Article

CT-based dosimetry calculations for I-125 prostate implants

Department of Radiation Oncology, University of California, Los Angeles 90095-6951, USA.
International Journal of Radiation OncologyBiologyPhysics (Impact Factor: 4.26). 12/1999; 45(5):1347-53. DOI: 10.1016/S0360-3016(99)00343-0
Source: PubMed

ABSTRACT

To evaluate the Monte Carlo code MCNP4B for low-energy brachytherapy calculations, including the effects of interseed attenuation and patient specific heterogeneities, on the calculated dose distribution from transperineal implantation of 125I.
The Monte Carlo code MCNP4B was used to model and benchmark the absolute dose distribution from two 125I brachytherapy seeds (model 6711 and 6702). Based upon the physical source model, the total photon intensity and differential energy spectrum were evaluated as a function of angle from the transverse bisector of the source. These spectral and intensity data were reformatted to produce probability distributions for sampling from a virtual point source. The virtual source model and a modified version of MCNP4B is then used for simulating arbitrary brachytherapy source configurations within a homogeneous or heterogeneous patient specific computed tomography (CT)-based lattice geometry.
Comparison with TG-43 data and the Monte Carlo calculations is excellent with MCNP4B predicting the radial dose function for the 125I 6711 and 6702 sources within 6% for all data points tested. Attenuation effects from neighboring seeds were investigated for pre- and postimplant seed distributions and found to be negligible. Preliminary dosimetry analysis of postimplant seed distributions comparing homogeneous water versus heterogeneous CT simulation geometries indicates an average decrease of approximately 5.6% for the volume of tissue irradiated to a prescription isodose line of 144 Gy.

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    • "The dose can be either lowered or raised depending of the tissues and materials encountered by the radiation (adipose tissue, muscular tissue, applicator, etc.). Hence, we concluded that unlike low dose rate sources like 125 I or 103 Pd [10], the dose distribution and treatment quality evaluation criteria [11] [12] [13] are not affected for "
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    ABSTRACT: To assess the dosimetric effects of the presence of the applicator, air pockets in clinical target volume (CTV) and OARs along with tissue heterogeneities using the Monte Carlo (MC) method in high dose rate (HDR) gynecologic interstitial brachytherapy with a Syed-Neblett template. The CT based dosimetry has been achieved with the Geant4 MC toolkit version 9.2. DICOM-RT files of 38 patients were imported into our own platform for MC simulations. The dose distributions were then compared to those obtained with a conventional TG-43 calculation. Taking account of heterogeneities has effects of the order of 1% on the HDR gynecological dose distributions. However, the exclusion of air pockets and applicator from the DVH calculation can lower the CTV D90 and V100 by as much as 8.7% and 5.0% in comparison with TG-43. Rectum dosimetric indices can also be lowered by approximately 3% compared with TG-43 for most cases. Differences for urethra and bladder are for most cases below 1%. Exclusion of non-biological material such as air pockets and applicator volume from the CTV is important for both TG-43 and MC calculations. It could be easily implemented and automated in treatment planning systems without affecting computation times.
    Full-text · Article · Oct 2013 · Radiotherapy and Oncology
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    ABSTRACT: Low-energy photon emitting radionuclides encapsulated for a permanent implant are routinely applied in prostate cancerbrachytherapy. Before clinical use, a new source design requires full dosimetric analysis and calibration standardization. The results of one such experimental measurement and analysis are reported here for a new design of 125 I source, model I125-SL. Dose measurements were made using standard methods employing thermoluminscent dosimeters in a water equivalent plastic phantom, in accord with the AAPM Task Group #43 recommendation of liquid water reference material. Precision machined bores in the phantom located dosimeters and source(s) in a reproducible fixed geometry providing for transverse-axis and angular dose profiles over a range of distances from 0.17 to 10 cm. The data were analyzed in terms of parameters recommended by AAPM TG43. The dose-rate constant, Λ, was evaluated by two methods, the first with reference to a 60 Cobalt standard, accounting for response variation with photon energy spectrum. Second, the dose-rate constant was determined with reference to phantom measurements using NIST traceable calibrated model 6702 and 6711 sources. The radial dose function, g(r), the anisotropy function, F(r,θ), the anisotropy factor, φ an (r), and the point-source approximation anisotropy constant, φ̄ an , were derived from one- and two-dimensional dose distribution data measured in the phantom, accounting for finite dosimeter volume and with attention to interchip effects. The results are compared to TG43 and other existing data for 125 I sources. The new source is comparable to the model 6711 source design.
    No preview · Article · Dec 2000 · Medical Physics
  • [Show abstract] [Hide abstract]
    ABSTRACT: Low-energy photon emitting radionuclides encapsulated for a permanent implant are routinely applied in prostate cancer brachytherapy. Before clinical use, a new source design requires full dosimetric analysis and calibration standardization. The results of one such experimental measurement and analysis are reported here for a new design of 125I source, model I125-SL. Dose measurements were made using standard methods employing thermoluminscent dosimeters in a water equivalent plastic phantom, in accord with the AAPM Task Group #43 recommendation of liquid water reference material. Precision machined bores in the phantom located dosimeters and source(s) in a reproducible fixed geometry providing for transverse-axis and angular dose profiles over a range of distances from 0.17 to 10 cm. The data were analyzed in terms of parameters recommended by AAPM TG43. The dose-rate constant, lambda, was evaluated by two methods, the first with reference to a 60Cobalt standard, accounting for response variation with photon energy spectrum. Second, the dose-rate constant was determined with reference to phantom measurements using NIST traceable calibrated model 6702 and 6711 sources. The radial dose function, g(r), the anisotropy function, F(r,theta), the anisotropy factor, phi(an)(r), and the point-source approximation anisotropy constant, phi(an), were derived from one- and two-dimensional dose distribution data measured in the phantom, accounting for finite dosimeter volume and with attention to interchip effects. The results are compared to TG43 and other existing data for 125I sources. The new source is comparable to the model 6711 source design.
    No preview · Article · Jan 2001 · Medical Physics
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