Radiation Physics

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TU_7_3177 - Assessment of Absolute Dose Rate in Tungsten-Shielded Applicators for High-Dose-Rate Brachytherapy: Comparing Results Between Advanced Dose Calculation Methodologies and Physical Measurements

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Assessment of Absolute Dose Rate in Tungsten-Shielded Applicators for High-Dose-Rate Brachytherapy: Comparing Results Between Advanced Dose Calculation Methodologies and Physical Measurements
C. J. Tien, and Z. Chen; Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, CT

Purpose/Objective(s): A critical task in commissioning Leipzig tungsten-shielded applicators for high-dose-rate (HDR) surface brachytherapy is the determination of absolute dose rate (i.e., dose delivered to a reference point per unit time) for each applicator; similar to the output calibration for Linac-based external beam radiotherapy. The purpose of this work was to share our recent commissioning results in the determination and validation of dose-rates produced in Ir-192 based Leipzig HDR surface brachytherapy applicators.

Materials/Methods: A Leipzig applicator along with four apertures, of 30, 35, 40, and 45 mm diameter, was obtained by our institution for HDR surface brachytherapy. The absolute dose-rates from each applicator were determined by (1) physical measurements, (2) a Monte Carlo (MC) based formalism, and (3) model-based dose calculation algorithms (MBDCA), respectively. Physical measurements utilized an A20 ion chamber, build-up cap and custom positioning jig. Readings were performed in both “face-down” as well as “face-up” orientations to determine intra-lumen source sag. MC results were derived from the Fulkerson et al. dose-rate formalism. MBDCA results were obtained from the AcurosBV 13.6 MBDCA using a planning software solid applicator library. The dose-rates determined from physical measurements were compared with those obtained from MC and MBDCA in order to cross-validate.

Results: Absolute dose-rates determined for all four apertures are shown in Table 1. The Leipzig applicator geometry is fixed at 12.5 mm SSD. Only depth=3.45 mm was achievable using the custom jig. Face-down measurements were ~8.5% higher than face-up, and intra-lumen sag was calculated to be 0.57 mm. After applying sag correction factor, physical measurements yielded dose rates of 2.988 Gy/min and 2.900 Gy/min in face-down and face-up orientations, respectively, averaged across all four apertures, at 3.45 mm depth. The MC formalism assumed air kerma of 40700 U for 10 Ci Ir-192, PDW=0.7087, and PDD(3.45mm)=0.9394 for all apertures. The average dose rate using MC formalism was 2.987 Gy/min. Recreating the geometry in planning software, average dose-rate was 2.944 Gy/min using MBDCA, and agreed within 2.2% of physical measurements.

Conclusion: After correcting for intra-lumen source sag, physical ion-chamber based measurements yielded dose-rates which were within 1.8% of MC-based results. MBDCA agreed within 3.9% of MC-based results, and within 2.2% of physical measurements. This mutual agreement is encouraging, which increases our confidence in MBDCA to be used in treatment planning for Leipzig surface brachytherapy.
Aperture [mm] Monte Carlo Acuros MBDCA (%dif vs MC) Measured Face-down (%dif vs MC, %dif vs MBDCA) Measured Face-down (%dif vs MC, %dif vs MBDCA)
30 3.020 2.902 (3.9%) 2.965 (1.8%, 2.2%) 2.884 (4.5%, 0.6%)
35 2.982 2.932 (1.7%) 2.996 (0.5%, 2.2%) 2.917 (2.2%, 0.5%)
40 2.963 2.958 (0.2%) 3.011 (1.6%, 1.8%) 2.900 (2.1%, 2.0%)
45 2.982 2.983 (0%) 2.980 (0.1%, 0.1% 2.900 (2.7%, 2.8%)

Author Disclosure: C.J. Tien: None. Z.(. Chen: Employee; Bristol-Myers Squipp Company.

Christopher Tien, PhD

Yale University School of Medicine

Biography:
Christopher Tien, PhD, DABR, is a clinical assistant professor with Yale School of Medicine and medical physicist with Yale-New Haven Hospital. Residency from Brown University/ Rhode Island Hospital in 2013. PhD (Medical Physics) from University of Florida in 2011. MS (Nuclear Engineering and Radiological Sciences) from University of Michigan in 2008. BS (Nuclear Engineering and Radiological Sciences) from University of Michigan in 2007. Originally from Troy, MI.

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TU_7_3177 - Assessment of Absolute Dose Rate in Tungsten-Shielded Applicators for High-Dose-Rate Brachytherapy: Comparing Results Between Advanced Dose Calculation Methodologies and Physical Measurements



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