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Abstract
The conventional delivery quality assurance (DQA) process for RapidArc (Varian Medical Systems, Palo Alto, USA), has the limitation that it measures and analyzes the dose in a phantom material and cannot analyze the dosimetric changes under the motional organ condition. In this study, a DQA method was designed to overcome the limitations of the conventional DQA process for internal target volume (ITV) based RapidArc. The dynamic DQA measurement device was designed with a moving phantom that can simulate variable target motions. The dose distribution in the real volume of the target and organ-at-risk (OAR)s were reconstructed using 3DVH with the ArcCHECK (SunNuclear, Melbourne, USA) measurement data under the dynamic condition. A total of 10 ITV-based RapidArc plans for liver-cancer patients were analyzed with the designed dynamic DQA process. The average pass rate of gamma evaluation was 81.55±9.48% when the DQA dose was measured in the respiratory moving condition of the patient. Appropriate method was applied to correct the effect of moving phantom structures in the dose calculation, and DVH data of the real volume of target and OARs were created with the recalculated dose by the 3DVH program. We confirmed the valid dose coverage of a real target volume in the ITV-based RapidArc. The variable difference of the DVH of the OARs showed that dose variation can occur differently according to the location, shape, size and motion range of the target. The DQA process devised in this study can effectively evaluate the DVH of the real volume of the target and OARs in a respiratory moving condition in addition to the simple verification of the accuracy of the treatment machine. This can be helpful to predict the prognosis of treatment by the accurate dose analysis in the real target and OARs.
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Fig. 2
Consideration of moving phantom plates in dose calculation: (a) Real structure of plates beneath ArcCHECK, (b) Virtual solid-water plate equivalent to three plates.
Fig. 3
RapidArc beam intensity which exhibit the same motion as the target and relatively equivalent condition of the target motion: (a) Real treatment condition in ITV-based RapidArc, (b) relatively equivalent condition by the application measured dose data using moving ArcCHECK.
Fig. 4
Effect of moving phantom plates on the DQA error evaluation in patient (B): (a) Increased error due to the effect of the plates in moving phantom, (b) Reduce the error by the application of virtual solid-water plate in reference dose calculation.
Fig. 5
Total calculated DVH data through dynamic DQA process. Solid line: DVH in the real volume of target and OAR, Dashed line: DVH calculated in the ITV-based RapidArc plan.
Fig. 6
Dynamic DQA process for ITV-based RapidArc designed in this study to evaluate DVH of the real volume of target and OARs.
Table 1.
Volume comparison of target and OARs between ITV-based plan and Gated plan with information on the period and range of respiratory motion in each patient.
Table 2.
DQA results of the ArcCHECK measurement in a static condition.
| Patients | A | B | C | D | E | F | G | H | I | J |
|---|---|---|---|---|---|---|---|---|---|---|
| Pass rate (%) | 99.5 | 98.9 | 99.2 | 98.4 | 99.3 | 97.6 | 99.0 | 98.4 | 99.0 | 99.1 |
Table 3.
DQA error analysis according to the effect of virtual SW plate under the static and moving condition.
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