A Method for Calculating the Alpha-ray Dosage to Soft Tissue-filled Cavities in Bone

Bone-seeking alpha-particle emitting radionuclides are common health physics hazards. Additionally, they are under consideration as an option for therapeutic molecular radiotherapy applications. Current dose models do not account for energy or bone-site dependence as shown by alpha-particle absorbed fractions given in ICRP Publication 30. Energy-dependent, yet bone-site independent, alpha-particle absorbed fractions have been presented by the models of Stabin and Siegel (2003 Health Phys. 85 294-310). In this work, a chord-based computational model of alpha-particle transport in cortical bone has been developed that explicitly accounts for both the bone-site and particle-energy dependence of alpha-particle absorbed fractions in this region of the skeleton. The model accounts for energy deposition to three targets: cortical endosteum, haversian space tissues and cortical bone. Path length distributions for cortical bone given in Beddoe (1977 Phys. Med. Biol. 22 298-308) provided additional transport regions in the absorbed fraction calculation. Significant variations in absorbed fractions between different skeletal sites were observed. Differences were observed between this model and the absorbed fractions given in ICRP Publication 30, which varied by as much as a factor of 2.1 for a cortical bone surface source irradiating cortical endosteum.

Section: Previous Dosimetric Models For Alpha-particles In Cortical Bonementioning

confidence: 99%

Absorbed fractions for alpha-particles in tissues of cortical bone

Watchman¹,

Bolch²

2009

“…Lawson (1967) presented similar calculations for the recoil proton dose at marrowbone interfaces isotropically irradiated by fast neutrons. Both Yamamoto and Lawson used a calculation model based on the work of Charlton and Cormack (1962b).…”

Section: Previous Work On Bone Dosimetrymentioning

confidence: 99%

The dosimetry of bone irradiated by fast neutrons

Montelius

Burlin

1986

The previous work on the dosimetry of bone is briefly reviewed. A dosimetric theory for the response of detectors irradiated by fast neutrons is applied to the problem of bone dosimetry. In the theory the detector or cavity shape is characterised by distributions of chord lengths along which the neutron-produced charged particles travel and deposit energy. Cavities of different convex geometries can be treated. A simplified version of the theory uses a single mean chord length to characterise the cavity. The absorbed dose to individual marrow cavities in trabecular bone is calculated over a large range of marrow cavity size for monoenergetic neutrons ranging from 0.5 to 7.0 MeV and for 252Cf neutrons. The influence of cavity shape is explored by considering spheres and cylinders of different elongation. The difference in absorbed dose is not great. Also the simplified model using a single mean chord length gives results in close agreement with the results obtained with chord length distributions. The mean marrow dose to different human bones has been calculated in three ways. First by using measured chord length distributions for the marrow cavities in the bones, second by using a sphere with the same mean chord length as the measured distribution and third by applying the measured single mean chord length. The difference between the three approaches is small and the agreement is good with results obtained by other workers who used the Monte Carlo technique. The dose to the endosteal cell layer has also been calculated by approximating the layer with an infinite slab.

“…He assumed that per Ra disintegration about 2.5 a-particles are generated, each with an average energy of 5.75 MeV. Charlton and Cormack (1962a) improved on Spiers' calculation using a variable LET derived from the exponential range-energy relationship and assuming true cylindrical geometry for the Haversian canals. Their calculated dose values across the diameter of the cylinder (Charlton and Cormack 1962b) were significantly higher than those of Spiers.…”

Section: Introductionmentioning

confidence: 99%

Radiation dose factors for alpha -emitters in osteons and some considerations on dose non-uniformity ratios and relative distribution factors

Polig

1989

Dose factors for locations within a tissue-filled cylindrical cavity bounded by an infinite medium of bone labelled with an alpha-emitter are calculated by means of a Monte Carlo procedure. The calculational approach is general and allows us to determine dose factors for specific distances or target volumes defined by concentric cylinders, as well as various source geometries including surface sources, buried surface sources, infinite and bounded volume sources, and also comprises plane (trabecular) surfaces as a limiting case. Values for the dose factors and the contribution of cross-fire traversals are calculated mainly for 239Pu and 226Ra and a typical Haversian canal diameter of 30 microns in beagles and 70 microns in humans. Also tables are given that allow dose factors to be derived for the relevant range of alpha-particle energies, diameters and target distances. The dose non-uniformity factors (local dose rate/average skeletal dose rate) for 239Pu are 27.2 in humans and 14.0 in beagles. The corresponding values for 226Ra are in the ranges 1.18-1.26 (humans) and 0.90-0.97 (beagles) for a 222Rn retention between 10-30%. The relative distribution factors 239Pu/226Ra are 21.5-23.0 for humans and 14.5-15.6 for dogs. General expressions for calculating non-uniformity factors and relative distribution factors are derived and implications are discussed in the light of some experimental findings.