Transperineal 125iodine implants of the prostate can be performed with ultrasound guidance, a simple technique that has met with widespread acceptance. However, ultrasound does not allow good visualization of the pubic bones in relation to the pelvic outlet, and the pubic bones may interfere with needle placement in the anterior peripheral aspect of the prostate. Adequate irradiation of the entire periphery of the prostate is important to assure tumor control, since most tumors are multicentric and may involve the anterior aspect of the prostate. A computerized tomography-based treatment planning procedure that allows for angulation of transperineal needles to avoid the pubic bones and still reaches the most peripheral aspects of the gland is described. The technique also allows for the use of transrectal ultrasound and fluoroscopy to verify correct needle placement in the prostate at the procedure. Early treatment results, based on prostate specific antigen and regression of palpable tumors, are encouraging.
Basic dosimetric parameters as recommended by the AAPM Task Group No. 43 (TG-43) have been determined for recently available IoGold 125I brachytherapy seeds. Monte Carlo methods (MCNP) were used in the calculation of these parameters in water, and results compared with soon to be published experimental parameters also for 125I IoGold seeds as well with parameters for model 6702 and 6711 125I seeds. These parameters were the radial dose function, anisotropy factor and constant, and the dose rate constant. Using MCNP, values for the radial dose function at 0.5, 2.0, and 5.0 cm were 1.053, 0.877, and 0.443, respectively. The anisotropy factor was 0.975, 0.946, 0.945, and 0.952 at 0.5, 1.0, 2.0, and 5.0 cm, respectively, with an anisotropy constant of 0.95. The IoGold dose rate constant was determined by excluding the low energy titanium characteristic x rays produced in the IoGold titanium capsule. Using this post TG-43 revised NIST air kerma methodology, the IoGold dose rate constant was 0.96 cGy h-1 U-1. These calculatively determined parameters for IoGold seeds were compared with those determined experimentally for IoGold seeds, and also compared with parameters determined for model 6702 and 6711 seeds as presented in TG-43.
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