ESTRO 38 Abstract book

S1064 ESTRO 38

Purpose or Objective Patients complain about the tightness of thermoplastic masks and experience anxiety when the mask tightly encloses the head. Especially for patients suffering from claustrophobia, wearing a mask can be intolerable. Hence, a treatment method without the necessity of fixating the head is of advantage for patients. In order to ensure a reproducible position and to control intra-fraction motion, an alternative is needed. For this purpose, optical surface imaging is used. The aim of the study is to investigate the clinical feasibility of whole brain radiation therapy without a mask. Material and Methods In total 30 patients received palliative, whole brain radiation therapy (5 fractions of 4 Gy) with the intention not to use a thermoplastic mask. The CTV-PTV margin is 5 mm. In case of participation, informed consent was signed. The surface scanner used for this study is the Catalyst™ (C-RAD AB, Sweden). Patients are instructed to lie as still as possible on the treatment couch. Positioning is done using surface scanning (see Figure 1) and an online matching procedure is performed. This is verified by making an anterior-posterior and a medio-lateral kV image. Motion monitoring starts as soon as the patient is positioned. The threshold for motion of the head is set to 3 mm, since this is an acceptable deviation taking the CTV- PTV margin of 5 mm into account. Figure 2 gives an example of the calculated iso-center shift. If the movement exceeds the threshold the radiation beam is interrupted and the patient may be repositioned. When more than two repositioning procedures are required, the fraction is labelled unsuccessful. Success of the treatment is defined when three or more fractions of one patient have been successful. Clinical feasibility in this study is defined as: more than 70% of the patients can complete the radiation treatment successfully, which is tested by using a one-tailed binomial test (p value < 0.05). Results Two patients out of the total population were not able to lie still and continued the treatment by using a mask. For one of them it was already decided at the CT to make a mask. The other patient completed a successful and an unsuccessful fraction without mask, before it was decided to use a mask for the remaining fractions. All other patients completed their treatment successfully with only one unsuccessful fraction (4 repositioning procedures) for one of these patients. With a probability of success of 93,3% (28 out of 30), we accept the hypothesis that more than 70% of the patients can complete radiation treatment successfully (p value is 0.0021, 95%-confidence interval 0.80-1.0). Intra-fraction motion data is available for further analysis.

Conclusion Patients receiving whole brain radiation therapy have been treated without a thermoplastic mask. In this study it is observed that patients are able to lie still below the threshold of 3 mm. Hence, it can be concluded that irradiation without a mask is a clinically feasible method for whole brain radiation therapy. EP-1952 Intra-fraction motion assessment of frameless intracranial radiosurgery using 1.5T MR simulator J. Yuan 1 , G.G. Lo 2 , O.L. Wong 1 , K.F. Cheng 3 , W.W.K. Fung 3 , Y.H. Zhou 1 , G. Chiu 3 , K.Y. Cheung 1 , S.K. Yu 1 1 Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong, Hong Kong SAR China ; 2 Hong Kong Sanatorium & Hospital, Department of Diagnostic & Interventional Radiology, Hong Kong, Hong Kong SAR China ; 3 Hong Kong Sanatorium & Hospital, Department of Radiotherapy, Hong Kong, Hong Kong SAR China Purpose or Objective The intra-fraction motion of intracranial radiosurgery is usually investigated by using the online X-ray-based imaging. We aim to retrospectively assess the intra- fraction motion shift of a thermoplastic mask immobilized intracranial radiosurgery using MRI data acquired on a 1.5T MR-simulator by taking the advantage of superior MRI image contrast. Material and Methods The simulation MRI scan data at 1.5T of 27 patients (brain metastases n=23, others n=4) prior to Cyberknife intracranial radiosurgery were retrospectively assessed. All patients were scanned in the identical treatment position with a thermoplastic mask fixation. MRI protocol included at least two (T2w and post-contrast T1w+c, ~10min) sequences for all patients. A third T1w+c sequence (~5min) was included in 10 patients. All MRI images had isotropic 1mm voxel size. T1w+c MRI (represented the position at 5min and 10min respectively)