What Are IMRT and IGRT?
Intensity-modulated radiation therapy (IMRT) is an advanced mode of high-precision radiotherapy that utilizes computer-controlled X-ray accelerators to deliver precise radiation doses to a malignant tumor or specific areas within the tumor. The radiation dose is designed to conform to the three-dimensional (3-D) shape of the tumor by modulating, or controlling, the intensity of the radiation beam to deliver a higher radiation dose to the tumor.
We carefully plan treatment using 3-D computed tomography (CT) images of the patient in conjunction with computerized dose calculations to determine the dose intensity pattern that will best conform to the tumor shape. Typically, combinations of several intensity-modulated fields coming from different beam directions produce a custom-tailored radiation dose that maximizes tumor destruction while protecting adjacent normal tissues.
Because of its pinpoint accuracy, IMRT allows for higher and more effective radiation doses to tumors while minimizing damage to normal tissue. This translates to fewer side effects when compared with conventional radiotherapy techniques. IMRT also has the potential to reduce treatment toxicity, even when doses are not increased. Radiation therapy, including IMRT, stops cancer cells from dividing and slows tumor growth. In many cases, radiation therapy is capable of killing cancer cells, thus shrinking or eliminating tumors.
Image-guided radiation therapy (IGRT) allows better localization as the trend to higher doses, a smaller number of fractions and tighter margins becomes an important part of radiotherapy. Image-guided localization provides improved precision using surgically implanted gold marker seeds, or anatomic landmarks, to even better localize the treatment volume.
Bone or soft-tissue implanted markers, known as implanted fiducials, sometimes lead to better image-guided localization. Implanted markers are broken into two main categories: bone and soft tissue. Bone markers are pure gold spheres. They are used in cranial and spinal applications. They image clearly on electronic portal devices (EPID). These markers are typically placed in a small hole in the bone and bone wax is used to keep them in place. Soft-tissue markers are cylindrical so that they can be easily inserted using a needle. These markers have been through a special knurling procedure so that the surface is crosscut to inhibit migration once the marker is placed.
Tumors are not stationary, unchanging targets; they move between and during treatments. Markers are imperative in all situations where the target moves with respect to external marks (e.g. tattoos). Prostate, liver and other internal organ tumors can be much more accurately targeted using implanted markers. IGRT uses daily X-ray scanning to create three-dimensional images that pinpoint the exact size, location and coordinates of the tumor. In the past, oncologists have had to compensate for tumor movements by making the radiation beam larger, exposing a significant volume of healthy tissue to radiation. With IGRT, images are verified daily, pinpointing the position of the cancer just prior to treatment. Increased precision allows higher doses of radiation – ultimately leading to higher cure rates.
Daily Image Guidance
With daily image guidance, we can track the position of normal tissue structures. This technique significantly reduces the rate of normal-tissue complications. For example, in the treatment of prostate cancer, radiation can be focused to minimize exposure to the rectum and bladder. Additionally, the radiation dose to the penile bulb can be minimized for potency preservation.