3D delivered dose assessment using a 4DCT-based motion model

Abstract

Purpose: The purpose of this work is to develop a clinically feasible method of calculating actual delivered dose distributions for patients who have significant respiratory motion during the course of stereotactic body radiation therapy (SBRT). Methods: A novel approach was proposed to calculate the actual delivered dose distribution for SBRT lung treatment. This approach can be specified in three steps. (1) At the treatment planning stage, a patient-specific motion model is created from planning 4DCT data. This model assumes that the displacement vector field (DVF) of any respiratory motion deformation can be described as a linear combination of some basis DVFs. (2) During the treatment procedure, 2D time-varying projection images (either kV or MV projections) are acquired, from which time-varying "fluoroscopic" 3D images of the patient are reconstructed using the motion model. The DVF of each timepoint in the time-varying reconstruction is an optimized linear combination of basis DVFs such that the 2D projection of the 3D volume at this timepoint matches the projection image. (3) 3D dose distribution is computed for each timepoint in the set of 3D reconstructed fluoroscopic images, from which the total effective 3D delivered dose is calculated by accumulating deformed dose distributions. This approach was first validated using two modified digital extended cardio-torso (XCAT) phantoms with lung tumors and different respiratory motions. The estimated doses were compared to the dose that would be calculated for routine 4DCT-based planning and to the actual delivered dose that was calculated using "ground truth" XCAT phantoms at all timepoints. The approach was also tested using one set of patient data, which demonstrated the application of our method in a clinical scenario. Results: For the first XCAT phantom that has a mostly regular breathing pattern, the errors in 95% volume dose (D95) are 0.11% and 0.83%, respectively for 3D fluoroscopic images reconstructed from kV and MV projections compared to the ground truth, which is clinically comparable to 4DCT (0.093%). For the second XCAT phantom that has an irregular breathing pattern, the errors are 0.81% and 1.75% for kV and MV reconstructions, both of which are better than that of 4DCT (4.01%). In the case of real patient, although it is impossible to obtain the actual delivered dose, the dose estimation is clinically reasonable and demonstrates differences between 4DCT and MV reconstruction-based dose estimates. Conclusions: With the availability of kV or MV projection images, the proposed approach is able to assess delivered doses for all respiratory phases during treatment. Compared to the planning dose based on 4DCT, the dose estimation using reconstructed 3D fluoroscopic images was as good as 4DCT for regular respiratory pattern and was a better dose estimation for the irregular respiratory pattern.

Evaluating the four-dimensional cone beam computed tomography with varying gantry rotation speed

“…1 cm. For a gantry rotation speed 200°min 21 , maximum deviation of 226.8% was observed in the target size 1 cm, whereas it was 217.8% for 3 cm target size.…”

“…Full paper: Evaluating the 4DCBCT with different gantry rotation speed BJR 210.8% when using the gantry rotation speed 36°min 21 . Among the three target sizes, the maximum deviation was observed for smaller target size i.e.…”

“…The ITV deviation increased with increasing gantry speed (210.8% vs 217.8% for 36°and 200°min 21 in 3-cm target) and decreasing target size (217.8% vs 226.8% for target diameter 3 and 1 cm in 200°min 21 ). Similarly, the image quality indicators such as spatial resolution, contrast-to-noise ratio and uniformity also degraded with increasing gantry rotation speed.…”

“…Respiratory motion creates artefacts in CBCT images, such as blurring, doubling, streaking and distortions, which greatly degrade the image quality and hence affect the target localization accuracy. [17][18][19] Nowadays, the four-dimensional CT (4DCT) scan is increasingly used in clinics for radiotherapy planning of thoracic and upperabdomen sites 20,21 owing to its superiority in eliminating the respiration-induced artefacts, which also enables a more accurate description of tumour motion. For a true four-dimensional (4D) treatment, ideally the verification imaging should also include a fourth dimension.…”

“…A dynamic thorax phantom with tumour mimicking inserts of diameter 1, 2 and 3 cm was programmed to simulate the respiratory motion (4 s) of the target. 4DCBCT images were acquired with different gantry rotation speeds (36°, 50°, 75°, 100°, 150°and 200°min 21 ). Owing to the technical limitation of 4DCBCT system, average cone beam CT (CBCT) images derived from the 10 phases of 4DCBCT were used for the internal target volume (ITV) contouring.…”