The first step is insertion of a Foley catheter to assist in urethral localization. Although the urethra can be bracketed quite closely by the implant needles, it is essential to avoid transfixing it. The afterloading devices (carrier needles or catheters) are inserted in parallel planes with equal spacing to create a uniform volume implant orthogonally to the longitudinal direction of the penis. Single-plane implants are discouraged because the isodose at a depth will be scalloped and may result in underdose to a part of the tumor. Generally, two to three
planes of needles or catheters are sufficient (21). For the template technique, individual needles (19.5 gauge for LDR and 17.5 gauge for PDR) are held in a parallel array using predrilled Lucite or plexiglass templates. When using brachytherapy catheters, the applicators are stabilizing devices such as Jackson–Pratt selleck inhibitor drains or fixing buttons. Appropriate spacing is chosen to cover the lesion, avoid the urethra, and provide an adequate margin. For LDR or PDR implants, spacing of 12–18 mm is acceptable, but 14–16 mm is preferred. Spacing should selleckchem be equivalent between adjacent needles and planes of needles. It should be noted that the closer the spacing, the less the lateral margin of high dose coverage lateral to the needles. Exterior planes of needles or “plesiocurietherapy”
(i.e., placed in space outside the penis) can be used to ensure adequate coverage of the surface and allow the most superficial of the “in-tissue” planes to be deep enough to avoid scarring Etomidate or necrosis from sources being too close to the skin. Tissue-equivalent bolus is placed between the exterior plane and the tissue surface to provide adequate radiation scatter (Fig. 2). The high-dose-rate (HDR) implant procedure is technically similar to the LDR brachytherapy, but it is not essential for the catheters to exactly follow a particular spacing system because source loading and dwell time adjustments (dosimetry optimization) can be used to
modulate the intensity of the radiation within the treatment volume within a certain range. Closer spacing is preferable for the HDR technique, generally 10–12 mm between needles or catheters because it improves the control and uniformity of the dosimetry. For instance, to minimize central dose to the urethra, periurethral needles can be more widely separated. A template that accommodates this flexibility is shown in Fig. 3. Holes are drilled on 3-mm centers (the closest possible to still have the enough template material between the holes for strength) allowing the needles to be spaced 9 or 12 mm apart as required. The bridge keeps the two templates parallel at all times. The parallel planes of needles can be either staggered or superimposed. Similar catheter spacing considerations can be applied to other stabilization techniques.