Room: Exhibit Hall | Forum 4
Purpose: To evaluate the accuracy and the dosimetric effect of heterogeneity corrected dose calculation based on the TG-186 report in the Oncentra Treatment Planning System (Elekta).
Methods: Using a remote after-loader with an iridium-192 source, doses were delivered to a single channel with 40 equally timed dwell positions, spaced 2.5 mm apart, and measured with a MapCheck 2 device. Four configurations of cork, tissue-equivalent bolus, and titanium dummy seeds placed over relevant detectors were recorded. Plans were calculated using CT scans of the device to match the irradiation, utilizing different material definitions within the planning software. Plans were compared to the measured data for 10 detectors within the irradiated volume. Additionally, non-heterogeneity corrected original plans from six patients treated with tandem and ring, vaginal cylinder, Capri, or interstitial needles were recalculated with heterogeneity corrections and compared dosimetrically.
Results: Measured doses were greater than calculated for most MapCheck 2 material setups. Using uniform metal correction for the detectors and seeds yielded a better result than HU based correction, improving the comparison by 17%. Similarly, for cork and tissue equivalent materials, uniform material correction yielded a better comparison than HU based correction by about 7% and 3% respectively. When comparing between TG-43 and TG-186 calculated plans, lower target dose coverage and critical structure doses resulted from the heterogeneity corrections. The average dose reduction was 3.7% for critical structure 2cmÂ³ volumes, and 4.7% for HRCTV D90.
Conclusion: Clinical plans calculated with heterogeneity-correction model resulted in reduced overall doses compared to TG-43 calculations. All measured doses were higher than TG-43 calculated doses and most were higher than TG-186 calculations depending on the material definition. The choices of material definition when setting up a heterogeneity corrected plan strongly affect the outcome and should be chosen carefully and consistently based on the accuracy of measured doses.