ESTRO 36 Abstract Book

S813 ESTRO 36 _______________________________________________________________________________________________

Conclusion The imaging doses on new TrueBeam accelerator is substantially lower than on previous iX platform. Manufacturer’s default IGRT protocols could be optimised to reduce the ESD to the patient without losing the necessary image quality for patient set-up correction. For patient set-up with planar kV imaging the bony anatomy is mostly used and optimization should focus on this aspect. Therefore, the current approach with anthropomorphic phantom is more advantageous in optimization over standard kV quality control phantoms and SNR metrics. EP-1515 A novel attachment system for cutouts in kilovoltage x-ray beam therapy M. Baumgartl 1 , G. Kohler 1 1 University Hospital Basel, Clinic for Radiooncology, Basel, Switzerland Purpose or Objective Customized shielding in superficial and orthovoltage therapy is a common procedure to spare healthy tissue and is nowadays mainly based on in-house cutouts attached to applicators or patients. However, the production of customized shields could be time consuming and does not always provide a promising result. Inaccuracies during the replacement of cutouts may arise if the same applicator is required to treat different patients or target volumes (TV) immediately one after the other. An adequate, fast and reproducible approach of shielding was developed to treat the TV in the low and medium energy (kV) range for In our department most of the cutouts have a straight- edged shape. Our developed in-house frame-based system can be used to attach and remove the cutouts to the applicator in an easy and fast manner. It is important to note, that the mounted frame must not interfere with the radiation field. Hence, the frame has an identical size as the used applicator. VELCRO® strips were deployed as an attachment modality between applicator, frame and cutout. Those were glued to a PMMA frame with the same size of a standard applicator which attach to VELCRO® strips elongating around the applicator. The skin facing side of the frame is also covered with VELCRO® strips. In addition, cutouts were covered on the one side with VELCRO® strips and on the other side with a plastic foil, respectively. Straight-lined cutouts can be adjusted within seconds to shape TVs. standard applicators. Material and Methods

Results All used straight-lined shields from our database could be exactly reproduced contouring 100% of the TV using the developed attachment system. Nevertheless, the system increases the distance of the applicator to the patient skin by non-negligible 8.8mm. This has to be taken into account by an output factor (OF). A comparison of the measured and calculated (by the ISL) OFs shows a maximum deviation of ±3.6% (12keV) and ±0.8% (>12keV) for our set-up. Conclusion Shielding healthy tissue in kilovoltage x-ray beam therapy using the reusable frame-based system promises a fast exchange of cutouts. It also ensures a high reproducible accuracy to irradiate multiple TVs in a row with the same applicator but with different frames and cutouts. Moreover, it provides a complete coverage of healthy tissue of straight-lined TVs. Furthermore, a measured OF should be considered to the prescribed dose using the presented frame-based attachment system. EP-1516 Prediction of secondary cancer risk from lateral electrons transport from pediatric radiotherapy A. Chaikh 1 , J. Balosso 2 1 CHU de Grenoble - A.Michallon, Radiothérapie et physique médicale, Grenoble, France 2 University Hospital of Grenoble- University Grenoble- Alpes, Department of Radiation Oncology and Medical physics, Grenoble, France