Benchmarking Doses to Inform Optimization of Radiological Protection for Verification Imaging in Radiation Therapy

Realization of the full potential of external beam radiation therapy requires accurate targeting, which in turn depends on frequent imaging of patients immediately before or during treatment. These verification images may expose normal tissues surrounding the tumor to additional radiation. Although imaging doses are substantially lower than therapeutic doses, irradiated volumes are larger and repeated exposure carries a non-negligible risk of inducing second primary cancers in adjacent tissues and other late effects, particularly in younger patients with longer life expectancies. Cone beam computed tomography with kilovoltage systems mounted on linear accelerators is the modality used most widely for verification in image guided radiation therapy. In contrast to diagnostic radiology, where imaging parameters are adjusted routinely to optimize radiological protection for individual patients, radiation therapy departments often use vendors' supplied default protocols with limited optimization. As imaging is often performed at every fraction, there is considerable scope for further optimization of radiological protection. However, information about doses from cone beam computed tomography imaging, which is a prerequisite, is frequently not available. In diagnostic radiology, imaging dose management is supported by routine surveys of standardized dose indices and the use of diagnostic reference levels for benchmarking practice. This paper reviews the methods used in diagnostic radiology and considers how a similar approach might be applied to patient imaging doses in radiation therapy. It identifies appropriate dose quantities such as the Cone Beam Dose Index and wide beam computed tomography dose index that might be used for surveying and benchmarking imaging dose levels. It sets out actions that will be needed to initiate optimization to balance image quality requirements against patient doses and discusses the next phase of setting up radiation therapy specific dose reference levels. Establishing such reference levels can raise awareness, support systematic imaging dose reduction, and improve the safety and effectiveness of image guided radiation therapy.