A phantom study on the effects of target motion in non-gated kV-CBCT imaging

A dosimetric phantom study of thoracic radiotherapy based on three-dimensional modeling of mediastinal lymph nodes

The aim of the present study was to investigate the optimal strategy and dosimetric measurement of thoracic radiotherapy based on three-dimensional (3D) modeling of mediastinal lymph nodes (MLNs). A 3D model of MLNs was constructed from a Chinese Visible Human female dataset. Image registration and fusion between reconstructed MLNs and original chest computed tomography (CT) images was conducted in the Eclipse™ treatment planning system (TPS). There were three plans, including 3D conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT), which were designed based on 10 cases of simulated lung lesions (SLLs) and MLNs. The quality of these plans was evaluated via examining indexes, including conformity index (CI), homogeneity index and clinical target volume (CTV) coverage. Dose-volume histogram analysis was performed on SLL, MLNs and organs at risk (OARs). A Chengdu Dosimetric Phantom (CDP) was then drilled at specific MLNs according to 20 patients with thoracic tumors and of a medium-build. These plans were repeated on fused MLNs and CDP CT images in the Eclipse™ TPS. Radiation doses at the SLLs and MLNs of the CDP were measured and compared with calculated doses. The established 3D MLN model demonstrated the spatial location of MLNs and adjacent structures. Precise image registration and fusion were conducted between reconstructed MLNs and the original chest CT or CDP CT images. IMRT demonstrated greater values in CI, CTV coverage and OAR (lungs and spinal cord) protection, compared with 3D-CRT and VMAT (P<0.05). The deviation between the measured and calculated doses was within ± 10% at SLL, and at the 2R and 7th MLN stations. In conclusion, the 3D MLN model can benefit plan optimization and dosimetric measurement of thoracic radiotherapy, and when combined with CDP, it may provide a tool for clinical dosimetric monitoring.

Phase-specific cone beam computed tomography reduces reconstructed volume loss of moving phantom

PURPOSE

The accurate volumetric calculation of moving targets/organs is required to use cone-beam computed tomography (CBCT) for replanning purposes. This study was aimed to correct the reconstructed volume losses of moving phantoms by phase-specific CBCT.

MATERIALS AND METHODS

Planning fan-beam CT (FBCT) of five hepatobiliary/gastrointestinal/pancreatic cancer patients were acquired under active breathing control and compared with free-breathing CBCT for kidney volumes. Three different-sized ball phantoms were scanned by FBCT and CBCT. Images were imported to a planning system to compare the reconstructed volumes. The phantoms were moved longitudinally on an oscillator with different amplitudes/frequencies. The phase-specific projections of CBCT for moving phantoms were selected for volume reconstruction.

RESULTS

The differences in reconstructed volumes of static small, medium, large phantoms between FBCT and CBCT were - 6.7%, - 2.3%, and - 2.0%, respectively. With amplitudes of 7.5-20 mm and frequencies of 8-16 oscillations/min, volume losses on CBCT were comparable with FBCT in large moving phantoms (range 9.1-27.2%). Amplitudes were more subject to volume losses than frequencies. On phase-specific CBCT, volume losses were reduced to 2.3-6.5% by reconstruction using 2-3 projections at end/midoscillation phase.

CONCLUSION

Amplitude had more impact than frequency on volume losses of moving phantoms on CBCT. Phase-specific CBCT reduced volume losses.