MIRD Pamphlet No. 21: A Generalized Schema for Radiopharmaceutical Dosimetry—Standardization of Nomenclature 5/15/2009

MIRD Medical Internal Radiation Dose –Committee of the Society of Nuclear Medicine –Original schema first issued in 1968 Provides a framework for the assessment of absorbed dose to organs, tissue subregions, etc. from internally deposited radionuclides Update to: –Standardize nomenclature –Adopts effective and equivalent dose –Highlights need to address deterministic effects

Mean Absorbed Dose Rate r T, r S : target tissue and source tissue D(r T,t): dose rate at time t in target tissue rTrT rSrS S S rSrS

Mean Absorbed Dose Equation describes the dose to target tissue r T for integration time T D after administration of radionuclide Time-dependent activity can be measured (e.g. w/ imaging, sampling) or modeled

S(rT←rS,t)S(rT←rS,t) Relates dose in target organ to activity in source organ Function of emission spectrum, relative position of organs, shape, and size of organs Tabulated for “standard” geometries and radionuclides

S(rT←rS,t)S(rT←rS,t) E i = energy of ith nuclear transition Y i = number of ith transition / nuclear transformation Δ i =E i Y i Ф i = fraction of emitted energy absorbed in target M(r T,t) is the mass of the target tissue

Equivalent Dose “The equivalent dose is a radiation protection quantity defined by the ICRP (7,8) and used to relate absorbed dose to the probability of stochastic health effects in a population exposed to radionuclides or radiation fields…” Stochastic effects include cancer and heritable disease Is not applicable to deterministic effects Unit = Sievert (Sv)

Equivalent Dose H(r T,T D ): equivalent dose w R : Radiation weighting factor for radiation type R (1 for photons & electron, 20 for  -particles)

Effective Dose “The effective dose E is a radiation protection quantity defined … for establishing annual limits of exposure to workers and members of the general public.” Accounts for internal and external radiation sources Unit = Sievert (Sv)

Effective Dose

Quantities Related to Deterministic Effects RBE-weighted dose BED EUD Isoeffective Dose

RBE-weighted dose RBE: Relative Biological Effectiveness RBE is analogous to w R (from equivalent dose equation) RBE depends on tissue and biologic endpoint

BED BED: Biologically Effective Dose Usually used to compare different fractionation schemes Accounts for dose rate variation in radionuclide therapy

EUD EUD: Equivalent Uniform Dose Dose delivered to an organ is generally not uniform EUD is the dose value that when uniformly delivered to an organ would yield the same response as the actual dose Unit: Gray (Gy)

Isoeffective Dose Recently proposed to allow comparison of high LET and low LET radiation.

Summary MIRD schema is used to calculate dose from internal radionuclides Stochastic effects –Equivalent dose: accounts for radiation type –Effective dose: accounts for radiation type and organs irradiated