ESTRO 36 Abstract Book

S992 ESTRO 36 2017 _______________________________________________________________________________________________

comparison of treatment plans whether Hexapod is applied. Results The average errors of the patients’ position were Lateral (X-axis) direction of 0.1±1.4 mm, Longitudinal (Y-axis) direction of 0.0±1.4 mm, Vertical (Z-axis) direction of - 0.4±1.2 mm Pitch of -0.29±0.61°, Roll of -0.42±0.98° and Yaw of -0.53±0.98°. If the position error takes absolute value, average error on three directions of translation was 1.06±0.14 mm. Rotation error was 0.82±0.14° which is larger than the translation error. Through DQA evaluation, the average error rate of point dose in the case of rotational error existed is 0.89±0.012%. and in the case without rotational error is 0.24±0.015%. Gamma pass rate in the case without rotational error is 99.71±0.328% in average and in the case of rotational error existed is 89.33±3.874% which is 10% lower so it is statistically significant.(p<0.05) The mean values of dose difference on each ROI before and after rotational error correction in treatment plan are 2.17 Gy of Brain_max, 0.28 Gy of brain_mean, and –3.58 Gy, -4.43 Gy of Brain_stem max and mean respectively. Also, the value of Lt_Eye_max is 1.34 Gy and the value of Rt_eye_max is –0.71 Gy individually. There is dose difference whether correction of rotational error is existed or not. Conclusion When VMAT with Hexapod Couch is applied for patients with brain tumors, it is considered to increases reproducibility on patients positioning and treatment efficiency and at the same time, decreases side effects. EP-1838 First IGRT results for SBRT bone and lymph node oligometastases within the pelvic region. L. Wiersema 1 , G. Borst 1 , S. Nakhaee 1 , H. Peulen 1 , T. Wiersma 1 , M. Kwint 1 , A. Smit 1 , M. Romp 1 , P. Remeijer 1 , A. Van Mourik 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, radiotherapy, Amsterdam, The Netherlands Purpose or Objective Purpose: There is a growing demand for application of stereotactic body radiation therapy (SBRT) to oligometastatic disease, like bone and lymph node metastases. Based on our clinical experience with common SBRT sites (such as lung, spine and liver), a comprehensive set of treatment execution guidelines was developed for bone and lymph node locations eligible for SBRT. To our knowledge, we present the first combined IGRT positioning data of bone- and lymph nodes SBRT treatments in the pelvic region. Material and Methods Materials and Methods: The IGRT data for 32 patients treated with SBRT in the pelvic region for oligometastases were reviewed; 16 on gland and 16 on bone. Radiotherapy schedules ranged from 24 -45 Gy in 3 fractions to 25-50 Gy in 5 fractions. These patients were immobilized with a personal vacuum bag, knee-fix, head rest and arm support. The Gross Tumor Volume (GTV) was expanded with a 5mm Planning Target Volume (PTV) margin for bone and 7mm for lymph node treatments. All patients were treated on an Elekta linear accelerator, with 10MV and a coplanar, dual arc, volumetric Modulated Arc Therapy (VMAT) technique. A Cone Beam CT (CBCT) based online imaging protocol was used for set-up, couch correction verification and intra-fraction motion (IFM) assessment. Rigid registrations were performed on the bony anatomy adjacent to the GTV. If the residual translation setup error (i.e. after couch correction) was larger than 2 mm, the correction-verification procedure was repeated and if residual rotation setup errors were larger than 3° the patient was repositioned. The coverage of GTV within PTV was checked visually. To calculate the IFM, the difference between translation and rotation errors of the inline (i.e.

during treatment) or post treatment CBCT and the residual setup errors was calculated. Results Mean, systematic and random components of residual setup and intra-fraction errors (translations and rotations) are summarized in the table 1 for bone and lymph node cases. The correction-verification procedure was repeated in 3.8% and 10% of the fractions for bone and lymph node cases respectively. Conclusion: The setup and IFM errors of patients treated with SBRT for oligometastatic disease in the pelvic region (for bone or lymph nodes locations) are very small, demonstrating the reproducibility and robustness of the positioning protocol. Consequently, the contribution of these errors to the GTV- PTV margin is limited and margins may be reduced. For the lymph node locations, research is ongoing to improve image registration methods (e.g. shaped region of interest registration). EP-1839 Towards planning organ at risk volumes for rectum and bladder using cone beam CvT in prostate cancer. R. Seuntjens 1 , T. Convents 1 , G. De Kerf 2 , A . Sprangers 2 , K. Van Belle 1 , D. Verellen 2 , P. Dirix 2 1 Odisee University College- campus T erranova, School of Medical Imaging, Brussels, Belgium 2 Iridium Cancer Network GZA Sint-Vincentius, Department of Radiation Oncology, Antwerp, Belgium Purpose or Objective To analyze planning organ at risk volumes (PRV) for the rectum and bladder using daily cone beam computed tomography (CB-CT) images acquired during prostate radiotherapy. Material and Methods From February 2015 to October 2015, 18 consecutive prostate cancer patients received daily CB-CT imaging after routine set-up imaging. All patients had intermediate- to high risk disease and received 37 fractions of 2.0 Gy to the prostate (CTV_high) and 1.5 Gy to the seminal vesicles (CTV_low) through volumetric arc radiotherapy (VMAT). Treatment simulation (with both CT and MRI on the same morning) was performed according to a strict protocol: patients were advised to place a Microlax ® Fleet enema 1 hour before the appointment. At the same time, they were asked to empty their bladder and then drink 400 cc of water. Bladder voidance should then be avoided until the CT was taken. First, the CT was taken. If the rectum was too filled and/or the bladder was too empty, the patient was removed from the table and advised to empty the rectum and/or drink some more water. Afterwards, patients were advised to empty the bladder and again drink 400 cc of water. About an hour later, the MRI was performed at the radiology department. Before each treatment, the same protocol was followed but a Glycerin ® suppository was subscribed instead of the A total of 666 CB-CT’s were evaluated. All CB-CT images were deemed of sufficient quality to identify the CTV’s as well as the rectum & bladder. Both CTV’s as well as the bladder and rectum(from the lowest level of the ischial tuberosities to the connection anteriorly with the sigmoid) were delineated on all CB-CT’s. The manual delineation of each CB-CT took around 18 minutes in total. There was considerable individual variation in rectal and bladder volume on CB-CT’s during treatment for each patient (see Figure). The mean rectal volume on daily CB-CT (84.1 cc) was not significantly different from the mean rectal volume during simulation (86.0 cc) on a paired student t- test. The mean bladder volume on daily CB-CT (160 cc) was significantly larger than the mean bladder volume during simulation (140 cc, p < 0.01). There were no significant differences in CTV volumes. PRV margins were enema. Results