Providence Prostate Implant Quality
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Quality Matters Challenges and Solutions
Why Quality Matters
During the l990s, great attention was placed on the quality of the implant
as determined by post-implant scans. Groups that did implants in the late l980s
reported their results following the changes in implant quality. Both major
groups reporting these results saw a marked improvement in their success
rates due to the improvement in implant quality.
Figure 1: Change in success rate from the Ultrasound Era to the
Post Implant Dosimetry Era in men treated in Seattle
Figure 2: Same analysis in men treated in Atlanta
The results presented in Figures 1 and 2 suggest that outcome improvements are directly related to improvements in implant quality. Finally, two large series have been reported by a group that pays close attention to dosimetry and implant quality. Both of these groups report success rates in excess of 90% for low- and intermediate-risk men. At the University of Michigan / Providence, similar results have been achieved.
Technical Challenges and Solutions
The most important consideration and final determinant of outcome in prostate implant therapy is implant quality. A high quality implant or acceptable implant delivers a continuous full dose of radiation to the entire prostate and limited dose to adjoining critical normal tissues (rectum and bladder). Achieving high quality implants remains a great challenge for the following reasons. Please note: In the following section, click on the image forfull graphic display.
Challenge: Needle Deviation from Ideal
As a needle is inserted through the skin and into the prostate, the track of the needle may not be perfectly straight but deviate by the varying densities of the tissues. Also, as the needle comes in contact with the prostate, the prostate itself may move to some degree, resulting in needle placement in a slightly different location than planned.
Solution: Assurance of Proper Needle Position
Multiple needles are placed within the prostate before any seeds are dropped off. Each needle tends to stabilize the prostate, resulting in more ideal needle positions. Needle depth is checked by both ultrasound and fluoroscopy (X-ray)—essentially a "double check" to ensure proper needle placement.
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Challenge: Seed Deviation from Normal
Even if the needle is in a perfect and ideal position, as seeds are dropped, there may be a shift in position compared to the ideal plan. Seeds may be placed within the prostate and immediately around the prostate. Some seeds can move away from the prostate if they are placed in a blood vessel near the prostate. Seeds perfectly spaced within the needle may spread out when they are released, resulting in a different distribution.
Solution: Confirm Correct Seed Position
Confirm seed drop-off by checking on X-ray or, if possible, visualize seeds on the ultrasound screen. This should be done as each needle is unloaded. Some groups check the implants by fluoroscopy at the end of the implant procedure and if a cold area is noted, seeds are added. It is impossible to tell which needle had poor drop-off with this technique, and adding seeds may not correct the problem if they are added after all planned seeds have been deposited.
Solution: Use of "Seeds on a String" Technology: 3-6 Seeds are Attached to One Another on a String
This prevents seed shift after placement and maintains a fixed spacing between seeds (currently available for 125I seeds only). It is less likely for seeds to be lost into the bladder or into blood vessels outside the prostate when they are attached to other seeds on a string.
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Challenge: Prostate Swelling
When many needles are passed into the prostate during the implantation procedure swelling occurs, resulting in a slightly enlarged prostate. Since the plan was designed to treat the prostate without swelling, some consideration to account for swelling must take place during implant planning.
Solution: Correct for Prostate Swelling
During the planning phase, the size of the prostate should be expanded to account for swelling. Plant enough seeds to cover the swollen volume.
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Challenge: Post-Implant Check
For all of the above reasons, the implant must be carefully checked following the procedure to make certain that the seeds are in close to ideal position and that a full dose of radiation is being administered to the prostate. This check usually includes a Computerized Axial Tomography scan (CAT scan or CT scan), done the day of the implant procedure or up to 30 days following the implant. The problem with the CAT scan post-implant check is that the definition of the prostate on the scan is not clear due to swelling and seed interference. This makes it difficult to define the actual prostate volume. If the prostate volume on the scan is well covered, however, there is a high certainty that the prostate is receiving full dose, since the image tends to overestimate the size of the actual prostate.
Solution: Combine MRI and CT Scans
The prostate is clearly visualized after an implant using an MRI. Unfortunately, the seeds are not as well visualized on MRI. Some groups combine the prostate picture on MRI with the seeds clearly displayed on a CAT scan to arrive at a more accurate picture of radiation dose to the prostate. At present, the use of MRI alone is not a standard practice and combining the two images (CAT scan and MRI) is a great technical challenge in itself. At the University of Michigan and Providence Hospital, MRI and CAT are used in combination to confirm seed placement. The scans are combined with the use of software called Mutual Information, which automatically combines like structures on the two scans.
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Challenge: Interval between the Plan and the Implant Procedure
For many groups, initial ultrasound images of the prostate are obtained and placed in a planning computer. It may take hours to plan an implantation procedure. For this reason, the individual undergoes an ultrasound and then there may be a several hour or several day waiting period before the actual procedure takes place. The problem with this interval is that when the individual is positioned later for the actual procedure, the angle of the probe and the position of the prostate may be slightly different than when the images were obtained and on which the implantation procedure is based. It can take a great deal of time to try to reproduce the earlier prostate position.
Solution: One-Step Plan and Implant
Some groups do an ultrasound and immediately develop a plan and carry it out while the patient is in the same position. If possible, it is preferable that the implantation be done in the same position as the plan was developed. On the other hand, it is perfectly acceptable to obtain the ultrasound, develop a plan, and carry it out hours or days later. Many excellent results have been reported using this technique. Personal preference of the implant plan and team often makes this decision. At the University of Michigan and Providence Hospital, the plan and implant procedure are done in a single step.
