Megavoltage imaging, megavoltage cone beam CT and dose-guided radiation therapy.

Elaborate methods of patient imaging for diagnostics, dose calculation, and radiation delivery are currently used to develop treatment plans with highly conformal patient dose distributions. However, the true delivered dose likely deviates from the planned distribution due to differences in patient position, anatomic changes due to weight loss or tumor shrinkage or variations of linear accelerator output during treatment. All the steps in a radiation treatment from diagnostics to the planning process are based on three-dimensional imaging, with the exception of treatment verification performed with electronic portal imaging devices (EPIDs) and two-dimensional images. Megavoltage cone beam CT (MV CBCT) generates an accurate three-dimensional representation of the patient anatomy, moments before the same X-ray beam is used for treatment. The three-dimensional images will provide additional information on the patient's treatment position and offer a wide range of opportunities to improve the delivery of radiation. The MV CBCT image can be registered with the planning CT for patient setup verification and correction. The periodic acquisition of three-dimensional images will allow the monitoring of anatomical changes over the treatment course due to tumor response or weight loss. The MV CBCT image can also be imported into the planning system to complement the regular CT in the presence of metallic objects or to measure the dosimetric impact of patient misalignment and anatomy modification on dose distribution. By combining exit dosimetry with the EPID and MV CBCT, this technology may play a key role in tracking the dose delivered to the patient, taking us into an era of dose-guided radiation therapy .