Abstract Book

S185

ESTRO 37

1 Academic Medical Center, Radiotherapie, Amsterdam, The Netherlands Purpose or Objective High-risk cervical cancer patients are treated on the pelvic as well as the higher para-aortic region (PAO) using a 0.5cm CTV-PTV margin, according to the EMBRACE ll protocol. The margins from the previous clinical protocol were reduced from 0.8cm to 0.5cm. Therefore, we decided to reduce the default restriction of 4 degrees rotation in the setup of the pelvic to 2 degrees to ensure the coverage of the PAO region. If this restriction was exceeded the patient was repositioned. The first aim of this study was to assess whether 2 degrees restriction ensures geometrical PAO region coverage. The second aim was to investigate the relation between the error in position of the PAO region and the rotation of the pelvic area. Material and Methods For this study we used 159 CBCT scans of 12 high-risk cervical patients who were recently treated in our clinic. As part of their treatment patients received daily online CBCT imaging. We also included scans that were not acceptable for treatment (>2 degrees rotation). To assess whether the PAO region is sufficiently covered, we first defined two regions of interest (ROI). The first ROI was defined in the pelvic area. The second ROI was defined from L5 to the upper border of the PTV area (figure 1a). The pelvic ROI was registered using an automatic bony anatomy registration (translations and rotations). This registration was converted into table correction values (translation only; excluding rotations). The PAO ROI was registered using a grey value algorithm with translations only. To assess the coverage of the PAO region, we calculated the difference between the registrations of the two ROI’s, based on translations only; the residual error. Sufficient geometrical coverage was assumed in case this difference is less than 0.5cm. Furthermore we investigated whether the residual error correlates with the X-rotation found in the pelvic area. Finally, a prediction of the residual error was made based on rotations found in the pelvic area assuming a rigid spinal column, and was compared with the residual errors found in the clinic.

Poster Viewing : Poster viewing 7: RTT

PV-0363 The effect of the use of CPAP on set-up errors in SBRT treatments in the thorax T. Katzman 1 , Z. Symon 1 , I. Weiss 1 , Y. Lawrence 2 1 Sheba Medical Center, radiation Oncology, Ramat-Gan, Israel 2 Sheba Medical Center, Center for Translational Research in Radiation Oncology, Ramat-Gan, Israel Purpose or Objective Over the last few years our centre has pioneered the use of continuous positive airway pressure (CPAP) for the treatment of thoracic tumours. We previously demonstrated that, compared with free-breathing, the use of CPAP increased total lung volume, while decreasing tumour movement leading to a considerable dosimetric advantage. A constant PTV expansion was applied to all cases. The decision of whether to use CPAP or standard free-breathing was based upon physician preference, but was the preferred option for lower lung tumours and those with multiple intrathoracic targets. The purpose of this study is to analyse whether the use of CPAP impacted upon the reproducibility of patient set- up. We hypothesised that the systematic and random errors would be reduced through use of CPAP, permitting use of a smaller PTV margin. Material and Methods 30 courses of SBRT treatments in the thorax were selected – 15 treated with CPAP, and 15 treated without CPAP. All patients were positioned similarly with a chest- board, and knee immobilization. Abdominal pressure was used when it was found to be effective for patients treated without CPAP. Altogether, 130 cone-beam CT (CBCT) scans performed prior to each treatment were analysed. Translational moves in the RL, AP, and SI planes were recorded. The systematic and random errors for each plane were evaluated, and the resulting PTV margin size using van Herk's recipe was calculated. Resulting PTV volumes were calculated using the standard ellipsoid formula. Results The mean (±standard deviation) of the set-up translational movements for the non-CPAP treatments were 0.4(±0.4) cm, 0.6(±0.4) cm, 0.7(±0.3) cm in the RL, AP and SI planes respectively. The mean (±standard deviation) of the set-ups for the CPAP treatments were 0.5(±0.4) cm, 0.4(±0.4) cm, 0.5(±0.5) cm in the RL, AP and SI planes respectively (all NS for comparison with CPAP). The PTV margins required in the non-CPAP arm were 1.3, 1.8 and 2 cm in the RL, AP and SI planes respectively. The margins required in the CPAP arm were 1.4, 1.3 and 1.6 cm in the RL, AP and SI planes respectively. The resultant CPAP PTV for a 1cm diameter target was 30% smaller than that obtainable without CPAP. Conclusion The use of CPAP increases the set-up reproducibility in the AP and SI directions, and slightly decreases the set-up reproducibility in the RL direction, although these differences were not statistically significant. Although the differences in each dimension were small, the overall difference in PTV volume was considerable, further improving the dosimetric advantages of CPAP use in thoracic tumours. PV-0364 The correlation between rotations of the pelvis and geometrical inaccuracy of the para-aortic region E. Ajanovic 1 , M. Kamphuis 1 , M.A.J. De Jong 1 , G.H. Westerveld 1 , J. Visser 1 , N. Van Wieringen 1

Results The mean difference of the residual error of the POA region and their standard deviations is shown in table 1 Of the 159 CBCT’s only 3 times the residual error