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Commissioning a CT Compatible LDR T&O Applicator Using Analytical Calculation with 1D and 3D Dosimetry

J Adamson

J Adamson1*, J Newton1, B Steffey1, J Cai1, J Adamovics2, M Oldham1, J Chino1, O Craciunescu1, (1) Duke University Medical Center, Durham, NC, (2) Rider University, Lawrenceville, NJ

SU-D-213AB-5 Sunday 2:15:00 PM - 3:00:00 PM Room: 213AB

Purpose: To determine the characteristics of a new commercially available CT-compatible LDR Tandem and Ovoid (T&O) applicator using 3D dosimetry.

Methods: We characterized source attenuation through the asymmetric gold shielding in the buckets by measuring dose with diode and 3D dosimetry and compared to an analytical line integral calculation. For 3D dosimetry, a cylindrical PRESAGE dosimeter (9.5cm diameter, 9.2cm height) with a central 6mm channel bored for source placement was scanned with the Duke Large field of view Optical CT-Scanner (DLOS) before and after delivering a nominal 7.7Gy at a distance of 1cm using a Cs-137 source loaded in the bucket. The optical CT scan time lasted approximately 15 minutes during which 720 projections were acquired at 0.5° increments, and a 3D dose distribution was reconstructed with a 0.5mm³ isotropic voxel size. The 3D dose distribution was applied to a CT-based T&O implant to determine effect of ovoid shielding on the dose delivered to ICRU 38 Point A as well as D2cc of the bladder, rectum, bowel, and sigmoid.

Results: Dose transmission through the gold shielding at a radial distance of 1-3cm from midplane of the source was 86.6%, 86.1, and 87.0% for analytical calculation, diode, and 3D dosimetry, respectively. For the gold shielding of the bucket, dose transmission calculated using the 3D dosimetry measurement was found to be lowest at oblique angles from the bucket with a minimum of ~51%. For the patient case, attenuation from the buckets lead to a decrease in average Point A dose of ~4% and decrease in D2cc to bladder, rectum, sigmoid, and bowel of 2%, 15%, 2%, and 7%, respectively.

Conclusions: The measured 3D dose distribution provided unique insight to the dosimetry and shielding characteristics of the investigated applicator, the technique for which can be applied to commissioning of other brachytherapy applicators.

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