ESTRO 36 Abstract Book

S802 ESTRO 36 2017 _______________________________________________________________________________________________

EP-1514 Planar kV imaging dose reduction study for Varian iX and TrueBeam linacs E. Gershkevitsh 1 , D. Zolotuhhin 1 1 North-Estonian Regional Hospital Cancer Center Radiotherapy, Radiotherapy, Tallinn, Estonia Purpose or Objective IGRT has become an indispensable tool in modern radiotherapy with kV imaging used in many departments due to superior image quality and lower dose when compared to MV imaging. Since, the frequency of kV images continues to increase (intrafractional imaging, etc.) the reduction of additional dose assumes high priority. Many departments use manufacturer supplied protocols for imaging which are not always optimised between image quality and radiation dose (ALARA). Material and Methods Whole body phantom PBU-50 (Kyoto Kagaku ltd., Japan) for imaging in radiology has been imaged on Varian iX OBI 1.5 and TrueBeam 2.5 accelerators (Varian Medical Systems, USA). Manufacturer’s default protocols were adapted by modifying kV and mAs values when imaging different anatomical regions of the phantom (head, thorax, abdomen, pelvis, extremities). Images with different settings were independently reviewed by two persons and their suitability for IGRT set-up correction protocols were evaluated. The suitable images with the lowest mAs were then selected. The entrance surface dose (ESD) for manufacturer’s default protocols and modified protocols were measured with RTI Black Piranha (RTI Group, Sweden) and compared. Image quality was also measured with kVQC phantom (Standard Imaging, USA) for different protocols. The modified protocols have been applied for clinical work. Results The default manufacturer’s protocols on TrueBeam linac yielded 9.4 times lower ESD than on iX linac (range 2.5- 24.8). For most cases it was possible to reduced the ESD on average by a factor of 3 (range 0.9-8.5) on iX linac by optimising imaging protocols. Further ESD reduction was also possible for TrueBeam linac. 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

irradiation measurements with the Multiplan’s DQA plan. It generates QA reports that feature overlays of isodoses in 2D and 3D, profiles, DVHs, voxel statistics, and pass/fail metrics for dose difference and distance-to-agreement according to gamma index criteria. In this study, we performed DQA for four CK patients who received treatment for brain metastasis, spine metastasis and trigeminal neuralgia as recommended by AAPM TG-135. For each patient, the DQA was done three times. Results Figures 1 and 2 show the CrystalBall phantom setup with OD/cm to dose auto-calibration, 2D and 3D overlay of isodoses for a patient, respectively.

Table 1 shows results of the study for gamma evaluation passing averages for the DQA of the four patients. For all patients studied, we found a passing rate of more than 96% with gamma index criteria of 2 % dose difference and 2 mm distance-to-agreement. For 3 % and 3 mm criteria, the passing rate is found to be above 99%.

Conclusion Our DQA results suggest that the newly developed CrystalBall QA phantom system for robotic radiosurgery can be ideal tool for 3D dose verification with isotropic sub-millimeter spatial resolution and film-equivalent accuracy. This 3D tool can offer unique advantage over other existing 2D tools and techniques in terms of high- resolution DQA necessary for radiotherapy with minimal additional physics resources.

Electronic Poster: Physics track: Radiation protection, secondary tumour induction and low dose (incl. imaging)

standard applicators. Material and Methods

In our department most of the cutouts have a straight- edged shape. Our developed in-house frame-based system