ESTRO 2022 - Abstract Book

S1644

Abstract book

ESTRO 2022

The recent introduction of SGRT (Surface Guided Radiation Therapy) in clinical practice allows to check real time patient’s positioning using a thermal camera, avoiding the use of ionizing radiations. The goal of our multicentric study is to investigate the advantages in the use of SGRT systems during the pre-positioning phase in SBRT lung treatments, comparing four different set-up protocols in terms of efficiency and accuracy. Materials and Methods SBRT Lung treatments represent a kind of procedure that requires great accuracy in set-up with a longer lead time compared to others conventional treatments due to the complexity of immobilization systems molding and setting. Three highly specialized radiotherapy centers belonging to the same network, analyzed 4 different initial set-up protocols with different immobilization systems in SBRT Lung treatments. A total number of 12 cases were selected, 3 for each protocol. Among the protocols used for the set-up, two out of four foresee the use of the SGRT as an initial experience for the real-time correction of the patient's set-up in the pre-positioning phase: • PROTOCOL 1: using SGRT • PROTOCOL 2: using 3 points and automated shifts to isocenter with SGRT real time check and corrections • PROTOCOL 3: using 3 points and manual shifts to isocenter; • PROTOCOL 4: using 3 points and automated shifts to isocenter. To assess the advantages in using SGRT in the patient's initial set-up, in terms of accuracy, data related to the corrections applied with CBCT following the initial pre-positioning were collected and analyzed. In terms of efficiency, positioning times were recorded for each of the four protocols adopted in the centers, from patient access to bunker to the start of CBCT acquisition. Results Shifts after SGRT setup and CBCT were 0.45 cm AP, 0.38 cm SI, 0.17 cm RL, 1.77 degrees PITCH, 0.57 degrees ROLL, 1.10 degrees YAW (average values for Protocol 1); 0.27 cm AP, 0.32 cm SI, 0.44 cm RL, 1.54 degrees PITCH, 1.55 degrees ROLL, 1.28 degrees YAW (Average values for protocol 2). Shifts after pre-positioning without SGRT were 0.44 cm AP, 0.33 cm SI, 0.29 cm RL, 1.43 degrees PITCH, 1.13 degrees ROLL, 0.85 degrees YAW (average values for Protocol 3); 0.44 cm AP, 0.27 cm SI, 0.40 cm RL, 1.10 degrees PITCH, 1.70 degrees ROLL, 0.60 degrees YAW (average values for Protocol 4). The average of positioning time recorded for SGRT set-up protocols were 2’14’’ for PROTOCOL 1, 3’ for PROTOCOL 2. The average of positioning time recorded for no SGRT set-up protocols were 1’49’’ for PROTOCOL 3 and 4’30’’ for PROTOCOL 4. Conclusion No significant gains were found with the use of the SGRT in terms of accuracy, as for each protocol CBCT detected shifts do not exceed 0.5 cm for translational axis and 2 ° for rotational ones. Regarding efficiency, the use of SGRT software has not still shown a real gain in terms of time saving but reduces the possibility of occasional mistakes sending manual shifts from the 3 points to the treatment isocenter. 1 The University of Manchester, Faculty of Biology, Medicine and Health, MANCHESTER, United Kingdom; 2 Christie NHS Foundation Trust, Clinical Oncology, MANCHESTER, United Kingdom; 3 Christie NHS Foundation Trust, Radiotherapy, Manchester, United Kingdom; 4 Christie NHS Foundation Trust, Clinical Oncology, MANCHESTER, United Kingdom; 5 Christie NHS Foundation Trust, Clinical Oncology, MANCHESTER, United Kingdom; 6 The University of Manchester, Faculty of Biology, Medicine and Health, MANCHESTER, United Kingdom; 7 The Christie NHS Foundation Trust, Clinical Oncology , MANCHESTER, United Kingdom Purpose or Objective Online adaptive radiotherapy based on MR Linac (MRL) can potentially address inter and intra-fractional changes using images acquired daily during treatment. We report the intra- and inter-fractional movement for the first 25 fraction cervical cancer treatment delivered on the MRL in the UK. Materials and Methods Following institutional approval, a woman with stage FIGO-IIB node-negative cervical cancer was consented to the MOMENTUM trial (NCT04075305) for 25 fractions of radiotherapy. Treatment was prescribed according to the EMBRACE II guidelines using adapt-to-shape (ATS) workflow. A comfortably full bladder was required for treatment. Three MRIs were acquired at each fraction: a setup image ( used for plan adaptation), a verification (verif) image (acquired immediately prior to ‘beam on’) and a post-treatment (post-treat) image (to assess intra-fractional changes). A single observer contoured the bladder and CTV on all available verif and post-treat MRIs. Intra-fraction changes in bladder and CTV were determined by measuring the volume differences between the setup and verif images and the volume differences between the verif and post-treat images. The inter-fraction changes in CTV and bladder volumes were determined by measuring the difference between the verif images from each fraction and the MRI used for initial treatment planning. Intra-fractional changes were reported using Dice coefficients (DC) and inter-fraction changes reported using volume changes in cm 3 . Statistics have been presented as descriptive statistics and boxplots. Results 22 fractions and 67 MR images were available for review. The mean treatment time was 68 min (range 51–82 min) from the time the patient entered the room until the patient left. The median time from the setup image to verif image was 43.5 min (range 31-72 min). The median time from verif image to post-treat image was 13 min (range 4-40 min). The median intra-fraction DC for the CTV was 0.62 (range 0.40-0.80) and 0.90 (range 0.57-0.95) for the setup to verif and verif to post- treat images respectively. The median intra-fractional DC in bladder volume from setup to verif images and from verif to PO-1853 Assessing the inter and intra-fraction changes utilising MR Linac for cervical cancer A. Alshamrani 1 , R. Chuter 2 , C. Nelder 3 , A. Choudhury 4 , L. Barraclough 5 , C. Eccles 6 , M. Aznar 6 , P. Hoskin 7