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Investigating the Impact of Inter-Fractional Variations to the Delivered Dose to Esophagus During VMAT for Lung Cancer Using CBCT-Based Deformable Image Registration

P Mavroidis1*, C Trustram Eve1, A Walters1, A Weiner1, S Stathakis2, J Hammers1, S Das1, K Wang1, (1) Univ North Carolina, Chapel Hill, NC, (2) Mays Cancer Center - MD Anderson Cancer Center, San Antonio, TX


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

Room: AAPM ePoster Library

Purpose: This study aims at estimating the accumulated dose delivered to esophagus throughout the course of radiation therapy examining the dosimetric impact of its inter-fractional variations in location and shape. Also, to examine the dosimetric impact in treatment delivery when contrast is used during CT simulation.

Methods: work involves ten lung cancer patients treated with VMAT to 60Gy in 30 fractions using daily Cone Beam CT (CBCT). A total of 120 fractions of four patients were analysed. Manually contours drawn on each CBCT were used to drive the contour/intensity-based deformable image registration (DIR). MIM (MIM Software, Inc) was used to calculate the cumulative dose in esophagus. The clinical dosimetric metrics (mean dose, D1cc) and the normal tissue complication probability (NTCP) were calculated for the comparisons with the treatment plans.

Results: The results were grouped based on whether the patient was CT simulated with or without contrast. The dose differences between the delivered doses and those in the treatment plan were less than 1.0 Gy in mean dose for all the patients. Regarding the clinical metric D1cc, the differences ranged between 0.3-2.2Gy and in most cases the doses were lower for the delivered doses. The NTCP values were 2.5% lower for the delivered doses for the patients with contrast compared to 0.1-1.0% for the patients without contrast (where the values were lower for the plans). The values of the biologically effective uniform dose followed the same pattern.

Conclusion: Internal organ displacements and deformations resulted in considerable deviations between the estimated delivered and planned doses between fractions. However, after accumulating the fractional dose distributions from all the fractions, the final dosimetric deviations were small. It seems that the use of contrast during simulation have some impact in the dose delivered to esophagus compared to treatment plan.

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Cone-beam CT, Dose Volume Histograms, Dose Response


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