ESTRO 36 Abstract Book

S811 ESTRO 36 _______________________________________________________________________________________________

The new IBA Dolphin (IBA Dosimetry, Germany) is a dose transmission detector (DTD) mounted onto the gantry for online treatment verification as well. Aim of this study is to compare the results of the Dolphin/Compass with the traditional 3D dosimetry phantom Delta 4 (Scandidos, Sweden) for lung stereotactic body radiation therapy treatmentsand to mesure the detector attenuation for At first the two systems were compared in terms of ability of error detection of leaf position. A box treatment was measured three times with introduction of a shift of one leaf bank in steps from 0 to 2 mm and the analysis of gamma index or DVHs was carry out. Afterward ten patients with lung cancer, treated by sbrt, were included in the study. All treatment plans were simultaneuslyverified with the Dolphin and the Delta 4.The treatment plans were generated by Monaco system (ver.5.0, Elekta AB, Sweden). Dolphin with the Compass software (v. 4.0) permits the 3D dose distribution reconstruction on a patient CT and the Compass itself is a model-based dose computation, with a collapsed cone dose engine; the beam model of the Compass was validated and accepted. For the quantitative analysis 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. online dose verification. Material and Methods

*D max

defined at 0.035 cc

fig 1 shift leaf detectability

Table 1. Comparison between Compass computed and reconstructed doses

Conclusion 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-