Evaluation of a planar diode matrix for SRS patient‐specific QA in comparison with GAFchromic films

Abstract

We validate the routine use of a two-dimensional (2D) diode matrix for patient specific pre-treatment verification for Cyberknife (CK) stereotactic radiosurgery and to compare it with film dosimetry. Materials and method: A total of 46 patients were selected according to the most frequent diseases treated at our institution with the CK system,that is,brain metastases, meningiomas, spine metastases, and prostate tumors. All cases were evaluated with GAFChromic EBT-3 films and SRS MapCHECK for Fixed cone, IRIS, and MLC collimators of the CK. Results:The highest mean passing rate was observed for the SRS MapCHECK system compared to films. In order to assess if the two techniques provide statistically different results, a Wilcoxon Signed-Rank non-parametric test was performed (p < 0.05) and we found gamma values significantly lower for EBT-3 films with respect to the SRS MapCHECK. We noticed a moderately significant association between the two techniques using Spearman's rank correlation coefficient (rs > 0.4). We also performed the Bland-Altman statistical method: less than 5% of the differences resulted outside the range (mean ± 1.96 × SD), so the two methods can be considered interchangeable within the combined inaccuracy. Conclusions: The use of SRS MapCHECK for CK patient specific quality assurance (QA) is feasible for a variety of clinical districts and could be reliably used as a replacement for radiochromic films.

Wobbling nature of gamma passing rate as a function of calibration field sizes in patient-specific quality assurance

“…While simple ion chamber measurements can provide one-dimensional (1D) information, it may not be sufficient for identifying errors in complex volumetric-modulated arc therapy (VMAT) and intensity-modulated radiation therapy (IMRT) plans. To obtain two-dimensional (2D) or three-dimensional (3D) dose information, many commercial array-based detectors are available, [3][4][5][6][7] equipped with built-in analyzing software. The planned dose and measured dose can be compared using the gamma index, quantifying the gamma passing rate (GPR) in percentage.…”