Temporary isotopes
In addition to permanent brachytherapy (discussed above), high dose rate brachytherapy has also been used. In this technique, the dose is delivered at a higher dose rate than is provided by a permanent implant. This delivers radiation to the prostate using temporary high dose rate radioactive implants. The most common isotope used for temporary brachytherapy is iridium (Ir)–192, which provides a higher dose of radiation than the I-125 or Pd-103 permanent implants. As such, the Ir-192 implants are not left in the prostate gland.

Using an imaging modality such as ultrasonography, MRI, or CT, temporary catheters that contain the implants are positioned in the prostate. Patients require hospitalization while the implants remain in place but may go home once the implants are removed. IMRT is usually used with this technique. The optimal patient population has not yet been determined. High dose rate brachytherapy is commonly delivered in 2 or more fractions of 810 Gy or more. Most series reported are from single centers.

Most brachytherapy for prostate cancer is performed using the permanent technique, which is the focus of the remainder of this article.

Relevant Anatomy
The prostate lies below the bladder and encompasses the prostatic urethra. It is surrounded by a capsule and is separated from the rectum by a layer of fascia termed the Denonvilliers aponeurosis. The blood supply to the base of the bladder and prostate is from the inferior vesicle, which is derived from the internal iliac. The capsular branches of the inferior vesicle artery help identify the pelvic plexus arising from the S2-4 and T10-12 nerve roots. The neurovascular bundle lies on either side of the prostate on the rectum. It is derived from the pelvic plexus and is important for erectile function.

Contraindications
Brachytherapy has several relative contraindications, as follows:

• Transurethral resection of the prostate: Initially, prior transurethral resection of the prostate was associated with increased symptoms and urinary incontinence rates as high as 50%, but more recent studies have reported incontinence rates of less than 10%.
• Pubic arch interference: Interference may occur because of a large prostate (a gland >40 g), and this interference may preclude adequate placement of seeds. Hormonal ablation, exaggerated lithotomy, horizontal probe position, and CT-guided placement are all potential solutions.
• Obstructive symptoms: Significant preoperative obstructive symptoms increase the likelihood of postoperative urinary retention. While glands larger than 40 g are more likely to have obstructive symptoms, symptoms can occur in anyone. Glands between 50 g and 60 g require downsizing. Hormone ablation has been reported to downsize the prostate gland by 25-40% and is used to facilitate brachytherapy in patients with large glands. One study randomized prostates of comparable size to brachytherapy alone or brachytherapy after hormone ablation.9 They found that the acute side effects of urinary retention and dysuria were actually greater in the hormone ablation group. Clinicians often compromise and use a 5-alpha reductase inhibitor rather than true androgen ablation for downsizing. Nonetheless, brachytherapy is not advisable in patients with glands larger than 60 g.
• Morbid obesity: Focusing on the target is feasible, but the equipment often cannot sustain the weight or is not long enough to reach the prostate.

• CBC count
• Prothrombin time
• Activated partial thromboplastin time
• CHEM-7
• Urine culture

• The amount of radiation to be delivered to the prostate and the configuration of the implants must be assessed prior to placement of the implants. As experience with the technique has broadened, the planning and dosimetry stage has evolved from preplanning days to weeks in advance to intraoperative planning. The ABS has defined the following terminology to clarify the differences in the techniques.
• Preplanning is the creation of a plan days or weeks before the implant procedure.
• Intraoperative planning is treatment planning in the operating room without moving the ultrasound probe.
• Intraoperative preplanning is the creation of a plan in the operating room, with immediate execution of the plan.
• Interactive planning is stepwise refinement of a plan using computerized dose calculations derived from images of needle placement.
• Intraoperative treatment planning does not eliminate the need for postimplant dosimetric analysis.
• Key issues related to planning and dosimetry are described as follows:
• In order to perform accurate dosimetry and real-time visualization of percutaneous source placement, the prostate and margins of adjacent organs (eg, rectum, bladder) must be well visualized. TRUS and CT scanning are the two major modalities currently in use.
• TRUS has the advantages of real-time imaging and sharp contour of the posterior prostate and rectal wall. Its disadvantage is that its accuracy depends on the operator's skill.
• The accuracy of CT scanning, on the other hand, does not depend on the operator's skill, but prostate margins are less well defined with this imaging modality.
• With either modality, initial 5-mm slices are obtained from the base of the bladder to the pelvic floor. A target, which includes the prostate contour, with a generous allotment to the apex and a tighter margin at the base, is developed from these images. The apex tends to allow for less seed migration because of the presence of the pelvic floor muscles here, as opposed to the looser periprostatic tissue at the base.
• Traditionally, a portion of the seminal vesicles is included in the target of radiotherapy.
• The information on the target volume and margins is then transmitted to a computer program, and the computer helps perform the dosimetry and helps to plan the number of seeds and to define their location on a 2-dimensional grid.
• The strategy of seed placement is somewhat controversial, with some advocating uniform distribution of seeds and others emphasizing placement on the periphery of the prostate, where most cancers arise.

Treatment
Medical Therapy
Forms of brachytherapy
IMRT and brachytherapy are treatment options for localized prostate cancer. They differ predominantly in the areas of dose distribution, total dose, and dose rate. The dose is the unit of absorbed energy per weight of tissue. For example, the basic unit of radiation, the gray (Gy), is 1 J/kg of tissue.

In brachytherapy, the sharp radiation dose fall-off allows for a high degree of rectal sparing and for a higher total dose to be delivered to the prostate gland itself. Similar advantages can be obtained with conformal EBRT or IMRT. While brachytherapy has a much lower initial dose rate than EBRT, the aggregate radiation delivery is higher. The average doses are 10 Gy/wk for EBRT and 40 Gy/wk for Pd-103 and 13 Gy/wk for I-125 brachytherapy implants. In addition, high dose rate implants, such as Ir-192, can range from 2-36 Gy/min.

Early experience with the high-dose rate revealed excessive toxicity, and, subsequently, adjustments were made to fractionate the dose into 4-7 treatments. Advantages of the high-dose rate include a short duration of treatment (10-15 min), minimization of applicator movement, and optimization of dose distribution because sources are mobile. Disadvantages include increased adverse effects and the need for hospitalization.

Source options
Brachytherapy was first performed in 1914, shortly after Marie Curie discovered radium. Various sources have been used over the years, which vary in half-life and effective energy, as listed below.

• Radium (Ra)–226 - 1620 years, 1.2 J
• Cs-137 - 30 years, 0.66 J
• Gold (Au)–198 - 2.7 days, 0.41 J
• Ir-192 - 74 days, 0.34 J
• I-125 - 60 days, 0.027 J
• Pd-103 - 17 days, 0.39 J

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