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Abstract
Generally, to evaluate gated radiation therapy, moving phantoms are used to simulate organ motion. Since the target moves in every direction, we need to take into account motion in each direction. This study proposes methods to evaluate gated radiation therapy using gamma index analysis and to visualize adequate gating window sizes according to motion ranges. The moving phantom was fabricated to simulate motion in the craniocaudal direction. This phantom consisted of a moving platform, the I'm MatriXX, and solid water phantoms. A 6 MV photon filed with a field size of 4×4 cm2 was delivered to the phantom using the gating system, while the phantom moved in the 1-, 2-, 3-, 4-, and 5-cm motion ranges. The gating windows were set at 40∼60%, 30∼40%, and 0∼90%, respectively. The I'm MatriXX acquired the dose distributions for each scenario and the dose distributions were compared with a 4×4 cm2 static filed. The tolerance of the gamma index was set at 3%/3 mm. The greater the gating window, the lower the pass rate, and the greater the motion range, the lower the pass rate in this study. In case treatment without gated radiation therapy for the target with motion of 2 cm, the pass rate was less than 96%. But it was greater than 99% when gated radiation therapy was used. However gated radiation therapy was used for the target with motion greater than 4 cm, the pass rate could not be greater than 97% when gating window was set as 30∼70%. But when the gating window set as 40∼60%, the pass rate was greater than 99%.
Generally, to evaluate gated radiation therapy, moving phantoms are used to simulate organ motion. Since the target moves in every direction, we need to take into account motion in each direction. This study proposes methods to evaluate gated radiation therapy using gamma index analysis and to visualize adequate gating window sizes according to motion ranges. The moving phantom was fabricated to simulate motion in the craniocaudal direction. This phantom consisted of a moving platform, the I'm MatriXX, and solid water phantoms. A 6 MV photon filed with a field size of 4×4 cm2 was delivered to the phantom using the gating system, while the phantom moved in the 1-, 2-, 3-, 4-, and 5-cm motion ranges. The gating windows were set at 40∼60%, 30∼40%, and 0∼90%, respectively. The I'm MatriXX acquired the dose distributions for each scenario and the dose distributions were compared with a 4×4 cm2 static filed. The tolerance of the gamma index was set at 3%/3 mm. The greater the gating window, the lower the pass rate, and the greater the motion range, the lower the pass rate in this study. In case treatment without gated radiation therapy for the target with motion of 2 cm, the pass rate was less than 96%. But it was greater than 99% when gated radiation therapy was used. However gated radiation therapy was used for the target with motion greater than 4 cm, the pass rate could not be greater than 97% when gating window was set as 30∼70%. But when the gating window set as 40∼60%, the pass rate was greater than 99%.
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Fig. 1.
Moving phantom consists of; (a) Moving platform, (b) I'm MatriXX, and (c) Solid water phantom. The platform moves by crank connected to the geared motor. This phantom designed to simulate respiratory-induced organ motion in craniocaudal direction.
Fig. 2.
Example of 4×4 cm2 field dose distribution for 5 cm motion range: the larger gating window, the larger penumbra to the motion directions. No gating for the moving target caused shift of dose distribution.
Fig. 3.
Results of gamma distribution for each gating window comparing to the reference (static field); white: pass/black: fail. Increase of motion range causes decrease pass rate.
Fig. 4.
(a) Gating window at 40%∼60% with 5 cm motion range, (b) Gating window at 30%∼70% with 5 cm motion range: Sum of beam on time for each gating level were compared with total treatment time, respectively.
Table 1.
Results of pass rate calculated from gamma index for each gating window comparing to reference (static field).
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