Capítulo IX
A Technique for Computed Tomography-Guided Brachytherapy in Prostatic Cancer
RICARDO A. LATOUR, MD
Cáncer de próstata. Braquiterapia. Implante de semillas de Iodo 125.
Radioterapia Externa Conformada y Modulada. Prostatectomía radical.
A computed tomography (CT)-guided technique for transperineal brachytherapy of organ-confined prostate cancer is presented, using a prone position and hip flexion. Because the entire prostate gland should be encompassed by an adequate dose of radiation if a high probability of local control is to be attained, spatial resolution is the prime factor in planning prostate brachytherapy. To identify different organs, optimum placement and spacing of the afterloading needles is mandatory for an adequate implant. Computed tomography is an excellent modality for imaging the prostate gland because of its resolution and ability to identify different organs. The periprostatic fat with the venous plexus within it can be demarcated, the prostate can be delineated, and the placement of seeds in transurethral prostatic resection defects can be avoided. Precise placement of the radiation sources requires accurate placement of the needles at the planned position and dropping off each source at the selected location.
Technique
Planning
A planning CT scan is obtained for all patients. The patient is prone on the CT table, in a modified knee-chest position with the pelvis supported by the tomography implant system (TIS) . This device allows adjustment to the height of the pelvis and position of the template (and therefore of the rectal marker). Axial scans are then obtained, beginning at the apex and up to the seminal vesicles. The CT is also used to calculate the prostatic volume and is available at the time of the implant to help in the placement of the needles.
A computerized plan is then developed to decide the number and strength of seeds and and number of needles and their position. To this effect, we first select a peripheral pattern of needles that are placed within 0.5 cm of the capsule to provide potential dose coverage of the gland, so that the gland is encompassed at the apex, center, and base, according to the activity calculated with the nomogram. Optimization is then done by trial and error, changing the position of the needles in increments of 0.5 cm until a desired isodose pattern is reached. These isodoses are checked against each CT slice so that the prescribed dose of 140 Gy is delivered to the periphery of the prostate gland.
Procedure
Preoperative preparation involves a low residue diet for five days. The day before the procedure: neomycin is given I g per os every four hours; enemas are given to clear the bowels the night before; toperamide is given in a 4-mg dose eight hours before the procedure, and standard local preparation. The patient is placed on prophylactic antibiotics at the beginning of the treatment in the CT room and for the following two days.
The implant is performed in the CT suite under general anesthesia and usually lasts about one hour. A Foley catheter is anchored in the bladder with iodinated contrast in the balloon, and the bladder is emptied. This permits excellent visualization oft he urethra and the bladder's position. The patient is then placed prone on the CT table (Figure 2), supported by the TIS, in a modified knee-chest position, with excellent exposure of the perineum. A rectal exam is done to find the inclination of the anterior rectal wall that will match the template. The field is then prepared and draped in the standard fashion.
The template is then positioned. It is affixed to a rectal probe with radiopaque markers I cm apart. With the rectal probe introduced 8 cm in the rectum, the probe and template are angled so that the needles placed through the template will traverse the prostate from apex to base, following the gland longitudinal axis. The angle may be changed, if necessary, to prevent significant pubic arch interference. It is important that the template be as close as possible to the perineum to prevent deviation of the needles from the planned track of insertion. Lateral stabilizing needles have not become necessary.
The patient placed in the tomograph implant system.
A guide needle is inserted through the template along the posterior aspect of the prostate; the position of this needle is verified by scout views and several axial scans and corrected as necessary .We use Mick applicator needles or disposable needles. If the mandril is put aside, these needles do not make artifacts.
In case of doubt, a few axial scans can be obtained and the placement of a given needle modified. After all needles have been placed, scout views and axial scans are obtained. The seeds are then dropped along the track of the needles as the needles are removed, using a Mick applicator (or Rapid-Strand). Finally, 5-mm axial scans and plain films of the pelvis are obtained to verify the number of seeds and their placement . Cystoscopy is unnecessary since the excellent visualization of the urethra and bladder allows us to avoid depositing seeds in those organs. Patients are observed for 24 hours, and the Foley catheter is removed before discharge. Appropriate radiation precautions are followed during the short admission and instructions given to the patient on discharge.
Tomographic cut (TAC) of control with all the needles placed.
Discussion
From October 1996 to December 2001, one hundred forty six stage A or B prostate cancer (prostate diameter no larger than 5 cm) were implanted with this technique. No acute complications have been observed thus far. No urinary obstruction requiring catheterization, hematuria, perineal hematoma or infection has occurred. Although a peripheral seed placement may be slightly more susceptible to seed placement error,' the accuracy of CT predicts less seed loss and therefore a better dose distribution during the life of the implant. We are repeating CT scans monthly for the first two months to assess the displacement of the I125 seeds with the peripheral placement strategy. High precision in seed placement and avoidance of the urethra may allow escalation to the total dose given to the target volume (140 to 160 Gy).'
Itraoperative check of seed position.
Conclusions
We have found this technique to be highly reproducible and cost effective, with postimplant dosimetry being comparable with the initiallly planned distribution. The prone position with hip flexion allows excellent exposure to the perineum and by decreasing the venous pressure in the pelvis, minimizes the likelihood of significant hematomas.
We have had no problems with pubic arch interference, possibly because we can change the angle of the template and rectal probe easily, or because the prostates were not too large. Computed tomography guidance ensures optimal placement of the tip of the needles and allows precise overlaying of the dose distribution on the CT images. The use of contrast in the Foley catheter permits an excellent visualization of the bladder and urethra, minimizing the likelihood of inadvertent penetration of those organs. The peripheral distributions of the seeds preclude excessive irradiation of the urethra and have noticeably decreased the intensity of irradiation symptoms observed in our previous experience.' We believe this technique is a good alternative to the ultrasound-guided transperineal approach.
References
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