ESTRO 37 Abstract book

ESTRO 37

S554

these OARs may not be included in the plan optimization. The user need to be mindful of the imaging field length as well when it comes to increase OAR doses. PO-0994 A fast automated sanity check for online plan adaptation in MR-guided RT J. Kaas 1 , W. Van den Wollenberg 1 , A.J.A.J. Van de

Schoot 1 , F.W. Wittkämper 1 , T.M. Janssen 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands

Purpose or Objective Clinical introduction of the MR-Linac (Elekta AB, Stockholm) includes online plan adaptation (i.e. simple dose shift) to correct for setup errors due to the fixed couch position. Pre-treatment dosimetric verification of the adapted plan will be unavailable with the patient on the couch. The online adaptation workflow is new and complex, so potentially prone to errors. Therefore, there is a clear clinical need for a fast QA tool to verify online adapted plans. The aim of this work was to develop a tool to perform an automated "sanity check" on online adapted plans to avoid gross errors. Material and Methods In the MR-Linac workflow, a reference plan is made offline, which can be dosimetrically verified pretreatment. Any adapted plan sufficiently similar to it will be considered safe to deliver. To determine the similarity between the reference plan and the adapted plan, we introduce the following metrics: total number of MU, largest segment area, the MU-weighted sum of segment areas and the center of mass of the fluence distribution, determined from the centers of mass of the MLC apertures of each beam, weighted by MU and corrected for the FFF profile shape. The difference in fluence center of mass between the reference plan and the adapted plan is compared to the known applied online plan shift. To judge the stability of these similarity metrics, a test data set of three rectal cancer patients and three prostate cancer patients was created. Reference IMRT plans were created in Monaco (v5.19, Elekta AB, Stockholm), and adapted based on simulated setup errors of up to 30 mm in steps of 3 mm (prostate) or 10 mm (rectum) in all 6 cardinal directions. Each adaptation was performed in 2 ways: Segment Weight Optimization (SWO), where segments are only rigidly translated and weights (MU) reoptimized, and Segment Shape Optimization (SSO), where segments are also reshaped, resulting in 466 adaptations in total. Also, to test whether errors can be caught, a H&N patient with a large (in CC-direction) PTV was adapted based on setup errors that take part of the PTV out of reach of the beam (due to MLC limits), resulting in poor PTV coverage. Results Observed deviations were small (Table 1). In particular, the MU-weighted sum of segment areas is well conserved during adaptation, with only -2% to +3% difference between reference and adapted plan. The fluence center of mass shift closely tracked the setup error, with differences less than 2.5 mm for both SWO and SSO. For the intentionally bad H&N plan adaptations, larger deviations from 3 mm up to 9 mm in the center of mass of the fluence distribution were observed (Figure 1).

Conclusion A proof of concept of an automated sanity check for online plan adaptation was successfully demonstrated. Our algorithm was able to identify poor quality plans resulting from the target moving out of reach of the beam, using the center of mass of the fluence distribution. Other parameters that are approximately conserved during plan adaptation were identified. PO-0995 Development of cylindrical applicator for CNT based miniature electronic brachytherapy source H.J. Kim 1 , J.H. Lee 2 , C.Y. Jeong 1 , J.W. Kwak 1 , S.O. Cho 2 , B.C. Cho 2 , S.W. Lee 1 1 ASAN Medical Center, Department of Radiation Oncology, Songpa-gu- Seoul, Korea Republic of 2 Korea Advanced Institute of Science and Technology, Department of Nuclear and Quantum Engineering, Daegeon, Korea Republic of Purpose or Objective Electronic brachytherapy source based on thermionic emission type is promising new technology to replace brachytherapy using isotopes because of its low energy and steep dose gradient. Recently, miniature X-ray sources using carbon nanotube (CNT) field emission is developed for some clinical applications. The purpose of this study was to fabricate cylindrical applicator of CNT based miniature electronic brachytherapy source with homogeneous X-ray dose distribution around cylindrical applicator for intravaginal brachytherapy. Material and Methods The miniature electronic brachytherapy source based on CNT field emission was fabricated for electronic brachytherapy. The diameter of the fabricated miniature brachytherapy source is 7 mm and the length is 47 mm. The cylindrical application was designed using a Monte Carlo N-Particle eXtended (MCNPX) to create homogeneous X-ray dose distribution along the cylindrical applicator. The optimized design was fabricated with 3D