Two fundamental implant techniques are used in prostate brachytherapy, permanent and temporary. Permanent implants use radioactive Iodine–125 or Palladium-103 radioactive sources (seeds) that are inserted and permanently left in the prostate. Temporary prostate implants use high dose rate (HDR) radioactive iridium-192. These sources are inserted into the prostate and removed after a period of time, hence, the term temporary implant. To best understand how a radiation oncologist might use one technique or the other, we will look at the technical differences between permanent and temporary implants.
For either permanent or temporary prostate implants, their surgical procedure to insert the radioactive sources is basically the same. Briefly, several needles are first surgically inserted into the prostate through the perineum (the area of skin below the scrotum and in front of the anus). Transrectal ultrasound (TRUS) is used to help guide the needles through the perineal skin into the prostate. After this step, the two implant techniques differ significantly in regards to treatment planning and radiation delivery.
With a permanent seed implant, planning where to place the radioactive seeds inside the prostate is done prior to needle insertion. This process is termed “preplanning”. After a preplan has been approved, seeds are ordered, and the physician inserts the needles into the prostate according to preplan instructions. Finally, the physician attempts to eject the seeds from the needles so that the resultant seed patter within the prostate matches the pre-plan specifications. T o evaluate the resultant seed implant, a post-implant CT of the prostate is obtained, and the seeds inside the prostate are located and the actual radiation dose calculated. This process produces a post-plan. An implant’s quality would be measured by how close the actual seed pattern matches the pre-plan pattern. Generally there is good correlation between the pre-plan and the post-plan with appropriate delivery of radiation dose to the prostate.
On occasion, problems result in a post-implant seed pattern that does not satisfactorily deliver radiation to the prostate. One problem that occurs during surgery is needle deflection that causes needles to end up at locations that are different from their planned location. Another problem is that as the radioactive seeds are released from the needles, subsequent seed movement or shirting can occur. Instead of uniform spacing specified by the pre-plan, clusters of seeds can result. This can cause hot and cold radiation spots within the prostate gland. Keep in mind that once the radioactive seeds are released for the needle, it is not possible to rearrange the seeds within the prostate. Seeds that are linked by semi-rigid surgical material are used to minimize this potential problem. Additional free seeds can be inserted to help correct cold spots but hot spots cannot be reversed. Physician experience and proper equipment are critical components to achieving quality permanent seed implants.
With high dose rate iridium-192 (HDR-Ir192) temporary implants, the planning of the radiation delivery is done after needle insertion. Following surgery, CT scan images are used to identify actual needle locations within the prostate tissue. Each needle’s location in the prostate and the degree of needle deflection is now known. Furthermore, actual needle locations relative to the prostate’s capsule, known tumor location and their relation to radiation sensitive structures such as the urethra, bladder and rectum can be determined. This information is used to devise an optimal HDR-Ir192 source pattern for that patient, prior to actual isotope insertion. Once a plan has been approved, a specialized HDR-Ir192 source delivery system called an “afterloader” precisely places and moves the radioactive source within the implanted needles to deliver the exact treatment specifications of the plan. With this technique, the HDR Ir-192 source is only temporarily inserted into each needle of the implanted array and then retrieved. Treatment usually takes between 5 and 15 minutes. No radioactive material is left in the prostate and after treatment the needles are removed.
While the term HDR refers to “high dose rate”, you can also think of this technique as “highly directed radiation”. Because there is more precise control of the entire process, HDR-192 implants can more reliably deliver comprehensive radiation coverage to the prostate gland compared to permanent seed implants. Therefore, there may be situations where physicians may feel that HDR treatment is the most appropriate implant technique. For example, when the patient is suspected to have moderate to extensive intra-prostatic carcinoma, there becomes an increased need for reliable comprehensive radiation coverage. In this clinical situation, some physicians may favor the HDR technique because of its inherent ability to consistently deliver comprehensive radiation coverage. Another example where HDR may be selected is when the patient’s cancer is primarily located at the base areas of the prostate. The base is sometimes a difficult are to reach with transperieneal insertion without significant needle deflection and seed migration. When one examines a series of patients implanted with permanent seed techniques, it is generally the base regions where radiation coverage can be inconsistent.
An HDR-Ir192 implant has its drawbacks as well. Currently, most treatment schemes employing HDR-192 are done in combination with external beam. Many patients with low volume carcinomas, especially at mid and apical locations, can be satisfactorily treated with a seed implant alone without the necessity of supplemental external beam radiation. HDR-Ir 192 implants are often done on an outpatient basis, but generally require two separate implant procedures or may require a one-night stay in the hospital to complete the brachytherapy treatment.
We perform both permanent seed and HDR-Ir192 implants. While there are no precise guidelines, general recommendations are given to the patient regarding the appropriateness of each implant type. This is base upon perceived tumor burden, tumor location, probability of extracapsular extension, physician preference, etc. For patients who are felt to have low or moderate volume tumors of low / intermediate grade and where tumor locations are probably mid and apically based, permanent seed implants are favored. For patients who are viewed to have more extensive tumors, higher Gleason’s grades, or if their tumors are primarily located in the base of the prostate base, then HDR-Ir192 technique becomes more of an option. The HDR-Ir192 implant may also be preferred when patients have prostates larger than 50 – 60 cc in volume on the planning ultrasound study. With lager prostates, pubic bone interference poses more technical problems for a permanent seed implant than for an HDR-Ir192. With HDR-Ir192 implants you are not radioactive once you return home so that there may be some advantages to this technique in avoiding radiation exposure to family members.
The most important point is to talk to your doctor at length about all possible treatment options and choose the best one for your particular situation.