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Image-Guided Radiation Therapy (IGRT)
Successful radiation therapy is dependent on the ability to deliver substantially more dose to a tumor than surrounding healthy tissues. Generally, the greater the tumor dose the greater chance there is of eradicating a tumor. To that end there have been many new developments in radiation therapy over the past few years to improve dose delivery including Image-Guided Radiation Therapy (IGRT).
Before the development of the CT scanner in the mid 1970s a radiation oncologist would use plain films, like a chest x-ray, and his understanding of how tumors spread to design treatment fields that would encompass only the tumor and spare nearby normal organs. However, plain films have poor soft tissue contrast. This means it would sometimes be difficult for the radiation oncologist to clearly delineate the tumor from healthy tissues. The oncologist would have to add a margin to the treatment field to ensure that the entire tumor was included within the portal. However, the larger fields would also irradiate more healthy tissues and could lead to an increase in side effects. The increase in side effects could limit the radiation oncologist's ability to deliver a dose large enough to destroy a tumor. Conversely, if the oncologist reduced the field margin in order to limit side effects the chance of missing part of the tumor could also limit the chance of cure. It would take the development of the CT scanner to address this problem.
The invention of the CT scanner was one of the most important developments in x-ray imaging. In radiation therapy, beginning in the 1980s, the CT scanner had a dramatic impact on the planning of radiation fields. CT images have far better soft tissue contrast than plain films and allow the oncologist to view the tumor from an axial or crosscut view. This made it much easier for the radiation oncologist to identify the full extent of a tumor and allowed for a potential reduction in the volume of normal tissue irradiated. This would give the oncologist more confidence to treat tumors to higher doses safely. CT based planning is still the standard of practice today. Although CT based planning has resulted in more accurate targeting of tumors the ability to further reduce field margins is limited due to tumor motion. It has been shown with a number of studies that tumors move during the course of therapy due to a number of factors including breathing, swallowing and bladder filling. This has resulted in a limit to which field sizes can be reduced because of the need to cover the tumor and its motion.
Because of this motion it has become necessary in some cancers to localize or precisely identify the location of a tumor relative to healthy tissues on a daily basis. The technology and processes that allows for daily tumor localization has come to be know as IGRT. Many different technologies have been developed to aid in the daily localization of tumors. These technologies include ultrasound imaging, daily CT imaging and implantable fiducials or markers. At the C.R. Wood Cancer Center we have recently adopted the implantable marker technology to treat prostate cancer.
The markers are very small gold seeds that are 3 mm long and 1 mm in diameter. Gold is used because it is radio-opaque or easily seen with x-rays. The markers are crosscut to prevent the seeds from moving once they are implanted. The seeds are implanted in the urologist office. The procedure takes about a half-hour. Under ultrasound guidance the urologist will place three seeds into the prostate gland. The placement of the seeds is very important. They must be placed only in the prostate and widely spaced to aid in their visualization.
After the seeds are implanted the patient is scheduled for simulation in the radiation oncology department. At simulation, the location of the markers relative to the center of the treatment fields is identified with the use of a CT scanner and planning computer. The coordinates of the seeds relative to the center of the treatment fields are programmed into the accelerator imager computer. Each day before treatment two images are taken using the accelerator imager, which is like a digital camera. Next the radiation therapist identifies each seed in each image using the imager computer. The computer software will then identify the location of the prostate gland relative to the treatment field and calculate shifts in the treatment couch position in order to place the prostate exactly in the center of the field. This technology is accurate to within 1 mm! Knowing exactly the location of prostate gland within the pelvis each day will allow the radiation oncologist to eventually reduce field margins to a minimum, which will allow us to increase tumor doses and reduce side effects. In the future we also hope to use this IGRT technology in the treatment of other cancers including head and neck, brain and lung tumors.