Attenuation of intracavitary applicators in 192Ir-HDR brachytherapy

Med Phys. 2004 Jul;31(7):2097-106. doi: 10.1118/1.1762791.

Abstract

Unlike the penetrating monoenergetic 662 keV gamma rays emitted by 137Cs LDR sources, the spectrum of 192Ir used in HDR brachytherapy contains low-energy components. Since these are selectively absorbed by the high-atomic number materials of which intracavitary applicators are made, the traditional neglect of applicator attenuation can lead to appreciable dose errors. We investigated the attenuation effects of a uterine applicator, and of a set of commonly used vaginal cylinders. The uterine applicator consists of a stainless steel source guide tube with a wall thickness of 0.5 mm and a density of 8.02 g/cm3, whereas the vaginal cylinders consist of the same stainless steel tube plus concentric polysulfone cylinders with a radius of 1 or 2 cm and a density of 1.40 g/cm3. Monte Carlo simulations were performed to compute dose distributions for a bare 192Ir-HDR source, and for the same source located within the applicators. Relative measurements of applicator attenuation using ion-chambers (0.125 cm3) confirmed the Monte Carlo results within 0.5%. We found that the neglect of the applicator attenuation overestimates the dose along the transverse plane by up to 3.5%. At oblique angles, the longer photon path within applicators worsens the error. We defined attenuation-corrected radial dose and anisotropy functions, and applied them to a treatment having multiple dwell positions inside a vaginal cylinder.

Publication types

  • Comparative Study
  • Evaluation Study
  • Validation Study

MeSH terms

  • Algorithms
  • Artifacts
  • Brachytherapy / instrumentation*
  • Brachytherapy / methods*
  • Computer Simulation
  • Equipment Failure Analysis / methods
  • Female
  • Genital Neoplasms, Female / radiotherapy*
  • Humans
  • Iridium Radioisotopes / analysis*
  • Iridium Radioisotopes / therapeutic use*
  • Models, Biological
  • Monte Carlo Method
  • Radiometry / methods*
  • Radiotherapy Dosage
  • Radiotherapy Planning, Computer-Assisted / methods*
  • Reproducibility of Results
  • Scattering, Radiation
  • Sensitivity and Specificity

Substances

  • Iridium Radioisotopes