ESTRO 36 Abstract Book

S801 ESTRO 36 2017 _______________________________________________________________________________________________

parameters of dose–volume based indices for PTV (V80%, D98%, mean dose, D2% and Gamma index 2%-2mm ) and OARs doses (Dmax and dose at the threshold volume according to AAPM TG101) were evaluated for Compass calculation and DTD reconstruction. At the same time gamma index (2%-2mm) was calculated based on Delta 4 measurements. The detector attenuation was estimated in a clinical context comparing the median dose inside the Delta 4 detector with and without the Dolphin mounted. Results Error detection ability : the fig. 1 shows the variation between difference % of mean dose in a Roi limited to the irradiation beams for DTD versus leaf position shift and the % of points with gamma index > 1 for Delta 4. Quantitative analysis: table 1 shows the results of the comparison between Dolphin/Compass and Delta 4 phantom. The PTV average gamma was 0.64±0.12; the mean percentage differences of V80%, D98%, mean dose and D2% were inferior to 3%. The difference in Gy for OARs were under or equal to 1 Gy, except for D(4cc) of trachea (1.15 Gy). The maximum difference was found for rib D max (4.4 Gy). The mean % of point with gamma < 1 for Delta 4 was 83.2±0.06; one patient was considered failed with 72% of points with g<1 in Delta 4. Detector attenuation : a value of 10.5±0.5 % was found.

fig 1 shift leaf detectability Conclusion

Table 1. Comparison between Compass computed and reconstructed doses

The DTD system seems to be more sensitive than 3D detector for error detection ability. The Dolphin/Compass system is a useful tool to perform QA patients in a SBRT context offering more clinical evaluable informations than 3D phantoms only. For the online dosimetry, the methodology proposed led to an attenuation correction factor not negligible but constant . EP-1513 CyberKnife robotic radiotherapy delivery quality assurance using CrystalBall 3D Dosimetry System M.A. Al Kafi 1 , A. Al Moussa 1 , M.J. Maryanski 2 , B. Moftah 1 1 King Faisal Specialist Hospital and Research Centre, Biomedical Physics, Riyadh 11211, Saudi Arabia 2 MGS Research- Inc., d.b.a. 3D Dosimetry, Madison- CT, USA Purpose or Objective Stereotactic radiosurgery/radiotherapy (SRS) and stereotactic body radiotherapy (SBRT) deliver high dose to the tumor accurately and precisely. With hypo- 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

*D max

defined at 0.035 cc