title: An innovative dosimetric model for formulating a semi-analytical solution for the activity-volume relationship in prostate implants.
abstract: An innovative (and yet simple) dosimetric model is proposed that provides a semi-analytical solution to the total activity-volume relationship in ultrasound-guided transperineal prostate implant. This dosimetric model is based on 4 simple assumptions. First, the prostate target volume is approximated as a sphere. Second, the urethra is presumed to transverse through the center of the prostate target volume. Third, peripheral loading is applied as the seed-loading technique. Fourth, as the major innovation of the proposed model, the radial dose function of the Iodine-125 125I seed is forced to fit a simple power function of the distance r. Pursuant to the third assumption, the peripherally-loaded seeds also define a spherical volume defined as the loading volume w. Also pursuant to the fourth assumption, the radial dose function is expressed as 1.139*r(-0.474) for r = 1.5 to 2.5 cm. Thereafter, a simple analytical power-law equation, A = 1.630* w(0.825), for the relationship between the total activity A in mCi and the loading volume w in cc is derived for 125I monotherapy. Isodose plans for loading volumes corresponding to r = 1.5, 1.8, 2.2, and 2.5 cm were performed. The maximal isodose coverage volume maxV100 was calculated for each case and was found to be on the average 65% larger than the loading volume w. Matching prostate target volume V to the loading volume w therefore yields a generous implant (with a margin of approximately 3.3 mm). Conversely, matching the prostate target volume V to the maxV100 yields a tight implant (with 0.0 mm or no margin). Matching the prostate target volume V to a midpoint between the loading volume w and maxV100 yields a moderate implant (with approximately 1- to 2-mm margin). Three individual equations are derived for each type of implants: A = 1.630* V(0.825), A = 1.288* V(0.825), or A = 1.078 V(0.825) for generous, tight, or moderate implants, respectively. Patient data at the Chicago Prostate Cancer Center are found to support the above dosimetric model and the 3 semi-analytically derived equations. The above equations are also compared favorably with some of the previously published equations from other authors. These results support the efficacy of the proposed dosimetric model.
authors.affiliation: Alexian Brothers Radiation Oncology Center, Elk Grove Village, IL 60007, USA. platolee@ix.netcom.com
authors.forename: Plato C
authors.initials: PC
authors.lastname: Lee
authors.forename: Eric K
authors.initials: EK
authors.lastname: Parks
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Moran
mesh_terms: D001918:Brachytherapy; D006801:Humans; D007457:Iodine Radioisotopes / Q000627:therapeutic use; D008297:Male; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D012007:Rectum / Q000528:radiation effects; D014521:Urethra / Q000528:radiation effects
chemical_list: D007457:Iodine Radioisotopes