ESTRO 36 Abstract Book

S466 ESTRO 36 _______________________________________________________________________________________________

acquired and analyzed using the offline image review workspace in Mosaiq (v1.60, Elekta, Stockholm, Sweden) to determine intrafractional patient movements. From each CBCT, 3 translational and 3 rotational coordinates were obtained. Results The average time between the patient setup CBCT and the post treatment CBCT was 9 minutes (range, 6-14). The average absolute translational variations (± 1 SD) obtained from the post-treatment CBCT was 0.7 ± 0.7, 0.7 ± 0.8 and 0.5 ± 0.6 mm in the lateral, longitudinal and vertical directions, respectively. The average absolute rotational angles were 0.8 ± 0.7, 0.7 ± 0.4 and 0.8 ± 0.6˚ along pitch, roll and yaw, respectively. Histograms of translational and rotational deviations for all patients are shown in figure 1.

Conclusion Near-rigid body immobilization, CBCT image guidance and six degrees of freedom correction yields minimal intrafractional motion and safe stereotactic spine radiosurgery delivery. It is not easy to determine the effect of rotational deviations. However, for treatment plans with the isocenter plased in the center of the target volume, which is the case for these patients, small rotations would not result in large deviations in dose to the target volume or adjacent OARs. There are different approaches that could result in less patient motion and increased precision in dose delivery. The combination of a polyethylene sheet with a vacuum cushion would presumably result in a more rigid immobilization. Intrafractional imaging during treatment is another alternative that could increase precision in dose delivery. PO-0858 Intra-fraction motion quantification of head- and-neck tumors using dynamic MRI T. Bruijnen 1 , R.H.N. Tijssen 1 , M.E.P. Philippens 1 , C.H.J. Terhaard 1 , T. Schakel 1 , J.J.W. Lagendijk 1 , C.P.J. Raaijmakers 1 , B. Stemkens 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective Previous research primarily focused on the effect of deglutition on the accumulated tumor do se. However, resting-state movements, such as respiratory-induced tumor motion, has been largely overlooked. Nonetheless, this may play an important role in the size of the treatment volume of head-and-neck cancer. Here, we investigate head-and-neck resting-state tumor motion in a radiotherapy treatment position in order to provide guidance for adequate internal target volume (ITV) determination. Material and Methods Acquisition: 46 patients with head-and-neck cancer (6 nasopharyngeal/ 25 oropharyngeal/ 15 laryngeal) underwent pretreatment clinical MRI scanning in a radiotherapy treatment setup, including a custom-fit immobilization mask. Two 2D sagittal dynamic acquisitions (RF- and gradient-spoiled gradient echo; TE/TR=1.5/3ms; voxel size=1.42x1.42x10 mm 3 ;158 ms temporal resolution), separated 10 minutes apart, localized to intersect the tumor were acquired on a 3.0T scanner. GTV delineations, as performed by a radiation oncologist, were obtained from the treatment plans.

Conclusion Systematic cranial and posterior intrafraction baseline shifts between setup CBCT and treatment delivery occurred for mediastinal LN targets in lung cancer patients and reduced the treatment accuracy. Intrafraction motion amplitudes were stable throughout each treatment fraction, as well as the treatment course. PO-0857 Analysis of Intrafraction Motion in Image- Guided Stereotactic Radiosurgery of Spinal metastases J.G. Svestad 1 1 Oslo University Hospital The Norwegian Radium Hospital, Department of Medical Physics, Oslo, Norway Purpose or Objective Stereotactic radiosurgery of spinal metastases include tight margins and steep dose gradients to the surrounding organs at risk (OAR). The proximity of the target to the adjacent spinal cord and the aim of keeping the dose to the spinal cord within tolerance require a high degree of precision in dose delivery. This study aimed to evaluate intrafractional motion using cone beam computed tomography (CBCT) image guidance, for immobilized spinal stereotactic radiosurgery patients, with correction in all six degrees of freedom. Material and Methods Intrafractional motion during spine radiosurgery treatment in 16 patients (26 fractions) was retrospectively analyzed. All patients were immobilized in the BlueBAG BodyFIX (Elekta, Stockholm, Sweeden) that uses a vacuum pump to create a precise mold of the patient’s position. Radiation treatment was performed using a Varian Truebeam STx linear accelerator equipped with a PerfectPitch 6 degrees of freedom couch (Varian Medical Systems, Inc., Palo Alto, USA). Following initial setup, a CBCT was acquired for patient alignment and patient position was corrected in all six degrees of freedom. Patients were resetup manually, and the process was reinitiated if the ± 3˚ rotational couch tolerance was exceeded. A post treatment CBCT was