ESTRO 36 Abstract Book

S895 ESTRO 36 _______________________________________________________________________________________________

Material and Methods Dataset for a total of 945 CBCT scans were obtained from 30 patients (13 with H mask and 17 with HS mask). For each fraction, patients were placed on the HexaPOD robotic couch and a CBCT was performed. For interfraction accuracy evaluation, the daily variations of the three principal axes (X, Y and Z) and three rotational movements (pitch, roll, and yaw) were extracted. Also, the type of thermoplastic mask was recorded. The following parameters were calculated: the mean of the setup corrections (M), the standard deviation (random error, σi), the group random error (σ) defined as the mean of all the individual patient random error σi and the systematic group error (∑) defined as the standard deviation of all the means measured for each patient. Results Results are shown in Table I. The overall mean displacements are larger for the HS mask patients than for the H mask. Also a trend toward a positive antero- posterior direction (1.19 mm) was observed in translational displacements for the HS mask patients. This could be explained taking into account the differences between the simulation and treatment couches as well as the accuracy of the room lasers. The remaining error components did not show any trend. For the rotational directions, the bigger error was in the pitch direction for both the H mask (0.76) and HS mask patients (0.85). Regarding the systematic group error, we have found a larger error for the HS mask patients (2.34) again toward a positive antero-posterior direction. The random group error shows the same behavior as well. Conclusion 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 two groups of patients. Material and Methods

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.

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