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Evaluating Daily Cone Beam CT Gamma Index as a Parameter for Decision Making in Adaptive Radiation Therapy Programs

M Bakhtiari1*, (1) William E. Kahlert Regional Cancer Center, Lifebridge Health, Westminster, MD


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Daily CBCT data sets are compared to original planning CT using a gamma-index pass rate. The trends of the gamma-index pass rate are investigated for several patients as a marker for triggering the re-planning.

Methods: Gamma density/distance criteria of 0.2 g/cc/3mm to compare daily CBC data to planning CT was used. This criterion allows to detect the large density discrepancies between planning CT and the daily CBCT. A commercial software system was used for calculating the gamma-index. Same CBCT data sets were used in a treatment planning system to investigate the changes in the DVHs looking for possible correlations. The maximum and mean dose to PTVs were used in the analysis. Both data sets were also analyzed for possible correlations with patient’s weight loss.

Results: The time history of gamma-index pass rate showed a trend toward lower numbers. The gamma-index compared to PTV’s maximum dose for the patients under study, resulted in negative correlation coefficients in comparison to DVHs calculated by TPS, e.g. lower the pass rate, higher the maximum dose to PTV is. A positive correlation coefficient of patient weight versus gamma-index pass rate was also observed (correlation coefficient: 0.5-0.8).

Conclusion: According to our analysis for selected patients, the gamma-index pass rate can be possibly used to indicate the needs for re-evaluation of the patients for re-CT and consequent re-planning.


Not Applicable / None Entered.


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