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Quality Control of Tandem-And-Ovoid HDR Brachytherapy Using Pre-Treatment CT Scout Images

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X Chen

X Chen1*, C Kim1 , J Wilson1 , J Fan2 , L Ariaratnam1 , V Lockamy1,3 , (1) Penn Medicine Virtua Cancer Program, Voorhees, NJ, (2) Fox Chase Cancer Center, Philadelphia, PA, (3) University of Pennsylvania, Philadelphia, PA


SU-I-GPD-T-22 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Typical tandem and ovoid (T&O) applicators have some metallic features which can be seen on CT scout images. The geometric distance of these features may be used to evaluate the dosimetric difference between inter-fraction treatments as an efficient way for quality control. The purpose of this study is to evaluate the relationship between the geometric distance of these feature points and the dose at point A in T&O HDR brachytherapy.

Methods: 10 CT dataset and 20 scout images were acquired during the course of T&O HDR treatment for 2 patients. Three feature points on the applicator (Varian fixed Ti Fletcher applicator) were identified, i.e., the tandem stopper and the knuckle on the ovoid shaft. The distances between these 3 points, which form a triangle, were measured on scout images. For research purpose, the source dwelling times on day one were duplicated on all 5 CT datasets for each patient. The calculated point A doses were compared between different fractions and were correlated to the geometric distance of the feature points on the applicator.

Results: The distance between tandem stopper and ovoid varied from 16.9-19.0 mm (18.1+/-1.1 mm; right side), 17.1-18.5mm (18.0+/-0.5mm; left side), and the inter-ovoid distances were 24.0-27.5mm (25.4+/-1.6mm) for one patient. These values were 16.4-19.8mm (18.1+/-1.3mm; right side), 15.5-20.9mm (17.9+/-2.0mm; left side), and 23.0-27.6mm (24.9+/-2.0mm; inter-ovoid) for the 2nd patient. Correspondingly, the mean point A doses were 596.8-610.6 cGy (604.3+/-5.2 cGy; 99.5-101.8% of prescription dose) for the 1st patient, and 582.8-596.6 cGy (589.0+/-6.3 cGy; 97.1-99.4% of prescription dose) for the 2nd patient.

Conclusion: The geometry of T&O applicator may change each fraction. This change could be quantified on pre-treatment CT scout images using the metallic features on the applicator and the point A dose could be estimated. Special caution may be needed for change over 2mm.

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