SU-E-T-147: Efficacy of Prostate SBRT When the Daily Anatomy Differs Significantly From the Baseline Anatomy

Authors


Abstract

Purpose:

To determine the effects of interfraction motion on the efficacy of prostate SBRT when the daily anatomy differs significantly from the baseline anatomy.

Methods:

A daily CBCT was acquired for a patient who was treated with 44–1.8 Gy fractions of a VMAT plan from Varian's Eclipse treatment planning system. This plan was then escalated into 2 hypofractionated regimes (4.3 Gy for 12 fractions and 7.25 Gy for 5 fractions) and applied to all datasets collected. For one particular fraction, the prostate anatomy was found to be significantly different from the simulated anatomy. A VMAT plan was created for this outlier dataset that had identical dosimetric goals as the original 44-fraction plan. This plan was also escalated into 2 hypofractionated regimes and applied to all datasets. To make comparisons between all plans, the biologically effective dose (BED) was calculated for each fraction for the bladder, rectum, and prostate. To increase the statistical significance of the average BED for each structure, the 44 individual fractions were uniquely combined for the hypofractionated regimes resulting in 21,090,682,613 instances of 4.3 Gy/fraction regiments and 1,086,008 instances of 7.25 Gy/fraction regiments.

Results:

Despite the large difference between the simulated and daily anatomy, the plan created from the outlier dataset resulted in a higher average BED for the prostate and a lower average BED for the rectum when compared to the original plan. The average BED for the bladder increased for the plan created from the outlier dataset. The BEDs varied more for the normal tissues than for the target when the daily anatomy was significantly different from the baseline.

Conclusion:

SBRT is still a viable option even when the daily anatomy differs significantly from the baseline anatomy. The dose to the prostate was not negatively affected, while the dose to the rectum improved.

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