Four-Dimensional Dose Reconstruction for Lung Cancer in Volumetric Modulated Arc Therapy

Abstract

The purpose of this study is to evaluate actual dose distribution for the cases with different respiratory motion using four-dimensional (4D) dose reconstruction system. In the thorax region, respiratory motion of tumor may lead changes in dose distribution due to interplay effects caused by motions of both multi-leaf collimators (MLCs) and tumors. It is important to assess actual dose distribution in each treatment. The authors have developed a 4D dose reconstruction system based on Linac log data and respiratory wave form during VMAT treatment. The 4D dose reconstruction was performed by reproducing the actual irradiation conditions on the treatment planning system using 4D-CT image sets and various field groups that were divided into multiple fields DICOM RT-Plan files created using log data according to each tumor position based on respiratory wave form and EPID cine images. The authors reported in the previous ASTRO annual meeting about results of the phantom experiment that this system could reconstruct the actual dose distribution within the error of 2%. In this study, the case of one lung cancer patient was evaluated. The patient received the 2-arc VMAT irradiation under free breathing (2Gy / fr). The reconstructed dose distributions with different respiratory motion were compared with each other, as well as the planned dose distribution. The patient was treated using Clinac21EX linear accelerator equipped with aS1000 EPID (Varian) and real time position management system (Varian) for acquisition the respiratory wave form. Dose calculation was performed using Eclipse 10.0.28 (Varian) on 4D-CT images (10 phases), and deformable registration was used to accumulate dose distributions of each respiratory phase. Dose distribution of each respiratory phase varied between different treatment sessions, i.e. different respiratory motion, while the end-of-exhale phase contained the highest point dose, and the phase included 30 to 40% of the total dose. The dose distributions and DVHs after accumulation of all respiratory phases were quite similar through different treatment sessions. Any effects of changes in respiratory conditions could not be observed. Additionally the reconstructed dose distributions and planned dose distribution showed similar shape, and the reconstructed CTV center doses were agreed with the planned dose within 0.5%. The change of the dose distribution was quite small due to small tumor motion. On the other hand, it was confirmed that the irradiation of the treatment plan has been correctly performed using (calculated on) this dose reconstruction system. Using the developed system, 4D dose distribution in the clinical case was successfully reconstructed, and the distribution difference between different respiratory motions was evaluated.