ESTRO 38 Abstract book

S1210 ESTRO 38

exposure to our paediatric patients, who are already at greater risk of secondary malignancies. It is therefore essential to make sure that the dose delivered as part of any verification procedure is optimised such that the dose is high enough to produce a useable image, but no higher. Material and Methods A 1 year old patient was referred for RT with an upper lip rhabdomyosarcoma with metastatic neck nodes. The patient was anaesthetised for treatment with a nasogastric tube fitted and laryngeal mask. Daily CBCT was required as IMRT was to be used and also daily repositioning was difficult due to the position of the nasogastric tube and laryngeal mask. A Varian TrueBeam linear accelerator with software version 2.0 was used for treatment. The CBCT mode editor was utilised and Varian default settings modified so that the gantry was forced to go under the couch, avoiding beam entry through radiosensitive structures such as the eyes. The mAs was reduced in steps to ensure image quality, such as homogeneity, contrast and noise were maintained in the reconstructed image. The geometry of a cone beam scan enables a large FOV to be viewed in one rotation. However this contributed to a large amount of scatter radiation. So in addition to a lower mAs, a smaller FOV was utilised to further reduce scatter radiation. Care was taken to not limit the total volume that could be reconstructed. Results CBCT images were reconstructed, evaluated, and compared. Visual comparison indicates that the image quality of CBCT(s) is comparable to that of the initial CBCT (#1-2). The results demonstrate that the scanning parameters can be manipulated to reduce the dose to the patient with minimal effect on the quality of the CBCT when comparing bony anatomy. This is done by employing lower mAs, less fps, smaller FOV, or by using 200° rotation below couch. Conclusion With optimal settings in use, the dose to the patient can be reduced by 95.5% when compared to the default setting with minimal impact on image quality. It is therefore safe and appropriate to use this optimised mode for all paediatric patients.

3 Helmholtz Zentrum München, Institut für Innovative Radiotherapie, Munich, Germany Purpose or Objective The aim of this study is to detect dose aberrations in cranial radiation therapy in the target volume and adjacent organs at risk (OARs) caused by insufficient rotational setup correction. 6 degrees of freedom (DOF) fusions were considered as the gold standard and compared to 4 DOF and 3 DOF registrations. Material and Methods Setup errors of 12 patients with cranial lesions undergoing radiotherapy and positioned with cone beam computed tomography (CBCT) were analyzed. The PTVs of the included patients were large and irregularly shaped and located near or overlapping cranial OARs such as brainstem, chiasm, optical nerves or lenses. A registration between planning CT and the first CBCT of the treatment course was performed using 6 DOF, 4 DOF and 3 DOF. Differences between the three registrations were implemented and the planning CT and associated structure set shifted and warped accordingly. To detect the DOF with the largest influence on the delivered dose a rotational error of 2°, 3° and 4° was simulated in one rotational dimension at a time. Using the original parameters, the treatment plan was copied and recalculated to the warped image set. Dmax and Dmean of the cranial OARs and D98 and D2 for the respective CTV and PTV were analyzed. Results Differences between 6 DOF and 4 or 3 DOF fusion ranged between -1.4° and 2° in pitch direction, -2.5° to 2° in yaw, and -4.4° to 4.2° in roll direction. Resulting median D98 in the PTV decreased by 0.7 Gy using only 3 DOF and increased by 0.08 Gy if 4 DOF were included. Maximum dose reduction of the PTV D98 was 2.96 Gy using 3 DOF and 0.39 Gy using 4 DOF. CTV dose decreased by 0.68 using only 3 DOF and by 0.03 Gy using 4 DOF. Resulting median dose effects in the OARs are displayed in Figure 1, with largest effects of additional 0.2 and 0.8 Gy for the left and right lenses, respectively. Doses for individual patients varied up to additional 6 Gy in the lenses and 3 Gy in the optic nerves. Effects on doses in the OARs and target volumes were generally larger with 3 DOF than with 4 DOF included, except for the brainstem with no visible difference. Pitch and roll setup errors had larger effects on the OARs compared to yaw setup deviations, as demonstrated in Figure 2. 2° roll setup errors caused more than 5 Gy dose effect on the ipsilateral optical nerve and chiasm, followed by 2° pitch errors resulting in over 3 Gy additional dose. The simulated 2° yaw error had an effect of under 1 Gy on dose delivery.

EP-2193 Influence of rotational setup errors on dose in target and organs at risk in cranial radiotherapy A.S. Gerhardt 1,2 , M.N. Duma 1,3 , M. Düsberg 1 , J.J. Wilkens 1 , S.E. Combs 1,3 , M. Oechsner 1 1 Klinikum Rechts der Isar, Department of Radiation Oncology, Munich, Germany; 2 Technical University Munich, Faculty of Medicine, Munich, Germany;