ESTRO 2020 Abstract book

S1023 ESTRO 2020

uncertainties were investigated both from a geometric perspective, by evaluating the entity of structures deformation, both from a dosimetric perspective, by comparing organs at risks (OARs) sparing between the actual CIRT+IMRT treatment and an conventional comparison treatment plan. Material and Methods As part of a prospective phase II study (AIRC IG-14300, “Carbon ions boost followed by pelvic photon intensity modulated radiotherapy for high-risk prostate cancer”, registered at ClinicalTrials.gov as NCT02672449), high-risk non metastatic PCa patients were treated with a CIRT boost receiving 16.6 Gy in 4 fractions followed by whole- pelvis IMRT of 50 Gy in 25 fractions (2 Gy/fraction), with following long term Androgen Deprivation Therapy. Deformable CT image registration (DIR) was performed and corresponding dose was used for plan sum. A comparative IMRT photon plan was obtained as whole-pelvis IMRT of 50Gy in 25 fractions followed by a sequential boost of 28Gy in 14 fractions (both 2 Gy/fraction). DIR performances were evaluated through structure-related validated parameters calculated using ImSimQA software (v4.1, Oncology Systems Limited, Shrewsbury, UK) and image characteristics validated parameters using Plastimatch (an open‐source, BSD‐style licensed software). Results Until now five out of ten total enrolled patient have terminated the treatment and thus are subject of this study. Dosimetric parameters were lower in CIRT+IMRT than IMRT-only plans for all organs at risks (OARs) considered: rectum, urinary bladder, anal canal and penile bulb. Conversely, the IMRT-only approach allows for lower dose to femoral heads whereas in CIRT+IMRT plans (Table 1). Regarding DIR evaluation, femoral heads resulted, as expected, the less deformed OAR. Penile bulb, bladder and anal canal showed intermediate deformation. Rectum resulted the most deformed (Table 2).

Conclusion Particle therapy allows reaching a better sparing of OARs, which is expected to reduce induced-toxicity, while increasing the efficacy due to the higher radio-biological effect of C-ions. However a mixed beam approach could introduce DIR problems in case of multi-centric treatments with different operative protocols or, for instance, in image-guided adaptive radiation therapy. The future development of this prospective trial will lead to more mature data concerning the clinical impact of implementing DIR procedures in dose accumulation applications for high-risk PCa treatments. PO-1749 Contour propagation using Velocity deformable image registration for re-scanned head & neck patients R. Fullarton 1 , G. Kidane 1 , D. Withers 1 , C. Liz 1 1 Queen's Hospital, Radiotherapy Physics, Romford, United Kingdom Purpose or Objective Head and neck patients undergoing radiotherapy can often require re-planning during treatment based on changes in anatomy. Delineating target volumes and organs at risk for the adapted treatment plan is time consuming for clinicians and planners. The deformable image registration software, Velocity (Varian Medical Systems, Palo Alto, CA), offers an opportunity to shorten this process by propagating the contours from the original planning CT through deformable transformation to create a basis for the new structures on the new planning CT. Material and Methods A retrospective study was carried out for 10 head and neck patients that required a new plan part-way through their treatment course. The initial planning CT and the planning CT used for the re-plan, along with the associated structure sets, were imported into Velocity. A deformable registration between the two planning CTs was carried out and the contours propagated from the initial planning CT to the new planning CT through the deformation vector field. All contours based on anatomy, and present on both scans, were included but not all contours were present for all patients. The contours were compared to the clinician drawn contours using Dice Similarity Coefficient (DSC) [1] and the mean distance between the two contour surfaces (MSD). Results