Quality assurance of volumetric modulated arc therapy: Evaluation and comparison of different dosimetric systems
Dedicated phantom for absolute point dose measurements: A 30 cm diameter water filled PMMA cylindrical container (4 mm thickness). The ion chamber can be inserted on the central axis as well as in any position 2.5 cm off-axis.
(a) MapPhan Phantom modified to permit the insertion of a film. (b) The same phantom used with MAPCHECK device: Film and 2D diode array are in the same coronal plane.
Average pass-rate values obtained by MAPCHECK, DELTA4, and SEVEN29 measurements performing a (3%, 2 mm) analysis. Results are shown for each detector separately for the two TPSs.
Example of dose deviations distribution obtained for one of the examined plan by DELTA4 system. Dose deviations are shown on the left. For completeness, for the same plan, DTA (center of the image), and gamma index (on the right) statistics are also shown.
Pass-rate [ (3%, 2 mm)] as a function of average leaf travel for 37 plans measured by DELTA4. A statistically significant correlation is observed between the two variables (, ).
(a) Sensitivity to translational shifts of ±3 mm, observed over 12 plans for MAPCHECK, DELTA4, and SEVEN29. The sensitivity is expressed as the averaged reduction in pass-rate obtained by index analysis, that is, the reduction in the percentage of points satisfying (3%, 3 mm) . A dependence on the shift directions is observed for the three detectors. (b) Sensitivity of DELTA4 to a 3° gantry offset, for seven examined plans [pass-rate reduction for (3%, 2 mm)].
Summary of the considered plans and corresponding dosimetric verifications, indicated separately for both TPS.
Absolute point dose measurements. Results obtained by an ion chamber inserted on the central axis of a dedicated cylindrical water phantom. Descriptive statistics is reported for the percentage differences between calculated and measured doses, for the totality of plans and separately for both TPS.
Comparison between calculated and measured dose distributions. Percentage of points satisfying (3%, 3 mm) : Statistics for all plans and all dosimetric systems. The average pass-rate obtained over seven and seven Oncentra plans optimized for the same treatment cases is shown separately, and indicated as comparative study.
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