ESTRO 36 Abstract Book

S192 ESTRO 36 2017 _______________________________________________________________________________________________

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 shifts of 1, 3 and 5mm in both the IN/OUT direction and 2D shifts in both IN/OUT and SUP/INF directions were introduced. Results Fig1b shows the 95% (of 7Gy) dose distribution of the combined BT and SBRT treatments. Analysis using 2%/2mm gamma criterion resulted in 99% agreement. Isodose line matching and a cross profile between measured and planned doses are shown in Fig1c-d. Fig2 shows the effect of 1D and 2D isocenter shift on D 98% , D 90% and D 2cc metrics of the CTV. A threshold of +10% was used as a gauge to compare dose values after shift with the zero shift baseline. The most changes in dose were for D 98% and D 90% , both exceeded the threshold for 3mm shifts and almost reached -20% for the 5mm shifts.

Conclusion Using the applicator as a guide, SBRT and BT for cervical cancer can be delivered on the same day. The isodos e gradient from BT is used to create dose shells needed to deliver doses by SBRT. 2D shifts were shown to affect D 98% the most and a positioning accuracy of 2mm results in dose variations within +10% from expected. OC-0362 Precision IORT – image guided IORT in cluding online CBCT based Monte Carlo treatment pl anning F. Schneider 1 , L.D. Jimenez 1 , F. Bludau 2 , A. Jahnke 1 , C. Illana 3 , J. Fleckenstein 1 , S. Clausen 1 , U. Obertacke 2 , F. Wenz 1 1 University Medical Center Mannheim, Department of Radiation Oncology, Mannheim, Germany 2 University Medical Center Mannheim, Department for Orthopaedics and Trauma Surgery, Mannheim, Germany 3 GMV, Innovating Solutions, Madrid, Spain Purpose or Objective The clinical use of intraoperative radiotherapy (IORT) is steadily increasing based on novel applications like breast and brain cancer and spinal column metastases. Traditionally, IORT has been eye and hand guided without treatment planning and inhomogeneity correction. This limits the precision of the application and the precise documentation of the location and the deposited dose in the tissue. Kypho-IORT is a novel treatment option for patients with spinal column metastases in which a minimally invasive kyphoplasty is combined with a sterilising dose of IORT. Here we present a set-up where we use image guidance by intraoperative cone beam CT (CBCT) for precise online