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A Few Dosimetric Quality Indicators Provide Enough Dose Volume Histogram Information to Calculate Accurate Radiobiological Parameters in a Randomized Prostate Brachytherapy Trial

W Butler*, G Merrick, Wheeling Hospital, Wheeling, WV

SU-E-T-296 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To use 6 brachytherapy dosimetric quality indicators to compare radiobiological indices between two arms of a randomized trial.

Methods: A 246 patient trial added 90 Gy 103Pd to 44 Gy external beam radiotherapy (EBRT) versus 115 Gy 103Pd added to 20 Gy EBRT. Six dosimetric quality indicators, D100, D90, V90, V100, V150, and V200, help define the shape of the brachytherapy dose volume histogram (DVH). Using TG-137 methodology, biological equivalent dose (BED) and tumor control probability (TCP) were calculated for each patient by applying an appropriate volume increment to each quality indicator. This approach was validated in 33 patients with accurate BEDs and TCPs calculated from the full DVH.

Results: In the validation group, paired t-tests showed no differences between BEDs calculated by the 6-indicator or detailed DVH approaches. There were significant differences between cohorts in the full population. In the 44 Gy arm, the total BED was 162 ± 7 Gy compared to 143 ± 8 Gy in the 20 Gy arm (p < 0.001). The TCPs also differed significantly, with TCP of 1.000 ± 0.001 in the 44 Gy arm vs. 0.994 ± 0.010 in the 20 Gy arm. In sensitivity analysis, V150, and V200 comprised > 75% of the volume but contributed < 1% of the BED. D100 and V90 accounted for < 2% of the volume but contributed most of the BED.

Conclusion: BEDs and TCPs calculated from 6 brachytherapy dosimetric quality indicators were nearly the same as those derived from detailed DVH analysis. Despite the clinically large difference of 19 Gy in total BED between arms, the difference in total TCP was small, 0.005. This is consistent with published trial outcomes showing no significant difference between the arms at any post-treatment time point, with 10-year progression free survival at 93% for both arms.

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