ESTRO 38 Abstract book

S1200 ESTRO 38

also subjectively experienced differences in handling the neck rests from the two manufactures. The staff found the material in neck rest A easier to handle because the amount of material was more appropriate. The amount of material in neck rest B resulted in too much spare material around the head which could contribute to steep gradients. The volunteers experienced neck rest A more comfortable and provided better support. Some of the volunteers observed smell from neck rest B in their clothes and hair for hours. Conclusion Challenges in customizing the individually neck rests for positioning brain tumor patients for proton therapy can be solved by being alert to the indentation on the HN board, when customizing the neck rest. The study showed no significant displacement using either of the neck rests. The staff found neck rest A was easiest to handle and without smell and most of the volunteers found neck rest A more comfortable. EP-2172 Evaluation of the pitch functionality and setup accuracy of the Solstice SRS Immobilization System C.L. Ong 1 , K. Hunnego 1 , F. Gescher 1 , J. Franssen 1 , E. Franken 1 1 Hagaziekenhuis, Radiotherapy, Den Haag, The Netherlands Purpose or Objective For stereotactic treatment of brain metastases, a good patient immobilization is crucial to ensure a highly accurate dose delivery with a limited PTV margin of 1-2 mm. Various commercial products offer great solutions for patient fixation and in combination with online daily imaging, translational and yaw-rotational setup errors can be corrected. Pitch-rotational setup errors correction is however only feasible with a 6D-couch, which is not commonly available in all institutes. The purpose of this study was to investigate the accuracy of the new Civco Solstice TM SRS immobilization system which allows manual corection of the pitch-rotational setup errors. Material and Methods The solstice mask comprised of a carbon fiber head support, customizable cushion, thermoplastic mask and a thermoplastic mouth-bite. A CT-scan of a Kyoto head phantom fixated with the Solstice mask was acquired (fig 1). Two spacers were position at the anterior and posterior side of neck area in order to allow a controlled pitch- rotational motion of the phantom within the mask. At the LINAC, CBCT’s were acquired and registered to the CT in order to determine the accuracy of the mask. Subsequently several pitch-rotational setup errors were introduced within the mask and a CBCT was acquired. Based on the match value, the pitch setup error was manually corrected with the head support (fig 1) and a new verification CBCT was made. Furthermore, this mask was also applied on one patient receiving whole brain irradiation. This patient was also fixated using thermoplastic mask and the customized cushion and mouth bite. For this patient, two CBCT’s were acquired before the treatment and one after. All scans were automatically registered on bony anatomy.

Results The match values of CBCT with planning-CT with the phantom placed within the mask un-rotated were smaller than 0.1mm/° in all directions. After the phantom was rotated within the mask, CBCT match showed pitch- rotational errors of 1.7° and -3.7° (Fig. 2). After manual adjustment of the pitch-rotation on the head support, the verification match showed rotational setup error of ≤0.1° and translational setup error of ≤0.2mm (Fig.2). The rotational match values of the CBCT are independent of the location of the correction reference point and correspond to the pitch-rotational axis of the head support. For the first patient, the pre-treatment and post- treatment verification CBCT showed translational setup errors of ≤0.4mm and ≤0.3mm, which is close to the accuracy of the couch motion accuracy of 0.2mm.

Conclusion Manual adjustment of pitch-rotational setup error is feasible with the Solstice mask. This correction can be clinically importance for SRS treatment as it permits a more frequent use of single isocenter to treat patients with multiple brain metastases. The use of single isocenter has shorter treatment time and therefore is more patient friendly and reduce the risk of interfraction motion. These preliminary results warrant further investigation on more patients. EP-2173 Bladder filling in patients undergoing prostate radiotherapy on the MR-linac G. Adair Smith 1 , T. Herbert 1 , R. Lawes 1 , H. Creasey 1 , A. Dunlop 2 , A. Mitchell 2 , A. Pathmanathan 1 , L. Bower 3 , I. Hanson 2 , D. McQuaid 2 , R. Huddart 1 , U. Oelfke 2 , S. Nill 2 , A. Tree 1 , H. McNair 1 1 The Royal Marsden NHS Foundation Trust, Radiotherapy, Sutton, United Kingdo; 2 The Institute for Cancer Research, Medical Physics, Sutton, United Kingdom; 3 The Institute for Cancer Research, Radiotherapy and Imaging, Sutton, United Kingdom Purpose or Objective The first patient in the UK was treated for prostate cancer on the Elekta Unity (Elekta AB, Stockholm, Sweden) in September 2018. Due to the longer treatment times, departmental guidance for bladder filling was altered. An audit is being undertaken to evaluate if the bladder filling guidance is appropriate and reproducible; and to determine the rate of bladder filling and the effect on dose constraints across three time points during a treatment session. Material and Methods The first cohort of patients, under PRISM (Prostate Radiotherapy Integrated with Simultaneous MRI) trial