Radioactive Seed Implant
CT-Ultrasound Interactive Prostate Brachytherapy: A new treatment method that allows real-time dosimetry analysis and correction
CT-ultrasound interactive prostate brachytherapy (CT-US brachytherapy) is a new method of prostate brachytherapy guidance that was pioneered in 2002 by Dr. Fuller at Radiation Medical Group (8). This method represents a form of real-time dosimetry guidance, meaning that the target volume dose coverage is analyzed and improved based on feedback from the CT, ultrasound and planning computer, before the procedure is concluded.
The CT-US brachytherapy real-time procedure method effectively addresses one of the most serious potential shortcomings of prostate brachytherapy – under dosage within and adjacent to the cancer-bearing target region. Because this method allows for more accurate dose sculpting, it allows for more consistent radiation coverage of cancer-bearing regions, while also better sparing adjacent critical normal tissues such as the urethra, from excessive radiation dose.
Description of the method:
The procedure is done in our office, where staff, computing devices, the CT-scanner, the ultrasound (US) machine and the operating suite are all located and coordinated. It begins with a standard pre-planned US-guided brachytherapy procedure, done under local anesthesia and conscious sedation in our operating suite. After that, immediate US and CT studies follow for interval prostate and urethral contouring, CT-based seed identification, co-registration of the CT and US image sets in 3 dimensions, and analysis of radiation dose coverage. The result of the co-registration process is shown in (FIGURE 1) and (FIGURE 2). The patient is then returned to our operating suite while the analysis proceeds.
After critical analysis of target volume radiation dose coverage strengths and deficiencies, additional “simulated” seeds are added on the computer if necessary, until target volume coverage is optimized and “cold spots” eliminated. During this process, normal tissue structures are also analyzed and care taken to design the supplemental seed pattern so as not to excessively irradiate those structures.
A " mini-plan" is then designed to guide the simulated supplemental seeds to their targets, using the computer-fused ultrasound grid system as the seed guidance mechanism. The supplemental seeds are then inserted per “mini-plan” instructions, and the procedure concluded. A final CT-scan is then done for final dosimetry analysis and the patient discharged. A summary of the process is illustrated in (FIGURE 3).
In total, this process currently adds approximately one hour to a standard brachytherapy procedure, involving two CT-scans (interval and final), one ultrasound-scan (interval), five additional computer steps (immediate US contour, immediate CT contour, image co-registration, interval computerized analysis of dose coverage, computer design of supplemental seed pattern). The result is a more precise, conformal final seed implant radiation coverage result.
Results:
As of May 19th, 2003, 26 patients have been implanted at Radiation Medical Group by the CT-US interactive method. More consistent coverage of the prostate target volume region is demonstrated by the coverage statistics. The median V100 (percent of prostate volume receiving 100% of the prescribed dose) in this cohort measures 98.24% and more importantly, the minimum V100 result has measured 96.10%, indicating that this method has effectively terminated “cold spots” in every patient implanted to date - an unprecedented result in the practice of permanent source brachytherapy.
Reducing complications is just as important as curing the cancer. In this regard, we have been able to more carefully control the median and highest urethral dose by this method, potentially decreasing the risk of long-term urinary tract complications. As we move forward with the method, our effort will be to continue to maximize the coverage score, while attempting to further minimize the urethral dose. An example of urethral-sparing from the high dose radiation volume is illustrated in (FIGURE 4).
