Radioactive Seed Implant
The implant dose is calculated by the computer to superimpose over every single CT slice of the prostate, for the most accurate display of the radiation dose relative to the prostate target volume and nearby normal structures such as bladder, urethra and rectum. In this example, the 100% radiation prescription dose is bright yellow, covering the prostate with 3-7 mm margins, and dipping in the middle to spare the bladder neck and urethra. The peripheral seed pattern produces the highest dose in the peripheral zone of the prostate, where the cancer is typically most extensive (orange lines indicate 150% of prescription dose level, engineered to cover the peripheral zone).
CT-Guided Post-Brachytherapy Dosimetry Study
After the implant procedure a pelvic X-ray, chest X-ray and post-operative CT scan is done (FIGURE 16). The X-rays allow the most accurate seed count and the CT scan displays the location of each of those seeds within the prostate target region, and relative to the nearby bladder, rectum and urethra (urethra is only visible on CT if a Foley catheter is in the urethra). After the prostate CT, one of our nurses removes the Foley catheter, and the patient is released back home just as soon as he demonstrates an ability to void spontaneously.
The physics staff then counts and localizes all seeds, after which the computer calculates the surrounding radiation dose. The radiation oncologist then contours the prostate, urethra, rectum and bladder on every relevant CT slice, after which the computer reconstructs each of these organs, relative to the seed pattern. Finally, the resulting radiation dose is displayed in three dimensions, superimposed over the CT contoured prostate and nearby organs (FIGURE 17).
A new imaging modality that may more accurately assess our post-procedural implant dosimetry in some patients is MRI-based prostate implant dosimetry. The MRI usually gives a clearer definition of the prostate and seminal vesicle anatomy, though the CT is still needed to identify the seeds and calculate the radiation dose lines. In these cases, the CT and MRI prostate images are fused into the same space on our advanced brachytherapy planning computer, allowing the full use of the strongest aspects of both imaging modalities(FIGURE 18).
A quantitative analysis of the radiation dose received by the prostate and normal tissues is then accomplished, allowing an objective assessment of implant quality. This quantitative tool is known as a dose volume histogram (DVH)(FIGURE 19). The DVH plots the exact radiation dose level received by the prostate target volume and adjacent tissues in graphic form, and is the major quantitative method used to assess the implant adequacy.
The nuances of DVH interpretation are beyond the scope of this format but in general terms, we strive for a prostate V100* value over 90%, prostate D90* value over 90% and a urethral D10* value under 125%. It takes more than a simple DVH number analysis to judge an implant, as temporary prostate swelling and other factors complicate the analysis, but the DVH analysis does provide an implant quality assurance starting point.
- *prostate V100: percent of CT or MRI-defined prostate volume receiving >= 100% of prescribed radiation dose
- *prostate D90: minimum dose received by 90% of CT-defined prostate volume
- *urethral D10: mimimum dose received by hottest 10% of CT-defined urethra volume
“Problem implants”
No matter what the level of brachytherapy expertise or experience, or where in the world it is done, these may happen from time to time. If a DVH analysis suggested that a significant area of radiation under-dosage was present in a given patient, the next step would be to assess its location on the CT prostate images.
In our brachytherapy experience, any areas of radiation dose below the prescription level have usually been small in magnitude and volume, and typically located centrally near the urethra, or anteriorly, near the anterior bladder neck or prostate apex. These locations typically do not harbor cancer cells, and are not further treated. Furthermore, our CT analysis is done on post-operative day one, which is commonly a time of significant prostate swelling, temporarily distorting the seed pattern and artificially degrading the apparent quality of the implant. We have had rare cases where the implant dosimetry was initially of concern, but subsequently looked much better after simply waiting for the prostate swelling to calm, and then repeating the CT-based DVH analysis.
If a “cold spot” were judged to be occurring in a clinically significant location and did not resolve after repeat CT dosimetry analysis, additional radiation treatment might be contemplated to correct this, particularly if the patient had an intermediate or unfavorable cancer characteristic. Unplanned additional radiation treatment has been an extremely rare event in our brachytherapy practice, and requires considerable dosimetry analysis and patient evaluation before proceeding. Less than 1% of our brachytherapy patients have had “unplanned” follow-up radiation treatment, though there is an ability to do it should the need arise, especially with the added radiation dose control afforded by the new IMRT technology, where a custom-engineered IMRT prostate “hot spot” could be superimposed right over a brachytherapy “cold spot,” effectively filling in the brachytherapy dosimetry deficit. With the CT-ultrasound interactive method, the risk of significant brachytherapy cold spots appears to shrink to zero.
Potters, et al have published Brachytherapy dosimetry quality guidelines that correlate with a good therapeutic outcome (6). Briefly, the dosimetry parameter known as the D90* has correlated with a superior 4-year PSA-defined disease-free survival, when it exceeded 90% of the prescribed brachytherapy dose. In our experience, this dosimetry requirement has been met in greater than 99% of implanted patients, and our median day one D90 value has been about 112% of the prescribed brachytherapy dose.
- *prostate D90: minimum dose received by 90% of CT-defined prostate volume
