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Abstract
One of the methods to consider the effect of respiratory motion of a tumor target in radiotherapy is to establish a treatment plan with the internal target volume (ITV) created based on an accurate analysis of the target motion displacement. When this method is applied to intensity modulated radiotherapy (IMRT), it is expected to yield a different treatment dose distribution under the motion condition according to the IMRT method. In this study, we prepared ITV-based IMRT plans with conventional IMRT using fixed gantry angle beams, RapidArc using volumetric modulated arc therapy, and tomotherapy using helical therapy. Then, the variation in dose distribution caused by the target motion was analyzed by the dose measurement in the actual motion condition. A delivery quality assurance plan was prepared for the established IMRT plan and the dose distribution in the actual motion condition was measured and analyzed using a two-dimensional diode detector placed on a moving phantom capable of simulating breathing movements. The dose measurement was performed considering only a uniform target shape and motion in the superior-inferior (SI) direction. In this condition, it was confirmed that the error of the dose distribution due to the target motion is minimum in tomotherapy. This is thought to be due to the characteristic of tomotherapy that treats the target sequentially by dividing it into several slices. When the target shape is uniform and the main target motion direction is SI, it is considered that tomotherapy for the ITV-based IMRT method has a characteristic which can reduce the dose difference compared with the plan dose under the target motion condition.
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Fig. 4
Example of the measured dose distribution and error analysis in the moving condition. (a) RapidArc, (b) FB-IMRT, (c) Tomotherapy.
Fig. 5
Graphs showing the effect of target motion according to the CTV increase due to CTV length increase. (a) Pass rate in 2 cm moving condition. (b) Pass rate in 4 cm moving condition.
Table 1.
The length of internal target volume (ITV) according to the motion range of clinical target volume (CTV) considered in this study.
| CTV length | Motion range | |
|---|---|---|
| 2 cm | 4 cm | |
| 2 cm | 4 cm | 6 cm |
| 4 cm | 6 cm | 8 cm |
| 6 cm | 8 cm | |
Table 2.
Dose constraints in the optimization process of the IMRT planning.
| ITV | V4,750 cGy> 98% |
| OAR_L | Dmax <2,500 cGy, Dmean <1,200 cGy |
| OAR_R | Dmax <2,800 cGy, Dmean <1,000 cGy |
| OAR_S | Dmax<2,500 cGy |
| OAR_I | D <2,000 cGy |
Table 3.
The calculated pass rates by the gamma evaluation for the CTV with 2 cm length in the condition of motion.
Table 4.
The calculated pass rates by the gamma evaluation for the CTV with 4 cm length in the condition of motion.
Table 5.
The calculated pass rates by the gamma evaluation for the CTV with 6 cm length in the condition of motion.
| ITV length | Static condition | 7 sec motion period | 4 sec motion period | |
|---|---|---|---|---|
| 2 cm range | 2 cm range | |||
| RapidArc | 8 cm | 99.5% | 83.1% | 86.1% |
| FB-IMRT | 8 cm | 100.0% | 88.4% | 86.0% |
| Tomotherapy | 8 cm | 100.0% | 94.5% | 95.8% |
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