ESTRO 38 Abstract book

S1095 ESTRO 38

p=0.5). Dmax to spinal cord was increased by 1.6Gy (40.3 Gy to 41.90Gy ; p=0.2). Conclusion Online CBCT correction ensures better coverage of targets (PTV) while reducing doses to normal tissue. There was significant reduction in PTV coverage if translational shifts wouldn't have been applied during the course of radiotherapy. CBCT-based online correction also increased the accuracy of IMRT in Head and neck cancer patients and provides scope to reduce irradiated margins, by decreasing the setup errors. EP-2002 Prostate treatment planning for the MR-linac: effect of online performance on template development R.A. Mitchell 1 , A. Dunlop 1 , A. Pathmanathan 2 , S. Nill 1 , A.C. Tree 2 , U. Oelfke 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom ; 2 The Royal Marsden NHS Foundation Trust and The Institute of Cancer Research, Clinical Oncology, London, United Kingdom Purpose or Objective The MR-linac (Elekta AB, Stockholm) requires creation of an offline reference plan in Monaco 5.4 (Elekta) to enable propagation of planning parameters into an online workflow and adaptation of the existing plan. Consequently, the effect of parameters defined offline must be evaluated in the online setting during plan template development. We report on two areas for PRISM trial (NCT03658525) prostate planning: plan robustness with a-priori information and timing. Material and Methods Monaco’s constrained optimisation mode prioritises organs-at-risk (OAR) over target dose coverage; setting generic, effective OAR sparing IMRT objectives may adversely affect target coverage. Application of patient- specific objectives in Monaco, to lower OAR doses (Figure 1), is possible in our workflow using a-priori information derived from backup plans created upfront in RayStation 7.0 (RaySearch Laboratories, Stockholm). Two prostate patients, each with CT scan and four T2-weighted diagnostic MR scans, were used. CTVs (prostate and seminal vesicles), bladder and rectum were clinician- delineated, and RayStation and offline Monaco plans generated as per protocol. Monaco bladder and rectum IMRT objectives were set 5% higher than equivalent DVH values achieved by the backup plans. Online plans were generated on each MR image using an adapt-to-shape workflow (ATS); reference plans are adapted to each MR scan’s anatomy. Further ATS plans were created using objective values at 10% above the backup plans to compare online plan robustness. Planning time is affected by the number of shape changes during segment shape optimisation (SSO); more changes (SSO loops) may improve plan quality but with increased time, which is practical offline. Online, patients would be on-set for longer, with increased probability of intra- fraction motion. A reference plan was created for a prostate patient. Two ATS plans were generated on each of four MR scans, using our offline setting of ten SSO loops and then five to derive a suitable number for use online.

Conclusion We demonstrated that, using our IGRT protocol with daily online corrections, anatomical changes, noted especially for patients with a shorter interval between chemotherapy and RT, affect target coverage in only a small proportion of the RT fractions. In addition, among the two uncorrected setup errors, pitch and yaw, only pitch is correlated to dose delivery changes. The clinical impact of these changes is likely negligible. EP-2001 Efficacy of CBCT guided IMRT for Head and Neck cancers and its dosimetric impact on other structures. K. Talapatra 1 , C. Medichelme 1 , R. Chauhan 1 , P. Chadha 1 , V. Mhatre 1 , V. Muthu 1 1 Kokilaben Dhirubhai Ambani Hospital and Medical Research Institute, Radiation Oncology -, Mumbai, India Purpose or Objective Online adaptive correction in image-guided intensity- modulated radiotherapy appears to be a promising approach for precision radiation treatment in head and neck tumors. This study is to evaluate the effect of online cone beam computed tomography (CBCT) guidance in Intensity Modulated Radiotherapy of Head and Neck Cancers. Material and Methods 38 Head and Neck cancer cases treated at our institute were selected for this study from prospectively maintained data (May 2017 to May 2018). Most of the cases were of Ca hypopharynx (23 cases). The Varian NovalisTx system which integrates an On Board Imager (OBI), was used to deliver radiation treatment . Daily/alternate day CBCT images were acquired and registered to the planning CT for online and offline analysis and to correct the patient setup errors. The setup errors were noted after matching of CBCT with planning CT images. New plans were generated in each patient using the mean lateral , vertical and longitudinal shifts. This DVH was compared to the original planning DVH and doses delivered to the target and various critical structures were evaluated. and the difference in doses received were computed. Statistical analysis was done using paired t- test. Results The mean setup errors in three directions were : 1.4mm longitudinal , 1.2mm vertical and 1mm lateral. Analysis of dosimetric change due to a translational isocenter shift , if no correction was applied showed : Mean PTV1 V95% coverage to be dropped to 94.95% from the original of 96.37% , if shifts weren't applied (p<0.0001 with standard error 0.007 and 95% CI -1.4331 to -1.406). Similarly PTV2 V95% coverage reduced by -1.06% (97.39% to 96.33%) with p<0.0001(standard error of 0.006 and 95% CI -1.0724 to -1.0476) . There was also considerable dosimetric changes to critical organs as follows : ipsilateral parotid mean doses were increased by 0.54 Gy (42.06Gy to 42.60 Gy ; p=0.94) ; Contralateral parotid mean doses were increased by 1.74Gy (28.12 to 29.96Gy ;