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A Calculator for Single-Seed Or Two-Seed I-125 Radioactive Seed Localization (RSL) Medical Event Dose Reporting

W Erwin*, R Wendt , UT MD Anderson Cancer Center, Houston, TX


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: To develop a practical tool for calculating and reporting the dose from a radioactive seed localization (RSL) procedure in which the planned duration of implantation is exceeded, perhaps because of delayed or cancelled surgery.

Methods: An Excel spreadsheet was written to compute the difference between calculated and prescribed dose. The cumulative transverse plane dose at 1 cm is calculated for the highest-activity seed. The estimated dose for a second seed in close proximity (e.g., for tumor bracketing) is added, based on its activity, a fit of AAPM TG 43 dose vs. distance normalized to that at 1 cm and the second seed-to-1 cm from first seed distance measured on mammography. Two power fit functions were used, using data points from 0.1 - 2 cm and 0.75 - 9 cm, respectively, that resulted in minimum average difference from expected at all points and minimized that at 1 cm (1%). The dose calculation for a second seed at less than 1 cm from the dose point employed the former function, while that a distance of 1 cm or greater employed the latter. The dose from two seeds was also simulated in the GATE Monte Carlo software. The calculator was applied to one single-seed and four two-seed medical events.

Results: The doses at 1 cm from one and from one of two seeds agreed well between the calculator and the simulation. Implant durations ranged from 6 to 80 days. Measured second seed-to-dose point distances were 1.2 to 5.5 cm. Dose minus the prescribed not-to-exceed 30 cGy at 1 cm (maximum 0.200 mCi seed implanted for 5 days) differences were -6 to 186 cGy.

Conclusion: This dose calculation and reporting tool is a practical means of assessing the dose from an RSL implantation that exceeds the prescribed duration.


I-125, Seed Localization, Dosimetry


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