Comparison of seed loading approaches in prostate brachytherapy

Since uniform seed loading in prostate brachytherapy can produce an intoler ably high dose along the urethra, some form of peripheral loading is common ly employed. We define three variants of peripheral loading and compare the m in a small, medium, and large prostate in terms of coverage of the planni ng target volume (PTV), homogeneity, and ability to spare critical structur es of excessive dose. Modified uniform loading has at least 2/3 of the seed s occupying sites on a 1 cm cubic grid keyed to the prostate base and the p osterior border of the prostate. Nonuniform loading explicitly spares the u rethra by using only basal and apical seeds in at least two centrally locat ed needles. Peripheral loading uses higher activity seeds with the posterio r implant plane 5 mm anterior to the posterior border of the prostate. The three prostate volumes (18.7, 40.7, and 60.2 cm(3) by ultrasound) were expa nded to planning volumes (32.9, 60.0, and 87.8 cm(3), respectively). The pl anning volumes (PTVs) were loaded with a I-125 seed distribution and activi ty sufficient to cover 99.7+/-0.3% of the PTV with the prescribed minimal p eripheral dose (mPD) of 145 Gy. Activities used ranged from 0.32 to 0.37 mC i/seed (0.41-0.47 U/seed) for the first two approaches and from 0.57 to 0.6 6 mCi (0.72-0.84 U) for peripheral loading. Modified uniform loading produc ed the most uniform distribution based on dose-volume histograms and the vo lume receiving >150% of prescribed dose, All the approaches are capable of constraining the superior-inferior dose profile (the urethral path) to less than 150% of the mPD, but the nonuniform approach with explicit urethral s paring kept the urethral dose below 120% of the mPD. Dose profiles for the three approaches along the posterior-anterior midline axis are comparable n ear the urethra, but peripheral and nonuniform approaches have extended reg ions where the dose is >150% of mPD. These regions approach within 10 mm of the rectum or urethra, so these two approaches require greater accuracy in intraoperative execution of the plan. Although each of the three planning approaches can achieve the treatment goals of adequate coverage and critica l structure sparing, modified uniform loading has a more homogeneous dose d istribution. This approach may be more forgiving of systematic errors in se ed placement. (C) 2000 American Association of Physicists in Medicine. [S00 94-2405(00)01202-5].