ESTRO 36 Abstract Book

S191 ESTRO 36 2017 _______________________________________________________________________________________________

(Fig. A). 100 points were evenly distributed over the intersection curve of each plane with the rectal wall. It is assumed that the most dorsal point of the rectum is fixed and also that the rectal wall only stretches perpendicularly to the central axis. For point pairs on rectum BT1 and rectum BT2 that were at the same location according to the PRM, the dose for BT1 and BT2 was added (D PRM ) and compared as a 'ground truth” to the DIR accumulated dose (D DIR ) in the BT2 point. For BT, the high dose regions in the OAR are most relevant and points within the 2 cm 3 volume receiving the highest dose should be correctly identified. We therefore evaluated the percentage of points where D PRM and D DIR were both >D 2cm3 .

Fig 1: Dose mean lineal energy ratios between Co-60 and 100 kVp Fig 2: Dose mean lineal energy ratios as a function of scoring diameter X-rays as a function of scoring diameter. The dotted line corresponds for various brachytherapy sources. to α-ratio of 1.20. Conclusion Currently, the International Commission on Radiation Protection (ICRP) assigns a radiation weighting factor of unity for all photon emitting sources, equating the RBE of high and low energy photon sources. However, the clinical RBE for lower energy brachytherapy sources are considerably above unity and should be taken into account during the treatment planning process, to ensure that the equivalent dose delivered to the tumour is similar for different sources. OC-0360 Dose warping uncertainties for the cumulative rectal wall dose from brachytherapy in cervical cancer L.E. Van Heerden 1 , N. Van Wieringen 1 , C. Koedooder 1 , C.R.N. Rasch 1 , B.R. Pieters 1 , A. Bel 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Brachytherapy (BT) is part of radiotherapy for women with locally advanced cervical cancer; nowadays, BT is commonly given in multiple applications to the tumour area. In clinical practice, the 2 cm 3 receiving the highest dose (D 2cm3 ) in the rectum is calculated by assuming that the high dose volumes overlap for each treatment. To account for rectal deformation due to differences in filling and/or the presence of air, many authors state it is preferable to sum the 3D dose distributions using dose warping after deformable image registration (DIR). However, little is known about the reliability of DIR for dose warping. The purpose of this study is to quantify the dose warping uncertainty in the rectum using a physically realistic model, which describes rectal deformation. Material and Methods Seven patients were studied, treated with MRI-guided PDR BT (two times 24 x 0.75 Gy, given in two applications BT1 and BT2). DIR was performed using the Feature-Based Deformable Registration (FBDR) tool, connected to a research version of Oncentra®Brachy (Elekta Brachytherapy, Veenendaal, the Netherlands). The delineated rectums were converted to 3D surface meshes, and a mapping was established to propagate elements on the surface of rectum BT1 to the surface of rectum BT2 . The transformation vectors were used to deform the BT1 dose distribution. Next, the BT1 and BT2 doses were summed voxel-by-voxel. To investigate the dose warping uncertainty a physically realistic model (PRM) describing rectal deformation was used. In this model the central axes of rectum BT1 and rectum BT2 were constructed. The axes were assumed to be fixed in length. For both rectum BT1 and rectum BT2 , orthogonal planes were reconstructed at 5 evenly spaced positions on the axis

Results Over all patients, D DIR

varied between 1.1-44.4Gy EQD2

and

D PRM

varied between 1.1-40.1Gy EQD2

(α/β=3Gy for late OAR

toxicity, T 1/2

=1.5 hours). For point pairs, the absolute

difference between D DIR (Fig. B). The 2 cm 3 volumes receiving the highest dose according to the two models have an overlap of 66% (Fig. C). and D PRM was 0-8.3Gy EQD2

Conclusion With the rectal model it is feasible to quantify dose warping uncertainties, which could be as high as 8.3 Gy EQD2 . Most points (>66%) in high dose regions were correctly identified as part of D 2cm3 .