PV QA 3 - Poster Viewing Q&A 3
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.