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Estimation of the Delivered Dose to Small Bowel During VMAT for Gynecological Cancer Based On Daily CBCT. Is the Impact of Inter-Fractional Variations Large?

A Weiner1*, C Trustram Eve1, T Rainer1, C Shen1, K Wang1, S Stathakis2, J Hammers1, S Das1, P Mavroidis1, (1) UNC at Chapel Hill, 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 to estimate the accumulated dose delivered to small bowel throughout the course of radiation therapy for locally advanced cervical cancer by examining the dosimetric and radiobiological impact of the organ’s inter-fractional variations in location and shape against treatment plan.

Methods: This analysis involves ten gynecological cancer patients treated with VMAT to 45Gy in 25 fractions using daily Cone Beam CT (CBCT) localization. A total of 125 fractions of five patients were analysed here. Small bowel was manually delineated on each CBCT. A MIM-UNC designed workflow (MIM Software, Inc) was used to calculate the cumulative dose in small bowel. 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 deviations between the estimated daily delivered and planned doses were considerable and had different patterns per patient. However, after accumulating the dose distributions from all the fractions, the final dosimetric deviations were small. The dose differences between the planned and delivered doses ranged between 0.5Gy to 2.5Gy in mean dose. Regarding clinical metric D1cc, the differences ranged between 0.3-2.4Gy and in all the cases delivered dose was lower than the predicted dose from the treatment plan on simulation CT. The respective differences in NTCP values ranged between 0.2% to 2.9%, which is translated to a biological equivalent doses of 0.1Gy to 1.5Gy.

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 cumulative dosimetric deviations were small. The differences had an observable expected clinical impact in only on one patient, where small bowel dose was 3% less than the treatment plan.


Not Applicable / None Entered.


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