Permanent prostate brachytherapy (PPB) is an excellent treatment option for prostate cancer patients, demonstrating durable cancer control rates with relatively low morbidity ( , , , ). Patient selection and risk stratification is an important consideration when deciding on PPB monotherapy with or without external beam radiation therapy (EBRT). Low-risk patients are eligible for PPB alone without the need for androgen deprivation therapy (ADT). Intermediate-risk patients generally receive neoadjuvant and adjuvant ADT given for 6 months (starting 3 prior to the implant), or a combination of PPB plus EBRT. In some instances, intermediate-risk patients who have some low-risk features can be considered for monotherapy ( ).
All patients with high-risk features are given trimodal therapy with PPB, EBRT, and ADT. Because the field of radiation is generally confined to the prostate, patients with T3 lesions are not typical candidates for PPB alone. Nine months of ADT (neoadjuvant and adjuvant) is typically sufficient for this high-risk population, although some clinicians will treat for 2 years; EBRT is generally administered 2 months post seed implant. In patients with seminal vesicle (SV) invasion, implantation of the SVs is feasible and should be part of the target volume for both components of treatment.
PPB as a salvage treatment is also an option for EBRT failures. A positive prostate biopsy should be confirmed by a pathologist experienced in reading postirradiation prostate biopsies. The patient should go through a rigorous staging evaluation prior to implantation.
Because the PPB technique relies on real-time imaging, visualization of the entire prostate and surrounding structures is critical when performing the implant. Therefore, patients who present with large prostates can present a problem. TRUS imaging of the anterior prostate or the intravesical component is difficult in glands larger than 60 cc. In such cases, preimplant androgen deprivation therapy for 3 months can be employed that will reduce prostate volume approximately 30% prior to initiating seed implantation ( ). There is no absolute upper limit for prostate volume, as even large prostates >100 cc are eligible for PPB, though technically challenging ( ). Additionally, patients with high IPSS scores, high post void residuals, or low peak flow rates should be counseled that they are at higher risk for post-PPB urinary retention. A prior transurethral resection of the prostate (TURP), though not an absolute contraindication, is important to consider, as a very large defect may not permit implantation of seeds throughout the entire gland.
The patient should undergo pretreatment imaging to determine the appropriate dosimetry prior to seed implantation. TRUS is considered the standard pretreatment imaging modality; however, MRI and CT can be utilized as well. The radiation oncologist (RO) is responsible for completing the study and determining the amount and type of activity to order.
The choice of radioactive isotope for implantation is based on the isotope half-life and dose rate. The two isotopes used for permanent implants are iodine-125 and palladium-103. In general, the iodine-125 seed consists of a titanium cylindrical shell measuring 4.5 mm in length by 0.8 mm in diameter containing I-125. The average energy of the radiation produced by the decay of the I-125 is 0.028 MeV. The half-life is 60.25 days. Similarly, Pd-103 is contained in a titanium shell of roughly the same dimension. Pd-103 is produced from thermal neutron capture in Pd-102. Pd-103 decays by electron capture to Rh-103 with the subsequent emission of 20–23 keV characteristic x-rays. The average x-ray produced has energy of 0.021 MeV. The half-life of Pd-103 is 17 days. The dose rate, or the quantity of radiation absorbed per unit time, of Pd-103 is higher than I-125. Recently cesium-131 has been introduced as a third prostate implant isotope. It has a similar energy to I-125 but a half-life of 9.7 days. No studies have demonstrated any advantage of one isotope over another.
While the American Brachytherapy Society does not recommend the use of one specific radionuclide, we consider the radio-biologic characteristics when selecting I-125 or Pd-103. The longer half-life of I-125 is ideal because it matches the growth rate of well to moderately differentiated tumors. With a half-life of 60 days, significant radioactive decay will occur over 1 year following the implant, with the majority of decay (87.5%) occurring during the first 6 months. The prescription dose for I-125 ranges from 144 to 160 Gy.
Pd-103 is selected for tumors with Gleason scores of 7 or greater. This isotope is chosen for its shorter half-life and higher dose rate to match the faster growth rate of poorly differentiated tumors. For Pd-103 monotherapy, the prescription dose is at least 124 Gy. In addition, Pd-103 is used for patients who are treated with combined EBRT and PPB (prescription 100 Gy) and is also used in the setting of salvage therapy (prescription 115 Gy). Notwithstanding the above, both Pd-103 and Cs-131 have been used in these clinical situations.
