Dose Verification for Tumor Motion with Different Treatment Planning Systems: A Dynamic Thorax Phantom Study
Geoffrey G. Zhang,
During radiation therapy for lung cancer, the respiratory motion of the target increases error and affects the treatment outcome. A four-dimensional computed tomography (4D-CT) technology developed recently can be used to obtain snapshot thoracic CT images during the entire respiratory cycle, which can be helpful in visualization of the tumor displacement in respiratory motion. This study employed a dynamic phantom to simulate tumor respiratory motion, with motion amplitudes of 2, 5 and 10 mm in the superior-inferior direction and motion periods of 4 and 6 s. 4D-CT imaging was applied to record the movement of the target, and maximum-intensity projection (MIP) imaging was used to define the internal target volume (ITV). Treatment plans were generated using three different treatment techniques—tomotherapy, intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). In addition, using interchangeable insert modules (an ionization chamber for point dose measurement and Gafchromic EBT3 film for planar dose distribution) in the dynamic phantom, the dose received by the target was measured. The dose received with motion was compared with that received under stationary conditions for dose errors due to respiratory motion. The point dose differences between moving and stationary conditions for tomotherapy, IMRT and VMAT were 0.48 ± 0.51%, 0.17 ± 0.45%, and 0.68 ± 0.70%, respectively; and at motion amplitudes of 2, 5, and 10 mm, the mean dose differences were 0.27 ± 0.38%, 0.37 ± 0.55% and 0.68 ± 0.74%, respectively. Based on planar dose analysis, at motion amplitudes of 2 and 5 mm, the dose distribution differences between moving and stationary conditions using the three treatment methods were all very small, and all passed the gamma index test with a criterion of 3%/3 mm, while at an amplitude of 10 mm, the edge of the ITV was associated with significantly greater dose errors. As studies have shown that in only approximately 10% of pulmonary tumors move with an amplitude >10 mm, 4D-CT MIP imaging in combination with delineation of the ITV may resolve the issue of dose error in most cases. For tumors with large motion amplitudes due to respiration, respiratory gating method may be used to reduce healthy lung tissues inside treatment field.