were retrospectively analysed. MR-guided plans were created by transferring the dose from the reference plan to the daily MR image and then recalculating without adapting to the shape of the day. We then compared the dose for MR-adapted and MR-guided plans for each structure on a daily basis using paired two-tailed t-test; p<0 .05 was considered significant. For MR-guided plan, any structure exceeding the dose criteria >2% was considered an adaptation. Results 92 MR-adapted and 92 MR-guided plans were reviewed. Available fractions (fx) were (22, 24, 22, 24) for patients (Pt) 1 to 4 respectively. The remaining fx were delivered on a conventional Linac. Dose comparison and statistical results are shown