Relative dose uniformity assessment in interstitial implants

Abstract Purpose: Two new indices, the peak index (PI) and the new geometry index (NGI), that quantify implant dose uniformity and quality are presented. Their advantages include independence to absolute treatment dose and high sensitivity compared with other adopted dose-uniformity measures. The applicability of these indices were evaluated through computer simulations and several clinically executed implant cases. Target coverage is assumed to be properly observed and will not be discussed herein. Methods and Materials: The natural volume–dose histogram serves as the basis of our investigation. The PI and NGI definitions are based on parameters derived from the histogram. Two computer-simulated implants and 12 clinically executed implants, using high-dose rate remote afterloading techniques, are studied. Various indices that quantify the dose uniformity of the implant, namely the quality index (QI), geometry index, as well as the PI and NGI, are computed, and the results are compared. Results: The PI demonstrated significantly increased sensitivity (up to 5 times) to dose-uniformity evaluation, compared with the QI. The deduced parameter NGI may thus offer a better measure of implant qualities, allowing a more meaningful assessment and correlation between implant qualities to the treatment results. The PI system also offers a guideline to the design of optimal implant geometry. Conclusion: The PI overcomes some of the shortcomings of the QI in that it provides more information about the peaking of the natural dose–volume histogram of a particular implant. The PI and NGI may offer better, more sensitive means to assess implant dose uniformity, independent of prescription dose, than other measures.