Image Guided Radiation Therapy
As the trend to higher doses, smaller number of fractions, and tighter margins becomes an important part of radiotherapy, better verifiable localization is a necessity. Image-guided localization provides improved precision using surgically implanted gold marker seeds, or anatomic landmarks to even better localize the treatment volume.
Fiducial Markers – Bone or Soft Tissue Implanted Markers
In some situations, image-guided localization is best achieved by utilizing implanted fiducials. 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). The markers are typically placed in a small hole in the bone, and bone wax is used to ensure that they stay in place. In thousands of cases there has never been a situation in which a gold marker moved from its seated position in the bone.
Soft tissue markers are cylindrical so that they can be easily inserted using a needle. The markers have been through a special knurling procedure so that the surface is cross-cut to inhibit migration once the marker is placed.
Markers are imperative in all situations where the target moves with respect to external marks (e.g. tattoos). Certainly prostate, liver and other internal organs can be much more accurately targeted using implanted markers.
Tumors are not stationary, unchanging targets; they move between and during treatments. 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. This increased precision allows for 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.