ESTRO 36 Abstract Book

S812 ESTRO 36 _______________________________________________________________________________________________

fractionation, even small relative errors can lead to serious complications to the normal tissue or recurrences of the tumor. So delivery quality assurance (DQA) in SRS/SBRT is very critical and poses unique challenges due to extremely high dose gradients and lack of electronic equilibrium. For this reason, dose rate independent dosimeters with precise, high spatial resolution and 3D capabilities are essential as reported by the Council on Ionizing Radiation Measurements and Standards (CIRMS). Material and Methods The new CrystalBall system (3D Dosimetry, Madison, CT, USA) is designed for DQA with sub-millimeter spatial resolution in 3D. The system is composed of a fast laser CT scanner (OCTOPUS, MGS Research, Inc, Madison, CT) and reusable tissue-equivalent radiochromic polymer gel sphere-mounted on a special QA phantom. Gold fiducial markers are affixed in different locations of the phantom for image guidance with fiducial tracking for CyberKnife (CK) robotic SRS/SBRT system (Accuray, Sunnyvale, CA). The CT images of the CrystalBall gel phantom were transferred to the CK Multiplan treatment planning system. A DQA plan was generated by superimposing a patient plan onto the gel phantom CT data set. The DQA plan was then sent for CK irradiation. The CrystalBall’s VOLQA software registers the plan DICOM CT dataset with the laser CT of the irradiated gel, creates OD/cm to dose calibration curve and then compares the CrystalBall 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.

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)

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.

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