Ross Berbeco, PhD
Dana-Faber Cancer Institute
Harvard Medical School, Boston

MV CBCT as a radiotherapy imaging solution for LMICs

Medical linear accelerators, producing high energy (MV) x-rays, are by far the most common tool for radiation therapy treatments of cancer and other diseases. Too often, however, the most advanced radiation treatments are not available in low resources settings. There are a multitude of reasons for this, and many people are working to solve the associated technical, logistical, educational, financial and political challenges.

One such challenge is the need for high-quality, low-dose imaging to support image-guided radiation therapy (IGRT) procedures at both the treatment simulation and treatment delivery stages. The incorporation of on-board kV-CBCT (cone beam CT) imaging has revolutionized IGRT1 in high resource settings, but the additional infrastructure requirements for power and maintenance, not to mention the additional costs associated with the equipment and training, can be a limitation. In addition, there is still a need for kV-CT simulation for treatment planning, requiring further resources which may be unavailable or in use for other procedures.

By contrast, the MV treatment beam itself may be used for CBCT imaging, reducing many of the challenges associated with the introduction of add-on imaging devices, or separate scanners. While MV-CBCT has the technical advantage of decreased metal artifacts and accurate electron density mapping, the major disadvantage is the lower soft-tissue contrast compared to kV-CBCT.

Advances in MV flat-panel detector technology have been pursued by several groups with the focus on increasing detector efficiency to minimize the amount of radiation dose needed for patient imaging, while simultaneously maximizing contrast2-5. Novel detector configurations have included innovative scintillator materials6 and stacking of conventional flat-panel layers7, to increase photon collection. These approaches have shown great promise, with image quality approaching that of kV-CBCT for the same amount of imaging dose.

By incorporating these detector innovations, future radiotherapy delivery devices could be delivered with reduced cost and infrastructure demands, without sacrificing quality. The added advantages of the metal artifact reduction and electron density mapping could provide additional capability for adaptive radiotherapy and “see-and-treat” procedures. While these technologies would be a boon to low resource environments, one would expect them to also be sought after in higher resource environments as well, providing access to equal treatment capabilities across economic differences. The goal is a simpler, cheaper, and better imaging solution that would be available to all cancer patients, regardless of geography.

Improvements in image quality with a four-layer multi-layer imager (MLI).  Reconstructed images of a phantom with material inserts are shown for (left) a standard kV on-board imager (OBI) and 125 kVp source, (middle) a standard single-layer MV detector (Varian AS1200) and (right) a four-layer MLI (all layers combined) and 2.5 MV beam delivery. All images were acquired at nominally the same dose.

(Myronakis M et al. Medical Physics 2020)


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