# IBA myQA SRS Detector for CyberKnife Robotic Radiosurgery Quality Assurance
## 4 Discussion  
The peculiarities of radiosurgery delivery pose a whole new set of challenges for DQA programs [3]. To date, an ultimate device capable of performing accurate and efficient periodic dosimetry verification and pre-treatment patient-specific QA in the field of SRS, in particular when delivered with robotic systems, is lacking. High-resolution solid-state detectors providing point measurements are inappropriate for achieving full information both in the target area and in the steep dose gradient regions, which also deserve verification. Although enabling highly-resolved bidimensional measurements, radiochromic films (Gafchromic(tm), Ashland ISP Inc., Kentucky, KY, USA) need special handling care, are time-consuming tools, and may require dose scaling in order to avoid signal saturation [23]. To overcome these limitations, in recent years, there has been great interest in the development of 2D and 3D detectors. An example is MapCHECK(r) (SunNuclear, Melbourne, Australia), a 7.7 cm \(\times\) 7.7 cm 2D array composed of 1013 diodes with a diagonal center-to-center separation of 2.47 mm. Each diode has 0.48 mm \(\times\) 0.48 mm planar dimensions. The MapCHECK was recently evaluated for the patient-specific verification of thirty-three intracranial CyberKnife(plans [7]). The equipment resulted in being an option for patient-specific QA, obtaining 3%/1 mm Gamma Index passing rates above 90%. Despite non-continuous resolution, the device was also employed for trigeminal neuralgia treatment verification, obtaining high Gamma Index passing rates. The calibration factors used in this study were manufacturer provided and have been analyzed in another paper along with dose rate and angular corrections [24]. The described array and dose calibration procedures seemed to be quite hard-working and error-prone. No tests were reported by the authors for machine DQA. The OCTAVIUS(r) 1000 SRS (PTW, Germany) is a 11 \(\times\) 11 cm\({}^{2}\) 2D array of 977 liquid-filled ionization chambers. The detectors have a 2.3 \(\times\) 2.3 mm\({}^{2}\) size with a pitch of 2.5 mm in the square central area of 5.5 cm on each side, and a 5 mm pitch outside. In a recent work, a complete characterization of the device was performed for robotic radiosurgery QA  
Figure 9: Patient no. 4 pre-treatment verifications after beam pruning: (**a**) myQA® SRS measured distribution; (**b**) calculated dose distribution; (**c**) superimposition of \(x\) dose profiles of measured (green) and calculated (orange) dose; (**d**) Gamma Index (local 3%/1 mm) graph. in terms of OF, beam profiles, source-detector-distance response, and angular dependence in comparison with other detectors [6]. Output factor measurements performed for the CyberKnife 5 mm and the 7.5 mm cones with the 1000 SRS, with the PTW 60019 synthetic diamond, and with the PTW 60017 small-field diode differed approximately by 3.0%, in good agreement with the myQA(r) SRS results for the same collimators. Although the work found that the relative response with source-detector-distance changes remained within 1% for the clinically relevant range, an average correction factor was applied to patient-specific verifications. Patient-specific Gamma Index analyses resulted in 3%/1 mm passing rates above 90% for the 55 head and body SRS plans considered. The 80\({}^{\circ}\) angle was considered by the authors to be a reasonable threshold for angular independence of the measurement. Deviations with respect to planned dose resulted in deviations being below 1% for beam angles between 0\({}^{\circ}\) and 80\({}^{\circ}\). Despite angles over 80\({}^{\circ}\) leading to greater differences, the proportion of beams incident between 80\({}^{\circ}\) and 90\({}^{\circ}\) was considered negligible and seemed not to influence the Gamma Index passing rates. The IBA myQA@SRS bidimensional CMOS array appears to address many of the robotic radiosurgery QA issues by providing pitch-less high spatial resolution, a large measurement area, and real-time results in an easy-to-use piece of equipment.