Is Cs-131 or I-125 or Pd-103 the “Ideal” Isotope for Prostate Boost Brachytherapy?–A Dosimetric View Point


      Currently three isotopes, namely Cs-131, Pd-103 and I-125, are commercially available for prostate seed brachytherapy. All three isotopes offer the advantages of low energy (21–31 keV) with rapid dose fall-off to minimize doses to normal structures, but differ in half-lives and initial dose rates. Cs-131 offers initial dose rate of ∼32 cGy/h at prostate periphery which is approximately 1.5× and 4× higher than Pd-103 and I-125 initial dose rates. Our study aimed to perform a dosimetric comparison of boost therapy treatment plans generated utilizing Cs-131, I-125 and Pd-103 seeds.


      TRUS Prostate volume studies of 15 patients previously treated in our institution with seed implantation and representing a wide range of prostate sizes were utilized for this study. The TRUS images were obtained intra-operatively using B&K Ultrasound unit in 5 mm steps. The median prostate volume was 27.2 cc (range 16.0 to 39.2 cc). The CTV and PTV were defined according to RTOG 0232 guidelines. For each prostate size, three optimized treatment plans were generated–by changing the isotope to Cs-131, I-125 and Pd-103 respectively. The boost therapy prescription doses were: 85 Gy for Cs-131, 110 Gy for I-125 and 100 Gy for Pd-103. The seed strengths employed were: 1.6 or 1.8 U (Cs-131 and Pd-103) and 0.54 U (I-125). Planning goals attempted were: V100∼ 95%, D90 ≥ 100%, and Prostatic urethra D10 ≤ 150%. To eliminate bias, auto seed loading and optimization tools of the planning system were utilized. All plans were evaluated for coverage (V100, V90, V80 and D90) and uniformity (V200, V150) using ABS recommended guidelines. Dose calculations were performed using Variseed Planning system using AAPM TG-43 formalism.


      A total of 45 treatments plans were analyzed. The results of the comparison study are summarized in Table 1. For similar dose coverage (V100 and D90), the percentage volume of the prostate receiving 200% and 150% of the prescription doses (V200 and V150) which is a measure of dose homogeneity in the prostate was highest for Pd-103 plans followed by I-125 plans and lowest for plans utilizing Cs-131 seeds. Urethral doses (UD10) were not significantly different between the three sets of plans. For comparable seed strengths and prostate sizes, the numbers of seed requirements were identical for Cs-131/I-125 seed plans but required ∼30% higher number of seeds for Pd-103 implants.


      From a dosimetric view point, boost brachytherapy implants utilizing Cs-131 seeds yielded “homogeneous” dose distributions within the prostate while providing desired dose coverage and acceptable normal tissue doses compared to I-125 or Pd-103 seed implants.
      Table 1Dosimetric Comparison of the isotopes
      IsotopeNMeanStandard DeviationStandard Error Meanp Value
      I-1251528.77.82.0p < 0.0005
      I-1251557.05.01.3p < 0.0005
      Dose Coverage
      I-12515109.22.70.7p = 0.3
      Normal Tissue Doses
       Urethra UD10Cs-13115125.68.72.3
       Rectum RD10Cs-1311568.06.31.6


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