Dose Constants for seed sources

Dosimetry of I-125, Ir-192 and Pd-103:

  • The dose rate constant Λ (cGy/U-hr) is defined as the dose rate to water at a distance of 1 cm on the transverse axis of a unit air kerma strength source in a water phantom. The constant includes the effects of source geometry, the spatial distribution of radioactivity within the source, encapsulation, and self-filtration within the source and scattering in water surrounding the source. The default values are taken from TG43 Table VI. To convert the TG43 dose rate constant for linear sources into the seed strength at 1 cm as used by PS (parameter S in Luxton et al, Dosimetric Calculations and Measurements of Gold Plaque Ophthalmic Irradiators using Iridium-192 and Iodine-125 Seeds, IJROBP 15, 1988) you must remove all effects of source geometry from Λ by enabling the Adjust by 1/G(1cm,90&Deg;) checkbox which divides Λ by the TG43 geometry factor G(1cm,90°). For example, G(1cm,90°) = 0.9926 for a 3 mm active length linear source. Disable this checkbox for non linear-source radionuclide geometries such as point or spherical sources and Ru-106 plaques.
  • Plaque Simulator extends the TG43 formalism with a water to medium conversion factor which allows for calculation of dose to tissue, or to a small mass of water in a phantom. Normally, this factor should be set to 1. If, however, you wished to calculate dose to a water calibrated TLD in an eye phantom made of PMMA (acrylic) exposed to a plaque containing model 6711 seeds, you would set the dose rate constant to the approriate value for PMMA (0.98 * 0.697 = 0.683) and then set the water to medium field to be the ratio of dose to water in medium versus dose to medium (1.607). The dose rate constant relative to water, and water in medium ratios for PMMA, WT1 and RW-1 may be found in Tables III and IV of the reference: Luxton, G., Comparison of radiation dosimetry in water and in solid phantom materials for I-125 and Pd-103 brachytherapy sources: EGS4 Monte Carlo study, Medical Physics, 21, 1994. These data were used to create the default isotope labeled I-125 (6711 PMMA).
  • Local name is the way this radionuclide source appears in menus.
  • Variant is a long integer used to define variants of Name.
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  • The Class menu selects the isotope, such as "I-125".
  • The Unique ID is a 4 character code that MUST be different for each physics entity.
  • The Type: menu selects the class emission type. For I-125, Ir-192, Pd-103, select gamma. For Ru-106 select beta. The dosimetry parameters for these isotopes are discussed below:
  • The Units of: menu selects how source strength must be entered when creating inventories of this isotope. For instance, for COMS calculations using the default I-125 (6711 COMS) isotope, source strength must be entered in units of air kerma strength U. You may optionally direct that source strength is to be entered as apparent activity in mCi or mgRaEq. Note: If you elect to enter source strength for gamma isotopes in units other than U, you must be sure to enter an apparent activity to air kerma strength conversion factor in the appropriate field. The apparent activity to air kerma strength conversion factor is used to convert source strength entered in mCi or mgRaEq to U. For I-125 seeds, this value is 1.270 U/mCi.
  • The half life is to be entered in days.
  • Active and physical lengths for each source must be specified. For instance, for most I-125 (e.g. IsoAid Advantage and similar) seeds, these values are 3.0 and 4.5 mm respectively.
  • RBE The relative biological effect parameter is used when calculating biologically effective dose (BED).

Dose Constants for Ru-106

Dosimetry of Ru-106 (Rh-106) plaques:

The default parameters (C,R,b,v,k) for Cross's beta point kernel equation are from Cross, W.G. et al, Calculation of beta-ray dose distributions from ophthalmic applicators and comparison with measurements in a model eye, Med. Phys. 28(7), 2001, page 191. By default, these parameters are ignored by Plaque Simulator in favor of using the tabulated data itself rather than attempting to fit an equation to the data. From the Radial Dose tab you may optionally elect to generate Plaque Simulator's beta kernel using Cross's formula and these parameters if you wish.

From the older MIRD formalism:

  • is the average number of beta particles emitted per disintegration.
  • k' is 0.0160 (1.6 x 10^-8 g-cGy/MeV * 10^6 dps/MBq).
  • Eav is the average energy of the beta spectrum in MeV.
  • The water to medium conversion factor described above may also be used to rescale the beta calculation. The default value for this factor is 1.
  • The 90 percentile distance is the distance in mm from the source within which 90% of the energy is absorbed.
  • The active and physical lengths for beta "point and patch" sources should be set to 0.1 mm.
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