ESTRO 36 Abstract Book

S883 ESTRO 36 2017 _______________________________________________________________________________________________

We can conclude that the patients with HS mask have larger displacements than the H mask patients, but further investigations should be done. All errors are below 3mm. This result agrees with the literature for H&N displacements. In summary, HexaPOD couch in combination with daily CBCT can considerably improve the accuracy of patient positioning during VMAT treatment for H&N treatments. EP-1649 Comparison of two thermoplastic immobilization shells for frameless stereotactic radiotherapy I. Gagne 1 , S. Zavgorodni 1 , A. Alexander 2 , I. Vallieres 2 1 BC Cancer Agency - Vancouver Island, Medical Physics, Victoria, Canada 2 BC Cancer Agency - Vancouver Island, Radiation Oncology, Victoria, Canada Purpose or Objective The primary goal of this study was to compare the inter- fraction 6DOF corrections applied between patients immobilized with an open-face shell system and patients immobilized with a reinforced closed-shell system. The intra-fraction motion was also compared between these Sixty patients have been treated with frameless radiotherapy on a Varian TrueBeam STx linear accelerator equipped with a six-degree of freedom (6DOF) couch. All patients had a planning CT scan with an immobilization system that comprised of a CIVCO head cup, a CIVCO customizable pillow and a thermoplastic shell. An open- faced shell from CIVCO was used on the first 15 patients in anticipation of the ALIGN RT optical tracking system installation. A closed-faced shell from AUBO with extra reinforcement was employed on subsequent patients due to delays in the approval of ALIGN RT in Canada and larger than expected setup and intra-fraction motions with the open-faced immobilization system. Two pre-treatment CBCTs were acquired; the first to correct using 6DOF bone anatomy matching the initial inter-fraction setup error, the second to correct using 4DOF the residual setup error following the 6DOF couch moves. A post CBCT was acquired to determine the intra-fraction motion using 6DOF bone anatomy matching. Results Datasets from 12 patients with an open-faced immobilization shell and 29 patients with a reinforced closed-face immobilization shell were obtained for a total of 2451 CBCT scans. Table 1 summarizes the population average of the patient average and largest 6DOF corrections (vrt, lng, lat, pitch, roll, rtn) from CBCT1 for both the open-face and closed-face shells. Also included is the population average of the average and largest intra- fraction motion (vrt, lng,lat,pitch, roll, rtn) recorded from CBCT3 for both the open-face and closed-face shells. A student t-test for uneven sample and variance was applied to determine which parameters had statistically significant differences at the p=0.01 level. No statistically significant differences were found between the two patient populations when patient 6DOF correction averages, however, statistically significant differences were found between the two patient populations when the patient largest rotational corrections were used in the test. Statistically significant differences between the two patient populations were also noted for the patient largest 6DOF pitch and roll intra-fraction motion as well as the patient average 6DOF pitch intra-fraction motion. two groups of patients. Material and Methods

Conclusion Moving to a reinforced closed-face immobilization shell from an open-faced immobilization system has significantly reduced the magnitude of the rotational corrections as well as significantly reduced the magnitude of the pitch and roll motion errors. EP-1650 Setup uncertainty in head and neck assessed by a 1.5T MR-sim with thermoplastic mask immobilization Y.H. Zhou 1 , W.W.K. Fung 2 , J. Yuan 1 , O.L. Wong 1 , G. Chiu 2 , K.F. Cheng 2 , K.Y. Cheung 1 , S.K. Yu 1 1 Hong Kong Sanatorium & Hospital, Medical Physics & Research Department, Happy Valley, Hong Kong SAR China 2 Hong Kong Sanatorium & Hospital, Department of Radiotherapy, Happy Valley, Hong Kong SAR China Purpose or Objective This pilot study aims to prospectively assess the setup uncertainty in head and neck (HN) using thermoplastic mask immobilization on a dedicated 1.5T MR-sim for radiotherapy based on a cohort of healthy volunteers. Material and Methods 11 healthy volunteers received a total of 142 scans (each scanned 4-40 times) on a 1.5T MR-sim in a treatment position immobilized with customized 5-point thermoplastic mask with shoulder fixation to simulate HN- RT fractions. Volunteers were carefully aligned and positioned by RT therapists using a 3D external laser system. The imaging protocol consisted of a T1w SPACE sequence (TR/TE = 420/7.2ms, FOV = 470mm, 256 coronal slices, isotropic voxel size of 1.05mm, acquisition time ~5min, geometric distortion correction ON). Six Degree- of-Freedom rigid body registration based on normalized mutual information (sampling factor = 5%) were used to pair-wisely register images with respect to the first session for each subject. Systematic and random errors of translation and rotation in positional setup were calculated. One sample t-test and box-plot were used to evaluate positional variation of translation and rotation with a significance level of 0.05. Results The overall positional setup repeatability results were presented in Table I. The group mean translation was <1mm and mean rotation was <1º, respectively. The systematic error in LR, AP and SI translation was 0.48mm, 0.23mm and 0.53mm, respectively. The systematic error in roll, pitch and yaw rotation was 0.07°, 0.002° and 0.27°, respectively. The random error in corresponding direction was 1.83mm, 0.62mm and 1.88mm for translation, and 0.36°, 0.01° and 1.09° for rotation, respectively. 20 (~14.0%), 0 (~0%), and 25 (~18.6%) out of all 142 scan sessions had displacements >1mm in the LR, AP, and SI directions, respectively. Displacements >2 mm were seen only in the SI direction in one session (~0.7%). One-sample t-test showed significance of group mean error in AP translation ( p< 0.001) and all rotations ( p = 0.02 in roll and p <0.001 in pitch and yaw). The box-plot of