Patient-Specific Quality Assurance in a Multileaf Collimator-Based CyberKnife System Using the Planar Ion Chamber Array

Abstract

This paper describes the clinical use of the dose verification of multileaf collimator (MLC)-based CyberKnife plans by combining the Octavius 1000SRS detector and water-equivalent RW3 slab phantom. The slab phantom consists of 14 plates, each with a thickness of 10 mm. One plate was modified to support tracking by inserting 14 custom-made fiducials on surface holes positioned at the outer region of 10×10 cm 2 . The fiducial-inserted plate was placed on the 1000SRS detector and three plates were additionally stacked up to build the reference depth. Below the detector, 10 plates were placed to avoid longer delivery times caused by proximity detection program alerts. The cross-calibration factor prior to phantom delivery was obtained by performing with 200 monitor units (MU) on the field size of 95×92.5 mm 2 . After irradiation, the measured dose distribution of the coronal plane was compared with the dose distribution calculated by the MultiPlan treatment planning system. The results were assessed by comparing the absolute dose at the center point of 1000SRS and the 3-D Gamma (g) index using 220 patient-specific quality assurance (QA). The discrepancy between measured and calculated doses at the center point of 1000SRS detector ranged from −3.9% to 8.2%. In the dosimetric comparison using 3-D g-function (3%/3 mm criteria), the mean passing rates with g-parameter ≤ 1 were 97.4%±2.4%. The combination of the 1000SRS detector and RW3 slab phantom can be utilized for dosimetry validation of patient-specific QA in the CyberKnife MLC system, which made it possible to measure absolute dose distributions regardless of tracking mode.

Commissioning and clinical implementation of the first commercial independent Monte Carlo 3D dose calculation to replace CyberKnife M6™ patient‐specific QA measurements

“…In the low resolution area, filling the remainder of the 11 × 11 cm 2 area, the distance between the chambers is 5 mm. The use of the 1000SRS array for the CyberKnife has been investigated and found sufficiently accurate for patient‐specific QA measurements 4,6 . Our 1000SRS array has a fixed geometry with a slab phantom in which a set of three fiducial markers are embedded.…”

“…The use of the 1000SRS array for the Cyber-Knife has been investigated and found sufficiently accurate for patient-specific QA measurements. 4,6 Our 1000SRS array has a fixed geometry with a slab phantom in which a set of three fiducial markers are embedded. Pre-treatment alignment on the CyberKnife is based on a fiducial match between two stereoscopic x-ray images with digitally reconstructed radiographs based on the treatment planning CT, ensuring optimal alignment between planned and delivered geometry.…”

“…1 Patient-specific QA measurements can be performed using chambers, film, or diode arrays. [2][3][4][5][6][7] Especially for stereotactic CyberKnife treatment plans, both measurement equipment and analysis require stringent quality criteria. Internationally acknowledged gamma criteria of 2% dose difference and 2 mm distanceto-agreement Γ(2,2) are insufficient to detect possible errors relevant during CyberKnife dose delivery.…”

“…Patient‐specific QA measurements can be performed using chambers, film, or diode arrays 2–7 . Especially for stereotactic CyberKnife treatment plans, both measurement equipment and analysis require stringent quality criteria.…”

“…The gamma pass rates in this study exceed those of similar work. [2][3][4]6 This can be attributed to the use of a dedicated CyberKnife QA phantom and the beam delivery perpendicular to the array surface, to avoid angular correction of the array response.…”