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Dosimetric Impacts of Various Uncertainties in Cervical Cancer HDR Brachytherapy: Are Conventional Point Doses Good Surrogates for 3D Dosimetry?

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X Liang

X Liang1*, D Zheng2 , X Zhang3 , G Narayanasamy4 , S Morrill5 , J Penagaricano6 , N Paudel7 , Z Li1 , (1) University of Florida Health Proton Therapy Institute, Jacksonville, FL, (2) University of Nebraska Medical Center, Omaha, NE, (3) University of Arkansas for Medical Sciences, Little Rock, AR, (4) University of Arkansas for Medical Sciences, Little Rock, AR, (5) University of Arkansas for Medical Sciences, Little Rock, AR, (6) University of Arkansas for Medical Sciences, Little Rock, AR, (7) University of Arkansas for Medical Sciences, Little Rock, AR

Presentations

SU-F-T-14 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall


Purpose:In the context of evaluating dosimetric impacts of a variety of uncertainties involved in HDR Tandem-and-Ovoid treatment, to study the correlations between conventional point doses and 3D volumetric doses.

Methods:: For 5 cervical cancer patients treated with HDR T&O, 150 plans were retrospectively created to study dosimetric impacts of the following uncertainties: (1) inter-fractional applicator displacement between two treatment fractions within a single insertion by applying Fraction#1 plan to Fraction#2 CT; (2) positional dwell error simulated from -5mm to 5mm in 1mm steps; (3) simulated temporal dwell error of 0.05s, 0.1s, 0.5s, and 1s. The original plans were based on point dose prescription, from which the volume covered by the prescription dose was generated as the pseudo target volume to study the 3D target dose effect. OARs were contoured. The point and volumetric dose errors were calculated by taking the differences between original and simulated plans. The correlations between the point and volumetric dose errors were analyzed.

Results:For the most clinically relevant positional dwell uncertainty of 1mm, temporal uncertainty of 0.05s, and inter-fractional applicator displacement within the same insertion, the mean target D90 and V100 deviation were within 1%. Among these uncertainties, the applicator displacement showed the largest potential target coverage impact (2.6% on D90) as well as the OAR dose impact (2.5% and 3.4% on bladder D2cc and rectum D2cc). The Spearman correlation analysis shows a correlation coefficient of 0.43 with a p-value of 0.11 between target D90 coverage and H point dose.

Conclusion:With the most clinically relevant positional and temporal dwell uncertainties and patient inter-fractional applicator displacement within the same insertion, the dose error is within clinical acceptable range. The lack of correlation between H point and 3D volumetric dose errors is a motivator for the use of 3D treatment planning in cervical HDR brachytherapy.


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