Low dose megavoltage cone beam computed tomography with an unflattened 4 MV beam from a carbon target

Abstract

Megavoltage cone beam computed tomography (MVCBCT) is routinely used for visualizing anatomical structures and implanted fiducials for patient positioning in radiotherapy. MVCBCT using a 6 MV treatment beam with high atomic number (Z) target and flattening filter in the beamline, as done conventionally, has lower image quality than can be achieved with a MV beam due to heavy filtration of the low-energy bremsstrahlung. The unflattened beam of a low Z target has an abundance of diagnostic energy photons, detected with modern flat panel detectors with much higher efficiency given the same dose to the patient. This principle guided the development of a new megavoltage imaging beamline (IBL) for a commercial radiotherapy linear accelerator. A carbon target was placed in one of the electron primary scattering foil slots on the target-foil slide. A PROM on a function controller board was programed to put the carbon target in place for MVCBCT. A low accelerating potential of 4.2 MV was used for the IBL to restrict leakage of primary electrons through the target such that dose from x rays dominated the signal in the monitor chamber and the patient surface dose. Results from phantom and cadaver images demonstrated that the IBL had much improved image quality over the treatment beam. For similar imaging dose, the IBL improved the contrast-to-noise ratio by as much as a factor of 3 in soft tissue over that of the treatment beam. The IBL increased the spatial resolution by about a factor of 2, allowing the visualization of finer anatomical details. Images of the cadaver contained useful information with doses as low as 1 cGy. The IBL may be installed on certain models of linear accelerators without mechanical modification and results in significant improvement in the image quality with the same dose, or images of the same quality with less than one-third of the dose.

Characterization of a 2.5 MV inline portal imaging beam

“…There are several Monte Carlo simulation studies involving low atomic number (Z) targets in Varian, 1 , 2 , 3 Elekta, 4 , 5 and Siemens 6 , 7 linacs. However, a literature review on this subject reveals lack of experimental data and guidance leading to clinical use of this important new modality.…”

Assessment of image quality and dose calculation accuracy on kV CBCT, MV CBCT, and MV CT images for urgent palliative radiotherapy treatments

“…Instead of using the treatment beam, it used a low MV energy and a carbon target during imaging to improve image contrast (14) . Images were acquired using between 5 and 15 monitor units (MUs) (about 5‐15 cGy per acquisition).…”

Dependence of intrafraction motion on fraction duration for pediatric patients with brain tumors

“…The CBCT used was an investigational low dose MV device which delivers 1 cGy per CBCT at isocenter. ( 3 , 4 ) If the magnitude of the patient's pre‐CBCT offset was $\ge 2\hspace{0.17em}\text{mm}$, the position was corrected. The difference between the patient's position (lateral, longitudinal, and vertical) based on the post‐CBCT and the assumed position at the start of treatment was determined and labeled intrafraction motion.…”