ESTRO 36 Abstract Book

S193 ESTRO 36 _______________________________________________________________________________________________

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 (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 .

the two models have an overlap of 66% (Fig. C).

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 . OC-0361 Commissioning of applicator-guided SBRT with HDR Brachytherapy for Advanced Cervical Cancer S. Aldelaijan 1 , S. Wadi-Ramahi 1 , A. Nobah 1 , N. Jastaniyah 2 1 King Faisal Specialist Hospital and Research Center, Biomedical Physics, RIyadh, Saudi Arabia 2 King Faisal Specialist Hospital and Research Center, Radiation Oncology, RIyadh, Saudi Arabia Purpose or Objective There is emerging evidence that dose escalation to the “GEC ESTRO defined” high-risk clinical target volume leads to improved clinical outcome in patients with cervical cancer. For those with large residual disease or with unfavorable topography of parametrial spread, achieving such high doses is limited by the dose to organs at risk. Options include a parametrial boost by EBRT which lack precision and lead to prolongation of overall treatment time or the addition of interstitial needles which require a specialized brachytherapy (BT) program. The option of combining brachytherapy with SBRT, using the applicator as a guide, is being explored at our institution. The purpose of this work is to show how this idea can be successfully implemented using an EBT3 Gafchromic film-based dosimetry system. The effect of positional inaccuracies on overall dosimetric outcome is studied as well. Material and Methods A cube phantom was constructed to snuggly accommodate an intrauterine tandem (IU), Fig1a. Pieces of EBT3 film were taped on both sides of the IU to capture the dose distribution. The phantom was CT-scanned and the physician contoured a CTV mimicking large residual parametrial disease, Fig1b. The plan was such that the 7Gy isodose adequately covers the near-distance CTV. The BT plan was used as input for the SBRT plan and the 7Gy to 2.0Gy dose gradient were used to create dose shells, each having its own dose objective and constraint. Three VMAT arcs were used to achieve the goal of D 98% > 95% to the entire CTV. Later, HDR BT treatment was delivered using microSelectron v2 and the SBRT was delivered using TrueBeam®. Positioning accuracy of the phantom was done using CBCT imaging with the applicator for image registration. Films were scanned with 10000XL EPSON scanner at 127 dpi and dosimetry was done using the green channel and an in-house MATLAB routine. Intentional

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 and D PRM was 0-8.3Gy EQD2