Preparation for PPB is similar to the preparation for prostate biopsy. Aspirin or nonsteroidal antiinflammatory medication is stopped 10 days before the procedure. The anterior rectal vault needs to be free of stool and mucus. A Fleet enema the night before and a light supper will help ensure no fecal matter will interfere with the transducer. Antibacterial prophylaxis should be given prior to the start of the procedure.
Patient Positioning and Setup
The patient is brought to the procedure room, anesthetized and placed in extended dorsal lithotomy position. A spinal or general anesthesia can be used. The rectum is checked for mucus or stool and flushed out several times with water if not clean. The probe is prepared by placing the special gel pad or water path over its end. A small amount of transducer jelly is placed over the tip of the probe, and the gel pad or off-set is milked forward, pushing all of the air away from the electronic pads. The gel pad is then secured to the probe with transducer rings or small rubber bands. A condom filled with ultrasound jelly is then placed over the probe, which in turn is placed in the stepping device. If the water path is used, all air must be removed from the chamber by repeated irrigation and aspiration, as residual air could interfere with the ultrasound signal.
Under aseptic conditions, a Foley catheter is placed and clamped to maintain 100–150 mL in the bladder. The scrotum is taped to the lower abdomen so that it will be out of the operative field. The probe, fixed in the stepping device, is directed into the rectum. A sterile, clear drape, covers the brachy-stand and affords the urologist the ability to manipulate the hardware and maintain sterility ( Fig. 82.1 ). With the probe in transverse imaging, the ultrasound template guide on the ultrasound image is observed. The lowest row on the grid should be close to the most posterior part of the gland. If it is not, adjust the Z axis down (toward the floor) to better align the probe. Advance the probe all the way to the bladder and retract it to the apex to make sure the entire field of view of the prostate can be appreciated and that no stool or mucus obscures imaging. Imaging is switched to longitudinal and the probe is rotated obliquely, scanning through the entire sagittal images of the prostate. With these maneuvers, the urologist observes the vascular anatomy (Santorini veins and dorsal venous complex, presence of intravesical lobe, TURP defect, rectourethralis muscle, and position of prostate apex).
The position of the sources in the gland is based on brachytherapy principles established by Paterson and Parker. The concept holds that a homogeneous dose distribution can be obtained by an inhomogeneous distribution of sources. In our technique, a homogeneous dose distribution is achieved by placing more activity in the periphery of the gland and less activity in the interior of the gland. In this way, the desired dose will cover the entire gland while avoiding large high-dose areas in the center of the gland. Using either isotope, placement of 75% of the total activity in the gland periphery with the remainder placed with interior needles will accomplish this. The majority of the seeds placed through the interior needles are also “peripheral” as they are inserted at the base and apex.
There are several techniques used for seed placement including the use of needles preloaded with seeds, seeds in strands, or inserted individually using a Mick applicator. In addition, the treatment plan can be developed in advance or “real-time” in the operating room as we prefer. We will describe our preferred choice of seed placement with the Mick applicator.
The prostate gland, urethra, and rectum images are captured at 5-mm intervals from base to apex and acquired into the treatment planning software (Variseed, Varian Medical Systems, Palo Alto, CA). The Variseed tools are used to contour the structures, creating a three-dimensional image of the gland. Sagittal imaging is used to determine the length of the prostate in the anterior, mid-, and posterior portion of the gland from the base to the apex. The RO uses these figures to perform dosimetry calculation computing the number of seeds to implant as well as the spacing between the needles and seeds.
The urologist looks for the largest transverse image of the gland, with the virtual image of the prostate, urethra, rectum superimposed over the ultrasound image. Virtual needles also appear and the urologist places the applicator or preloaded needles through the template (grid) in a clockwise circumferential fashion so they end up at or near the intended positions. The implant is divided into two phases, with placement of the peripheral needles first. It is not necessary to get an exact match, because the physicist can drag the virtual needles to the “flash” positions of the inserted needle ( Fig. 82.2 ). In this way, the virtual image accurately represents the structures and inserted needle positions. After all of the peripheral needles have been placed, the prostate is recaptured into the planning system. This is a crucial step because the needle placement has significantly altered the original plan. The needles cause the prostate to move cranial, displace it off the rectum, and distort its edges ( ). The RO and physicist update the plan by adjusting the contours and adding or deleting seeds for each needle ( Fig. 82.3 ). Next, the RO can start inserting the seeds.