Radiation oncologists use image guided radiation therapy, or IGRT, to help better deliver the radiation to the cancer since tumors can move between treatments due to differences in organ filling or movements while breathing. IGRT involves conformal radiation treatment guided by imaging, such as CT, ultrasound or X-rays, taken in the treatment room just before the patient is given the radiation treatment on a daily basis.
All patients first undergo a CT scan as part of the planning process. The information from the CT scan is then transmitted to a computer in the treatment room to allow doctors to compare the earlier image with the images taken just before treatment. During IGRT, doctors compare these images to see if the treatment needs to be adjusted. This allows doctors to better target the cancer while avoiding nearby healthy tissue. In some cases, doctors will implant a tiny marker in or near the tumor to pinpoint it for IGRT. This helps to account for organ/tumor motion even if the body is immobilized by a casting device.
IGRT has been evolved to enable clinicians to treat the tumours that move with internal motion. IGRT provides a very effective means for mitigating the risk of tumor motion. Real time image guidance and adaptive radiation therapy involves constantly imaging the motion of the tumour during treatment delivery and changing beam delivery on the fly to compensate for undesired motion. IGRT is most suitable for Tumours of the Prostate, Urinary Bladder, Lungs and Gyneccological cancers.
Objective of image guidance or IGRT is to achieve highly accurate targeting of the tumour. Use of Kilovoltage cone beam CT (KV CBCT) allows three dimensional identification and matching of target (on a daily basis) to achieve sub-millimeter accuracy. Such accuracy in delivery allows reduction in normal tissue margins while planning treatment, leading to high cure rates with low radiation toxicity.
There are two types of variation in tumour position; interfraction and intrafraction. Interfraction variation can be caused by many factors; variation in patient positioning, deformation of body tissues, relative movement of organs e.g. bowel bladder, uterus etc., changes in patient′s weight due to nutritional factors, changes in tumour size due to response etc. IGRT eliminates interfraction variation and can help in minimizing intrafraction variation too.