ESTRO 2020 Abstract book

S1082 ESTRO 2020

to be implemented online or offline. The purpose of this work is to define an offline ART procedure capable of guaranteeing a correct replanning in prostatic treatments according to objective evaluation parameters. Material and Methods The treatment protocol of prostate patients involves emptying the rectum and filling the bladder. Daily checks are performed using CBCT images. The IGRT protocol involves the rigid fusion of the images acquired in the bunker with those derived from the CT simulation. For the study, 18 prostate patients were selected (medium and low risk) The offline ART workflow required the use of pretreatment daily images, compared with simulation images and calculating changes in rectum and bladder filling. The analysis was carried out using Velocity v4.0 software (Varian Medical System) on patients subjected to replanning (Group B) and others (Group A). At the end of the automatic deformable registration, adapted images between TC and CBCT (aCT) were made available. Contouring of OAR on aCT took place automatically. In order to allow an effective quantitative comparison, the coefficient of DICE and statistical indices of dispersion and distribution have been taken into consideration. Results Dispersion of percentages linked to the volume of the rectum is greater in the cases related to group A (IQR=5.72%; Q1=‐3.98%; Q2=‐0.66%; Q3=1.74%) while in group B (IQR=5.05%; Q1=‐2.66%; Q2=0.02%; Q3=2.39%). Distribution of bladder percentage changes in group A produced IQR=11.80% (Q1=‐10.12%; Q2=‐5.49%; Q3=1.67%) while in group B IQR=9.07% (Q1=‐3.57%; Q2=0.95%; Q3=5.51%). The DICE coefficient in group A showed a daily overlap of the bladder on average equal to 0.92 ± 0.13 while in group B, 0.93 ± 0.07. The volume of the rectum, in both groups, had an average Dice coefficient equal to DICE coefficient is a useful index to establish whether localization of the volumes is superimposable to CT sim data. Therefore, while performing an Offline ART workflow, at least every five fractions of therapy there should be a verification, in order to estimate in good time replanning needs. Since DICE coefficient does not consider volumes but only their geometrical overlapping, it is advisable to carry out also a check of OAR volumes averages, especially for what concerns the bladder, which is more subject to changes of this type, rather than to variations in spatial localization. 0.85 ± 0.14 Conclusion PO‐1848 Non‐invasive Cardiac Radioablation for Ventricular Tachycardia K. Pilling 1 , M. Wilkinson 1 , N. Richmond 1 , C. Walker 1 , L. McQuillan 1 , R. Brooks 1 , A. Ogilvie 1 , E. Shepherd 2 , D. Greenhalgh 2 , N. Seller 2 , R. McStay 3 , P. Atherton 1 1 Newcastle upon Tyne Hospitals NHS Foundation Trust, Radiotherapy Dept. Northern Centre for Cancer Care, Newcastle upon Tyne, United Kingdom ; 2 Newcastle upon Tyne Hospitals NHS Foundation Trust, Cardiology dept, Newcastle upon Tyne, United Kingdom ; 3 Newcastle upon Tyne Hospitals NHS Foundation Trust, Radiology Dept, Newcastle upon Tyne, United Kingdom Purpose or Objective There is a large body of experience from the field of radiation oncology in delivering Stereotactic ablative radiotherapy (SABR) to extremely small targets with very high accuracy. However, recent evidence suggests that SABR may have an emerging role in the treatment of patients with abnormal heart rhythms, Ventricular Tachycardia (VT), a non‐ malignant condition with high mortality. (1,2,3,4) Using SABR to treat VT represents a novel treatment alternative for cardiac patients too sick for invasive treatments or in whom conventional therapies, including invasive cardiac catheter ablation, have failed.

Conclusion The inter‐fraction variation of rectal volume was low. From our results we conclude that more effort should be done in bladder preparation. Either adaptive RT to account for bladder variations or a lower bladder filling on planning CT should be explored. Performing ultrasound before the treatment could be useful to assess bladder filling in all treatment fractions. PO‐1847 ART in prostate cancer: how to use an offline approach with daily CBCT and deformable registration L. Capone 1 , F. Cavallo 2 , F. Lusini 1 , D. Di Minico 2 , B. Nardiello 1 , N. Leonardo 1 , P. Gentile 1 1 UPMC San Pietro FBF, Radiotherapy, Rome, Italy ; 2 UPMC Hillman Cancer Center Villa Maria, Radiotherapy, Mirabella Eclano, Italy Purpose or Objective AdaptiveRT is a consolidated clinical practice, especially in some types of treatments subject to rapid changes due to Organs at Risk (OAR) that influence the movement of the target. The process of adaptability of the treatment to the single case combines tools such as the acquisition of pretreatment images, the clinical evaluation of the real need for adaptation, the planning of the new treatment and the guarantee of the final quality of this process. Modern radiotherapy equipment allow different strategies