ESTRO 38 Abstract book

S156 ESTRO 38

motion and increased sensitivity to range uncertainties may affect target dose coverage and dose to OAR. This in silico planning study was performed to evaluate robustness and the potential of IMPT to reduce dose to OAR in locally advanced cervical cancer, compared to the current photon treatment (VMAT). Material and Methods Five weekly repeated CT scans from 5 cervical cancer patients treated with CHRT were available for this study. The target volumes, with the para-aortic region included, were delineated according to the EMBRACE II intermediate IGRT protocol and three different primary treatment plans were created for 25 fractions of 1.8 Gy RBE using RayStation 6.99: a two-field IMPT (2F), a four-field IMPT (4F) and a two-arc VMAT plan. Robustness evaluation using a 5-mm setup and 3% range uncertainty margin was performed. Each repeated CT scan was contoured and registered to the plan CT scan and subsequently the primary treatment plan was recomputed on each repeated CT scan. The voxelwise minimum dose (worst case dose) delivered to 98% of the GTV (D 98% GTV) and lymph nodes (D 98% nodes) as well as the nominal OAR doses were evaluated on each of the CT scans. The selected clinical relevant OAR dose parameters were bone marrow V 10Gy and V 20Gy [1], femoral head D mean [2], sacrum D mean [2] and D 50% [3] and whole bowel V 15Gy [4] . In addition, entire DVHs for the whole bowel were compared. Results IMPT (2F and 4F) and VMAT showed similar plan robustness with regard to target coverage (GTV and nodes), with a mean dose deviation from the planCT of -0.11 ± 0.43 Gy RBE (IMPT 2F), -0.40 ± 0.58 Gy RBE (IMPT 4F) and 0.04 ± 0.90 Gy RBE (VMAT) (Figure 1A). The DVHs of the whole bowel showed a lower mean dose in the range of 0 – 40 Gy RBE for the IMPT plans compared to VMAT (Figure 1B). Bone marrow V 10Gy and whole bowel V 15Gy were significantly lower for IMPT 2F and 4F compared to VMAT (Figure 1C and 1D). For both dose parameters the dose on the planCT was predictive for the resulting dose on the repeated CT. Mean bone marrow V 10Gy and V 20Gy , femoral heads D mean , sacrum D mean and D 50% and whole bowel V 15Gy were markedly lower in both types of IMPT plans compared to VMAT treatment plans (Table 1). IMPT 4F resulted in significantly lower bone marrow V 20Gy in comparison to IMPT 2F.

Conclusion Robustly optimized IMPT treatment plans for cervical cancer patients show equivalent target coverage robustness when compared to VMAT treatment plans, but offer significantly better OAR sparing. Potentially, this could reduce bowel and hematologic toxicity for this young patient population. OC-0306 Using CBCT and VelocityTM Software for delivered dose verification during head and neck radiotherapy L. Hay 1 , A. Duffton 1 , P. McLoone 2 , E. Miguel 1 , S. Currie 1 , C. Paterson 1 1 NHS Greater Glasgow and Clyde, The Beatson West of Scotland Cancer Centre, Glasgow, United Kingdom ; 2 The Institute of Health & Wellbeing, University of Glasgow, Glasgow, United Kingdom Purpose or Objective Anatomical changes during head and neck radiotherapy (RT) can increase dose to OAR and under dose target volumes. On-treatment dose verification with Image Guided RT can aid the reduction of set-up uncertainties, ensuring planned dose is delivered to the correct structures. Currently a mid-treatment (#16) CT2 scan is compared with the initial planning CT (pCT1) to verify dose distribution. IGRT using cbCT and deformable image registration (IR) may allow the development of a process to reduce the number of patients receiving #16CT2 scans during RT. The study aimed to determine the suitability of Velocity TM Software in creating synthetic CT (sCT) images for mid-treatment dose verification, in patients receiving RT for locally advanced SCC of the oropharynx. Material and Methods 20 patients treated with VMAT, 65Gy in 30#’s, underwent weekly cbCT scans #’s 1, 6, 11, 16, 21 and 26, post- treatment. Image registration between the pCT1 and cbCTs was undertaken and the structure set duplicated to the cbCTs. Within Velocity TM the pCT1 was deformed with the weekly cbCT volumes, and re-sampled to create a new primary sCT volume, which had the unit values of the pCT1 volume. IR between the cbCT and sCT1 was performed and the structures applied to the sCT volumes. VMAT #16sCT1 plans were re-calculated in Eclipse TM applying the pre-set values of the original plan optimisation. #16sCT2 verification plans were generated by the #16cbCT being deformed with the mid-point #16CT2, to compare plan