Permanent Implant Before or After Radiation?

Some radiation specialists may risk an unstable permanent prostate implant by the way they treat their patients. Many patients who undergo permanent prostate implants will also receive several weeks of external radiation treatment. The overwhelming majority of patients in the United States are treated with external radiation first followed by permanent implant. External radiation given before, rather than after permanent implant, offers significant potential benefit to the patient.

External radiation reduces the size of the prostate gland. A recent review of patients treated by Gulf Coast Cancer Treatment Center used CT scans obtained before and after five weeks of external radiation treatment to document just how much the prostate gland changes. As shown in Table 1, the typical patient had a one third reduction in the size of their prostate.

The benefit of prostate shrinkage prior to an implant was highlighted in an article from doctors at the University of Pennsylvania (Kenneth Blank et al, December, 1999). Men in their study had treatment to reduce the size of their prostates prior to permanent implant. It was then possible for the Pennsylvania physicians to use in the implant approximately half the number of seeds that would have been needed prior to gland reduction.

A smaller prostate simply requires fewer seeds for thorough treatment than a larger prostate. Remember that each seed gives off radiation to all the tissues that surround it, not just the cancer cells. As such, placing fewer seeds should result in less radiation dose to critical surrounding organs such as the bladder and rectum.

Indeed, this was demonstrated in the University of Pennsylvania report. Patients who received treatment to shrink their prostate glands prior to permanent implant had significantly fewer bladder and rectal side effects than comparable patients who had not received such therapy.

Patients who undergo permanent prostate implant prior to external therapy not only lose the opportunity to receive fewer radiation seeds as described above, but also risk several other problems.

Implant first patients will often need two or three weeks more external radiation than patients treated with external therapy prior to permanent implant. The number of external treatments given after a permanent implant is changed to account for the dose of radiation given off each day by the implanted seeds. In other words, a lower dose of external radiation is given each day to patients undergoing implant first, so to reach an effective total amount of radiation more days of external treatment must be given. The risk and intensity of radiation side effects are directly related to the number of external treatments given, with basic radiation biology dictating that more treatments typically result in more side effects.

Another concern for patients who undergo permanent implant prior to external radiation relates to changes in gland size. External radiation treatment potentially causes the same reduction in prostate size after implant as it does before implant. As shown in figure 1A, the implant initially has perfect spacing of “seeds” evenly distributing radiation throughout the prostate gland while limiting radiation dose to the bladder, urethra, and rectum. Figure 1B, however, shows the changes to the gland surrounding tissue closer to the seeds, and the seeds closer to the urethra. The result is greater radiation dose than originally planned to the bladder, rectum, and urethra.

The point could be made that if the implant causes some swelling and the external radiation causes some gland shrinkage, then the end result will be a gland approximately the correct size. In truth, however, the only certainty following an unpredictable amount of swelling and an unpredictable amount of shrinking is that the implanted “seeds” will end up in a distribution very different from what was originally planned.

Next: Temporary Implants (HDR)