# IBA myQA SRS Detector for CyberKnife Robotic Radiosurgery Quality Assurance
## 4 Discussion
Since CMOS components are continuous, no dose reconstruction and interpolation algorithms are needed. As a result of the 0.4 mm contiguous spatial resolution, beam profile measurements can be performed simultaneously with OFs, enabling effective constancy DQA tests without the use of point detectors, film irradiation, motorized scanning devices, and water phantoms. The signal for OFs estimation was collected in a 3 \(\times\) 3 pixels area (i.e., 1.44 mm\({}^{2}\)) whose dimensions are comparable to the most recent diamond and solid-state point detector sizes. Nevertheless, with smaller collimators, a 2 \(\times\) 2 pixels ROI (i.e., 0.64 mm\({}^{2}\)) seemed to be more appropriate. With respect to the standard procedure for the Iris(tm) variable collimator field size measurement, which involves multiple radiochromic films irradiation and several analyses, myQA SRS coupled with the homemade MATLAB script results in a very powerful tool, allowing us to go film-less for constancy QA checks. This study confirms the dose rate dependence of the CMOS array. However, given that the dose calibration is performed in the same path/distance conditions, the dependence for CyberKnife nodes belonging to a single treatment path is quite negligible (less than or equal to 2.5%). The dependence of the signal on beam angular direction affecting bidimensional detector arrays is of major concern when the verification of a complex SRS plan has to be performed. Indeed, the AAPM Task Group No. 218 recommendations state _\"Since the 2D arrays are designed for perpendicular measurements, the array detector's radiation response is angularly dependent. [... ] The angular dependence may be smeared out when using beams from many angles... \"_[14]. Manufacturers faced this issue by implementing correction factors based on gantry angle sensors' information or developing motorized equipment that makes the dosimetry device rotate following the linac head. On the other hand, several research groups are developing angular-independent devices for small field and SRS dosimetry [4; 25; 26; 27; 28]. As CyberKnife(r) delivers non-isocentric non-coplanar beams, the angle sensor cannot be employed since the information it gives makes no sense for a 6-axes robotic manipulator. We investigated myQA(r) SRS angular response by running a comprehensive test that involved the whole straight solid angle over the couch, from left to right with respect to the patient's supine position. Unlike previous studies that used traditional linacs for arc delivery [24; 26; 29] and other CyberKnife(r)studies that used its isocentric modality [6], we rotated the myQA@SRS phantom while keeping the linac manipulator in a fixed position. Because beams outside of a 50\({}^{\circ}\) amplitude cone with respect to the patient's anterior-posterior direction cause more than 5% signal decay, we proposed performing patient-specific QA on plans that had been purged of those beams. Besides the machine DQA, patient-specific QA is the most critical test preliminary to clinical administration of an optimized SRS plan. While the Recommendations of AAPM Task Group No. 218 suggest _\"Universal tolerance limits: the gamma passing rate should be \(\geq\)95%, with 3%/2 mm and a 10% dose threshold\"_[14] for VMAT and fixed-gantry IMRT QA using 2D measurements devices, in this study a different criterion was chosen among those collected in a paper focused on TG-218 application to SRS QA [15]. Given SRS specificity in terms of delivered doses and localization accuracy/precision, similarly to other works focused on pre-treatment SRS plan QA [4; 7] an evaluation by Gamma Index with dose-difference 3% and distance-to-agreement 1 mm was considered appropriate.