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The Impact of Dose Shift-Invariance Assumption On Dosimetric Indices of the Thoracic Cavity in Radiation Therapy

H Nourzadeh*, C Hui , M Ahmed , W Watkins , E Aliotta , J Siebers , University of Virginia, Charlottesville, VA


(Sunday, 7/29/2018) 2:05 PM - 3:00 PM

Room: Davidson Ballroom A

Purpose: To quantify the dosimetric error introduced by assuming dose shift invariance in radiation treatment plans of locally advanced lung cancer patients.

Methods: A cohort patient of four locally advanced lung tumor patients all treated with intensity modulated radiation therapy was examined. Nominal static 3D dose was calculated for each patient using a superposition-convolution algorithm on 3D low-pitch CT scans. To investigate the impact of assuming dose shift-invariance on clinically relevant dosimetric indices, dose was calculated with 124 rigid displacements applied in left–right, anterior–posterior and superior–inferior directions were considered using two different
methods: (1) by re-computing dose for each displacement (2) by translating the nominal dose according to the applied displacement, i.e. assuming dose-shift invariance. For each patient and each offset, differences between re-computed and shift-invariant point dose, total dose per region of interest, and clinically important dose-volume metrics (CTV: D98 and D95, Organs at risk: D75, D50, D15, Dmean, Dmax) were determined to compare the scenarios.

Results: The standard percentage error between dosimetric indices in (1) and (2) were D98: -0.15±1.86, D95:-0.09±1.36 for target volumes, and Dmean: 0.04±1.47, Dmax: 0.63±2.56, D75:0.45±3.92, D50: 0.24±3.10, D15:0.45±3.92 for organs at risk. Assuming the full treatment 6600 cGy delivery, the mean voxel-wise maximum absolute errors were 708±192 cGy and 593.90±579.91 cGy for target volumes and organs at risks, respectively. The observed 98th percentile important dosimetric variation due to the dose shift invariance assumption in thoracic cavity was about 4.3% in tumor region and about 6% in normal tissues.

Conclusion: Dose-shift invariance is a poor assumption for our locally-advanced lung patients as dose errors exceed 5%. The impact of this on probabilistic treatment planning schemes remains to be determined, but indicates that dose recalculation may be necessary to have acceptable dose coverage estimates.


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