Isotope choice and the effect of edema on prostate brachytherapy dosimetry

In prostate brachytherapy, post implant dosimetry quality parameters may be strongly affected by edema brought on by the trauma of the implant procedure since the amount of edema and the time course of its resolution are highly variable from patient to patient. Edema was simulated from preplans on three prostates which had ultrasound prostate volumes of 18.7, 40.7 and 60.2 cm3 expanded to planning volumes of 32.9, 60.0 and 87.8 cm3, respectively. The preplans were designed so that identical seed distributions for a given prostate gave virtually identical target dose coverage of 99.7+/-0.3% of the planning volume when using either 125I or 103Pd. Simulated CT edema volume expansions of 0%, 10%, 20% and 30% were imposed anisotropically in accordance with clinical observations so that the expansion in the superior-inferior direction was twice that of the transverse dimensions. Dose-volume histograms (DVHs) were analyzed for each prostate as a function of isotope and degree of edema. The 103Pd implants were more greatly affected by fixed amounts of edema than 125I implants, and the slopes of the DVH curves indicate less homogeneity from 103Pd implants. The DVHs were then weighted according to the portion of the isotope decay curve occupied by each edema step for half-lives of edema resolution of 5, 10 and 20 days which are within the range of clinically observed resolution times. The weighted DVHs were summed to give a net DVH corresponding to the overall dynamic effect of edema. A greater fraction of the defined prostate volume received doses in the range of likely therapeutic significance, from 75% to 125% of the prescribed minimal peripheral dose (mPD), from 125I implants than from 103Pd implants. These differences in dosimetric quality arise from two differences in the physical properties of the isotopes: more rapid attenuation of 103Pd photons with distance creates cool spots in an edematous prostate, and the shorter half-life of 103Pd causes a greater fraction of the isotope decay to consist of the prostate in an edematous state. An increase in 103Pd seed strength by about 10% beyond that required to achieve equal coverage with an identical seed distribution using 125I should minimize the differences brought on by edema.