Commissioning a CT-compatible LDR tandem and ovoid applicator using Monte Carlo calculation and 3D dosimetry

Justus Adamson, Joseph Newton, Yun Yang, Beverly Steffey, Jing Cai, John Adamovics, Mark Oldham, Junzo Chino, Oana Craciunescu

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


Purpose: To determine the geometric and dose attenuation characteristics of a new commercially available CT-compatible LDR tandem and ovoid (TO) applicator using Monte Carlo calculation and 3D dosimetry. Methods: For geometric characterization, we quantified physical dimensions and investigated a systematic difference found to exist between nominal ovoid angle and the angle at which the afterloading buckets fall within the ovoid. For dosimetric characterization, we determined source attenuation through asymmetric gold shielding in the buckets using Monte Carlo simulations and 3D dosimetry. Monte Carlo code MCNP5 was used to simulate 1.5 × 109 photon histories from a 137Cs source placed in the bucket to achieve statistical uncertainty of 1 at a 6 cm distance. For 3D dosimetry, the distribution about an unshielded source was first measured to evaluate the system for 137Cs, after which the distribution was measured about sources placed in each bucket. Cylindrical PRESAGE® dosimeters (9.5 cm diameter, 9.2 cm height) with a central channel bored for source placement were supplied by Heuris Inc. The dosimeters were scanned with the Duke Large field of view Optical CT-Scanner before and after delivering a nominal dose at 1 cm of 5-8 Gy. During irradiation the dosimeter was placed in a water phantom to provide backscatter. Optical CT scan time lasted 15 min during which 720 projections were acquired at 0.5° increments, and a 3D distribution was reconstructed with a (0.05 cm)3 isotropic voxel size. The distributions about the buckets were used to calculate a 3D distribution of transmission rate through the bucket, which was applied to a clinical CT-based TO implant plan. Results: The systematic difference in bucket angle relative to the nominal ovoid angle (105°) was 3.1°-4.7°. A systematic difference in bucket angle of 1°, 5°, and 10° caused a 1 ± 0.1, 1.7 ± 0.4, and 2.6 ± 0.7 increase in rectal dose, respectively, with smaller effect to dose to Point A, bladder, sigmoid, and bowel. For 3D dosimetry, 90.6 of voxels had a 3D γ-index (criteria 0.1 cm, 3 local signal) below 1.0 when comparing measured and expected dose about the unshielded source. Dose transmission through the gold shielding at a radial distance of 1 cm was 85.9 ± 0.2, 83.4 ± 0.7, and 82.5 ± 2.2 for Monte Carlo, and measurement for left and right buckets, respectively. Dose transmission was lowest at oblique angles from the bucket with a minimum of 56.7 ± 0.8, 65.6 ± 1.7, and 57.5 ± 1.6, respectively. For a clinical TO plan, attenuation from the buckets leads to a decrease in average Point A dose of ∼3.2 and decrease in D2cc to bladder, rectum, bowel, and sigmoid of 5, 18, 6, and 12, respectively. Conclusions: Differences between dummy and afterloading bucket position in the ovoids is minor compared to effects from asymmetric ovoid shielding, for which rectal dose is most affected. 3D dosimetry can fulfill a novel role in verifying Monte Carlo calculations of complex dose distributions as are common about brachytherapy sources and applicators.

Original languageEnglish (US)
Pages (from-to)4515-4523
Number of pages9
JournalMedical Physics
Issue number7
StatePublished - Jul 2012
Externally publishedYes


  • 3D dosimetry
  • LDR tandem and ovoid
  • Monte Carlo
  • brachytherapy

ASJC Scopus subject areas

  • Biophysics
  • Radiology Nuclear Medicine and imaging


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