Isotope selection for permeant prostate implants? An evaluation of 103Pd versus 125I based on radiobiological effectiveness and dosimetry

A. P. Dicker, C. C. Lin, D. B. Leeper, F. M. Waterman

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Abstract

Transperineal interstitial permanent prostate brachytherapy (TIPPB) has become an increasingly popular treatment for early-stage/favorable-risk adenocarcinoma of prostate. Within TIPPB, permanent implants often use either 103Pd (T(1/2) = 17 days) or 125I (T(1/2) = 60 days). This review compares the radiobiological and treatment planning effectiveness of 103Pd and 125I implants by using the linear-quadratic model with recently published data regarding: prostate tumor cell doubling times, T(pot) α and α/β ratio. The tumor potential doubling times (T(pot) were determined based on recently published proliferation constant (K(p). The initial slope of the cell radiation dose survival curve, α, the terminal slope β, and the α/β ratio were taken from recent published clinical and cellular results. The total dose delivered from each isotope was the dose used clinically, that is, 120 Gy for 103Pd and 145 Gy for 125I. Dale's modified linear-quadratic equation was used to estimate the biological effective dose, the cell- surviving fraction, the effective treatment time, and the wasted radiation dose for different values of T(pot). Treatment plans for peripherally loaded implants were compared. The T(pot) reported for organ-confined prostate carcinomas varied from 16 to 67 days. At short T(pot) both isotopes were less effective, but 103Pd had much less dependence on T(pot) than 125I. However, at long T(pot) both isotopes produced similar affects. The minimum surviving fraction for exposure to 103Pd decreased from 1.40 x 10-4 to 1.31 x 10-5 as the T(pot) increased from 16 to 67 days. By contrast for exposure to 125I, the minimum surviving fraction decreased from 3.98 x 10-3 to 1.98 x 10-5 over the same range of T(pot). A comparison of treatment plans revealed that 103Pd plans required more needles and seeds; however, this was a function of seed strength. Both isotopes had similar dose-volume histograms for prostate, urethra, and rectum. The theoretical prediction of effectiveness using the linear quadratic equation for the common clinically prescribed total radiation doses indicated that 103Pd should be more effective than 125I because it had less dependence on T(pot). The greatest benefit of 103Pd was shown to be with tumors with a short T(pot). Although the regrowth delay would be longer with 125I, the benefit was inconsequential compared with the very slow doublings times of localized prostate cancer. Treatment planning with either isotope revealed no significant differences. These findings may explain why clinically there seemed to be no clear difference in treatment outcome with either isotope. Based on these predictions, we recommend a clinical trial to compare the efficacy of the two isotopes. (C) 2000 by W.B. Saunders Company.

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Keywords

  • Brachytherapy
  • I
  • I
  • Prostate cancer

ASJC Scopus subject areas

  • Urology

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