1. Cone Beam CT at the Ghent University Hospital: first clinical results and evaluation of the selected workflow. G. Pittomvils 1,,M. Coghe 1, A. Impens 1, S. Nechelput 1, G. Boon 1, G. De Meerleer 1, T. Boterberg 1 and W. De Neve 1 (1)Division of Radiotherapy, Ghent University Hospital, Belgium De Pintelaan 185, B-9000 Gent Belgium At Ghent University Hospital the Elekta Cone Beam CT is used on the Synergy platform for patient positioning prior to treatment. An action level of 2 mm for patient repositioning was selected in an adapted ‘no action level’ protocol [1,2]. As proposed in literature a weekly follow-up measurement technique was introduced [3]. The amplitude of this action level has been evaluated by analysing the systematic errors using a specially developed phantom and by post processing the clinical data for different pathologies with different patient immobilising devices. Introduction Conclusion Results Material and Methods  Test phantom - A test phantom was constructed to evaluate the “re”positioning precision. This phantom consists of 14 14x14cm² PMMA slabs of 1 cm thickness. The middle of the phantom is indicated with a metal sphere with a diameter of 2.4 mm. Orthogonal wrinkles on the phantom, indicating the positioning of the sphere were used as positioning tool on the imaging and treatment machines. - 1 mm thick CT slices are registered to evaluate the precision of the positioning on the Cone Beam CT guided Elekta Synergy. - The geometrical precision of the overall treatment process is compared to ‘no action level’  Patients results  The number of fractions used to calculate the correction is dependent on the total number of fractions but for routine treatment this number of fraction for the calculation of the correction never exceeds four [2]. This number was therefore applied for all treatment protocols.  Intracranial and Head and Neck Lesions - Intracranial lesions are immobilised using the micro-perforated Posicast thermoplastic 3 point fixation system. The average mean of the displacements, their standard deviation (∑) and the group mean of the standard deviations (σ) give an estimation of the random and systematic errors [4]. - The limited organ movements and the good fixation should result in averages close to the phantom measurements. The results of the weekly follow-up are compared to the selected threshold. 190° image acquisition using a S 20 filter is used for the cone beam imaging. - The head and neck patients are immobilised using the Posicast 5 point fixation system. The results are analysed using the same protocol.  Lung tumours - The patients with lung tumours are immobilised using the Posicast 4 point fixation system and the Sinmed knee fix. Only the patients selected in the NAL protocol for the treatment margins are included in the evaluation. 360° images acquisition using an M 20 filter is used for the cone beam imaging.  Adjuvant and salvage prostate treatments - Only patient with adjuvant radiotherapy after radical prostatectomy are included in this study. - Patients are positioned using the Sinmed knee fix and feet fix and a daily image guided patient positioning is applied while large day by day variations are expected.  Test phantom – Verification of the overall treatment using manual matching of the Cone Beam CT data and the reference CT resulted in a registration precision smaller than 1 mm. – Using soft tissue matching of the head and neck protocol daily displacements in left-right, anterior-posterior and cranial-caudal direction remain below 1 mm for the head and neck protocol. Using the lung protocol or the pelvis protocol a maximal deviation of 2 mm was observed.  Intracranial lesions (N = 13) – The 3D average displacement registered during the four initial fractions for the intracranial lesions is 1.1 ± 1.2 mm. The applied displacement was 1.0 ± 1.1 mm (figure 2 a). – During the follow up the selected threshold was never exceeded and the remaining average difference was reduced to 0.35 ± 0.9 mm (figure 2 b). The analysis of the follow up data resulted in a treatment margin of 3 mm (M = 2.5 ∑ + 0.7 σ) [4].  Head and Neck tumours (N = 31) - An average displacement of 1.1 ± 0.6 mm was observed and the applied displacement was 1.0 ± 0.6 mm (figure 3). - During follow up the selected threshold was never exceeded and the remaining average displacement was reduced to 0.8 ± 0.6 mm resulting in an remaining treatment margin of 3 mm [4].  Lung tumours (N = 48) - A larger average displacement is expected for lung tumours especially in the CC direction. An average displacement of 2.7 ± 7.4 mm was observed of which 2.4 ± 5.3 mm is due to CC displacement (figure 4). - For the lung patients selected for the NAL protocol (N = 20), after correction the remaining average difference is within 0.3 ± 1.0 mm and the obtained set-up margins were 5 mm for the LR direction, 6 mm for the AP direction and 9 mm for the CC direction [4].  Adjuvant and salvage prostate treatments (N = 39) – The average displacements are very small (1.3 ± 0.8 mm) and the error on the average AP displacement was the largest. – If an adaptive NAL protocol should be selected the set-up margins should be 7 mm in the LR direction, 5 mm in the CC direction and 11 mm in the AP direction. Figure 4: average displacements recorded during the four initial sessions Figure 5: average displacements recorded during the four initial sessions Figure 2: (a) average displacements and applied displacements recorded during the four initial sessions (b) remaining average displacements recorded during follow up The selected level of 2 mm for the adapted NAL protocol was appropriate for our clinical treatment protocols. Phantom measurements and clinical data for strongly immobilised treatment techniques illustrate that the differences between the applied corrections and the real corrections are negligible and that the expected repositioning accuracy on the cone beam CT is between 1-2 mm depending on the imaging modality used. This study supported our philosophy concerning the frequency of the cone beam guided image set-up, the adapted NAL protocol for head and neck and intracranial lesions, daily imaging for prostate treatments and an individual evaluation for the lung treatments Figure 1 : The PMMA slab phantom References [1] de Boer HCJ and Heijmen BJM, Int.J.Radiat.Oncol.Phys. 50 (2001), 1350-1365 [2] Bortfeld T, van Herk M, Jiang SB, Phys.Med.Biol. 47 (2002), N297-N302. [3] de Boer HCJ and Heijmen BJM, Int.J.Radiat.Oncol.Phys. 67 (2007), 1586-1595 [4] van Herk M, Seminars in Radiation Oncology 14 (2004), 52-64. Figure 3: (a) average displacements and applied displacements recorded during the four initial sessions (b) remaining average displacements recorded during follow up