Dosimetric and volumetric criteria for selecting a source activity and a source type (125I or 103Pd) in the presence of irregular seed placement in permanent prostate implants
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Fig. 1
The coverage index (CI) as a function of 125I seed activity and displacement. The seed displacement is obtained by shifting the original position randomly and uniformly within a cube with side equal to 2σ, where σ is 0, 0.25, 0.5, 0.75, or 1.0 cm.
Fig. 2
The external volume index (EI) as a function of 125I seed activity and displacement parameter, σ.
Fig. 3
D90 as a function of 125I seed activity and displacement parameter, σ.
Fig. 4
D80 as a function of 125I seed activity and displacement parameter, σ.
Fig. 5
The CI as a function of 103Pd seed activity and displacement parameter, σ.
Fig. 6
The EI as a function of 103Pd seed activity and displacement parameter, σ.
Fig. 7
D90 as a function of 103Pd seed activity and displacement parameter, σ.
Fig. 8
D80 as a function of 103Pd seed activity and displacement parameter, σ.
Abstract
Purpose: The dosimetric merit of a permanent prostate implant relies on two factors: the quality of the plan itself, and the fidelity of its implementation. The former factor depends on source type and on source strength, while the latter is a combination of skill and experience. The purpose of this study is to offer criteria by which to select a source type (125I or 103Pd) and activity.
Methods and Materials: Given a prescription dose and potential seed positions along needles, treatment plans were designed for a number of seed types and activities, specifically for 125I with activities ranging from 0.3 to 0.7 mCi, and for 103Pd with activities in the range of 0.8 to 1.6 mCi. To avoid human planner bias, an automated computerized planning system based on integer programming was used to obtain optimal seed configurations for each seed type and activity. To simulate the effect of seed-placement inaccuracies, random seed-displacement “errors” were generated for all plans. The displacement errors were assumed to be uniformly distributed within a cube with side equal to 2σ. The resulting treatment plans were assessed using two volumetric and two dosimetric indices.
Results: For 125I implants a coverage index (CI) of 98.5% or higher can be achieved for all activities (CI is the fraction of the target volume receiving the prescribed or larger dose). The external volume index (EI) (i.e., the amount of healthy tissue, as percentage of the target volume, receiving the prescribed or larger dose) increases from 13.9% to 20% as the activity increases from 0.3 to 0.7 mCi. For implants using 103Pd, the external volume index increases from 10.2% to 13.9% whenever CI exceeds 98.5%. Volumetric and dosimetric indices (coverage index, external volume index, D90, and D80) are all sensitive to seed displacement, although the activity dependence of these indices is more pronounced for 125I than for 103Pd implants.
Conclusions: For both isotopes, the lower activities studied systematically result in lower EIs. If seeds can be placed within approximately 0.5 cm of their intended position 103Pd should be preferred because its EI is lower than that of 125I. For all activities the coverage indices and D90 are within the required range. If seed placement uncertainties are larger than 0.5 cm, 125I provides slightly better target coverage; however, in terms of external volume (healthy tissue) covered, 103Pd is superior to 125I.
Keywords:
Palladium-103, Iodine-125, Brachytherapy, Prostate implants, Automated computerized planning, Seed displacementTo access this article, please choose from the options below
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