AbstractID: 8965 Title: Dose-Guided Radiation Therapy using Low-Dose Megavoltage
Cone-beam CT
The objective of this work is to demonstrate that Megavoltage Cone Beam CT (MVCT) can be performed with a very low exposure to
the patient, opening the possibility to use it clinically and perform dose guided radiation therapy.
Using a standard linac, the 6 MV dose rate was lowered to expose an a-Si flat panel using from 0.04 to 0.08 monitor units (M.U.) per
image. An acquisition mode was designed to produce a high signal to noise ratio without pulsing artifacts. Several acquisitions were
performed with either an anthropomorphic head phantom or a frozen sheep head, using a combination of 90 to 180 images
incremented by 2 to 4 degrees. For each experiment, MVCT reconstruction was performed to visualize the 3D bony anatomy and
some soft-tissue details. The MVCT image was registered with the planning CT. A partial cone beam reconstruction was then
performed using the portal images acquired during treatment delivery in order to initiate dose reconstruction. Finally, registration
between the planning CT and the MVCT was performed using either a manual alignment tool or a mutual information algorithm.
This work shows how a conventional linac with stable low dose rate and an a-Si imager can be used to obtain MVCT reconstructed
images with as little as 5 M.U. We believe that the quality of these images, along with rapid acquisition and reconstruction times,
suggests that clinically useful MVCT can be performed using a standard linac and flat panel imager.
Research supported by Siemens Oncology Care Systems