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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 cm2. 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 mm2. 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.
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Fig. 1.
The position of fiducial-inserted RW3 slab in patient-specific QA phantom: (a) Target zone overlapped by two x-ray imagers and (b) RW3 plate including 14 custom-made fiducials.
Fig. 2.
Patient-specific QA phantom including fiducial-inserted solid water phantom and Octavius 1000SRS detector.
Fig. 3.
QA template plan: (a) Fiducial identifications in fiducial-inserted RW3 plate and (b) a volume of interest representing ion chamber array.
Fig. 5.
Comparison between measured planar dose and calculated 3D dose distributions: (a) calculated point dose at the center of Octavius 1000SRS detector and (b) measured planar dose distribution in Verisoft.
Fig. 6.
Quantitative comparisons using 220 patient specific QA plans: (a) point dose difference and (b) gamma agreement.
Table 1.
Tracking mode compatibility between patient plan and QA template plan.
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