Image-guided radiation therapy: Physician's perspectives

Abstract

The evolution of radiotherapy has been ontogenetically linked to medical imaging. Over the years, major technological innovations have resulted in substantial improvements in radiotherapy planning, delivery, and verification. The increasing use of computed tomography imaging for target volume delineation coupled with availability of computer-controlled treatment planning and delivery systems have progressively led to conformation of radiation dose to the target tissues while sparing surrounding normal tissues. Recent advances in imaging technology coupled with improved treatment delivery allow near-simultaneous soft-tissue localization of tumor and repositioning of patient. The integration of various imaging modalities within the treatment room for guiding radiation delivery has vastly improved the management of geometric uncertainties in contemporary radiotherapy practice ushering in the paradigm of image-guided radiation therapy (IGRT). Image-guidance should be considered a necessary and natural corollary to high-precision radiotherapy that was long overdue. Image-guided radiation therapy not only provides accurate information on patient and tumor position on a quantitative scale, it also gives an opportunity to verify consistency of planned and actual treatment geometry including adaptation to daily variations resulting in improved dose delivery. The two main concerns with IGRT are resource-intensive nature of delivery and increasing dose from additional imaging. However, increasing the precision and accuracy of radiation delivery through IGRT is likely to reduce toxicity with potential for dose escalation and improved tumor control resulting in favourable therapeutic index. The radiation oncology community needs to leverage this technology to generate high-quality evidence to support widespread adoption of IGRT in contemporary radiotherapy practice.

Keywords: Conformal radiotherapy; high-precision; image-guidance; verification.

Figure 1

Process map and workflow of IGRT showing a series of inter-connected steps of treatment planning, delivery, and verification with a feedback loop

Figure 2

Images for volumetric verification on contemporary computed-tomography (CT) based IGRT systems in head neck cancer. Note the better image quality with kilovoltage cone-beam CT (a) compared to megavoltage CT (b)

Figure 3

Adaptive radiotherapy for bladder cancer. Clinical target volume is expanded by 5mm incremental margins isotropically (a) to generate six planning target volumes (PTVs) with a separate radiotherapy plan for every PTV. Verification CT prior to treatment delivery showing changing PTV (arrow) due to change in bladder filling (b). The plan with appropriate margin encompassing the entire bladder safely (arrow) is selected (c) and treatment delivered. Immediate post-treatment imaging (d) confirms adequate coverage of the PTV (solid yellow line) by prescription isodose (red dashed line)