Relative dose uniformity assessment in interstitial implants

Purpose: Two new indices, the peak index (PI) and the new geometry index (N GI), that quantify implant dose uniformity and quality are presented. Their advantages include independence to absolute treatment dose and high sensit ivity compared with other adopted dose-uniformity measures. The applicabili ty of these indices mere evaluated through computer simulations and several clinically executed implant cases. Target coverage is assumed to be proper ly observed and will not be discussed herein. Methods and Materials: The natural volume-dose histogram serves as the basi s of our investigation. The PI and NGI definitions are based on parameters derived from the histogram. Two computer-simulated implants and 12 clinical ly executed implants, using high-dose rate remote afterloading techniques, are studied. Various indices that quantify the dose uniformity of the impla nt, namely the quality index (QI), geometry index, as well as the PI and NG I, are computed, and the results are compared. Results: The PI demonstrated significantly increased sensitivity (up to 5 t imes) to dose-uniformity evaluation, compared with the QI. The deduced para meter 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 hist ogram of a particular implant. The PI and NGI may offer better, more sensit ive means to assess implant dose uniformity, independent of prescription do se, than other measures. (C) 1999 Elsevier Science Inc.