Image-guided radiotherapy: rationale, benefits, and limitations.

Technological advances have greatly enhanced the specialty of radiation oncology by allowing more healthy tissue to be spared for the same or better tumour coverage. Developments in medical imaging are integral to radiation oncology, both for design of treatment plans and to localise the target for precise administration of radiation. At planning, definition of the tumour and healthy tissue is based on CT, augmented frequently with MRI and PET. At treatment, three-dimensional soft-tissue imaging can also be used to localise the target and tumour motion can be tracked with fluoroscopic imaging of radio-opaque markers implanted in or near the tumour. These developments allow changes in tumour position, size, and shape that take place during radiotherapy to be measured and accounted for to boost geometric accuracy and precision of radiation delivery. Image-guided treatment also enhances uniformity in doses administered in a population of patients, thus improving our ability to measure the effect of dosimetric and non-dosimetric factors on tumour and healthy tissue outcomes in clinical trials. Increased precision and accuracy of radiotherapy are expected to augment tumour control, reduce incidence and severity of toxic effects after radiotherapy, and facilitate development of more efficient shorter schedules than currently available.