Clinical Megavoltage Cone-Beam CT
Dr. Pouliot's group, in collaboration with Siemens Oncology Care Systems, has been working on the clinical implementation of megavoltage cone-beam computed tomography (MV CBCT) for the last five years. The system consists of a standard linear accelerator equipped with an amorphous-silicon flat panel electronic portal-imaging device adapted for mega-electron volt (MeV) photons. An integrated computer workspace provides automated acquisition of projection images, image reconstruction, CT to CBCT image registration, and couch shift calculation. The system demonstrates submillimeter localization precision and sufficient soft-tissue resolution to visualize structures such as the prostate. In the last few years, we have used the MV CBCT system to detect nonrigid spinal cord distortions, monitor tumor growth and shrinkage, and locate and position stationary tumors in the lung. MV CBCT has also greatly improved the delineation of structures on CT images for patients with hip prosthesis. Click on each image for more details.
3D Patient Alignment using MV CBCT and the Planning CT
MVCBCT provides a 3D image of the patient in treatment position. This image in pair with the reference CT used for planning provides all the patient information needed to perform patient alignment based on bony anatomy and soft tissues. Our MV CBCT imaging system uses a conventional linac and a flat-panel EPID adapted for the detection of MeV photons. In 45 seconds, the gantry rotates 200° around the patient acquiring one image per degree. The table shift required to register the daily MV CBCT with the reference planning CT is available approximately 3 minutes after the beginning of the image acquisition.
A comparison of a reference planning CT (top) of a prostate patient, an MV CBCT (middle) of the patient lying on the treatment table, and a fusion of the 2 images (bottom). With an MV CBCT of 14.4 MUs, structures such as the prostate, rectum, fat, muscles, bone, and gold seeds are visible. Registration can be done automatically or manually using the 3 typical views (axial, sagittal, and coronal). The planning contours and the points of interest can be displayed on the MV CBCT to facilitate the registration.
MVCBCT Image Quality
Image Quality In The Presence of Metallic Objects
MVCBCT provides good quality 3D images of the patient for planning purposes even when metallic objects are present such as patients with hip prosthesis.
Monitoring Anatomical Changes
MVCBCT provides a fast and convenient way to monitor the anatomical changes over the course of the treatment, which may have dosimetric consequences.
Dosimetric Impact of Anatomical Changes
The anatomy of the patients may change during the course of treatment due to weight loss, tumor shrinkage or anatomy variability. These changes may have dosimetric consequences that necessitate re-planning. MVCBCT can be used to provide a quantitative answer on when it is time to re-plan.