Elsevier

Seminars in Nuclear Medicine

Volume 38, Issue 5, September 2008, Pages 367-383
Seminars in Nuclear Medicine

Small-Scale Dosimetry: Challenges and Future Directions

https://doi.org/10.1053/j.semnuclmed.2008.05.003Get rights and content

The increased specificity of targeting agents has resulted in an interest in the use of radionuclides that emit particulate radiation: alpha particles, beta particles and Auger electrons. The potential advantage of these radionuclides is the ability to deliver therapeutic doses to individual tumor cells while minimizing the dose to the surrounding normal tissues. However, the dosimetry of these radionuclides is challenging because the dose must be characterized on a scale that is comparable to the range of these emissions, ie, millimeters for beta particles, micrometers for alpha particles, and nanometers for Auger electrons to. In this review, each class of particulate emitter is discussed along with the associated dosimetric techniques unique to calculating dose on these scales. The limitations of these approaches and the factors that hinder the clinical use of small-scale dosimetry are also discussed.

Section snippets

MIRD Method

In 1968, the MIRD Committee established the formalism for dose calculation from internally deposited radionuclides reducing the complex nature of the absorbed dose calculation into a simple mathematical form.10 The MIRD schema provides methods for calculating the absorbed dose from the source-activity distribution and the physical properties of the radionuclide. This calculation is simply the conversion of activity in a source organ into the energy absorbed per unit mass in the target organ. In

Beta Particles

Beta emitters have been used for several decades within the context of targeted radiotherapy.1 Even though 131I is still the beta emitter most used within that context, 90Y has also been widely used, for example, in RIT of non-Hodgkin lymphoma.2 Several beta emitters have been investigated as potential candidates for targeted radiotherapy47; however, as stated in an early report by Rogus and Wessels,48 dosimetric behavior is just one criterion among many governing the selection of a particular

Alpha Particles

In recent years, a number reports have characterized the use of alpha particle emitters in therapeutic applications.3, 4, 5, 6, 84, 85, 86 Alpha particles consist of a helium nucleus with a charge of + 2, and are often the result of the decay of nuclei with an atomic number (Z) greater than 82.49 Symbolically, the decay is expressed as:XZAYZ2A4+α+γ where α represents the alpha particle and γ is a gamma emission that often accompanies the decay.49 The alpha particles considered for therapy

Auger Electrons

There are many radionuclides that decay by a process known as electron capture. Electron capture is a beta decay process that competes with positron emission. For nuclei, which are unstable because of an excess of positive charge, this positive charge can be reduced by the transformation of a proton to a neutron with the emission of a positron, or alternatively by nuclear capture of an orbital electron.49p++en+νe Whereas there are no emissions directly from the nucleus, the residual inner

Summary

The principles of small-scale dosimetry for alpha particle, beta particle and Auger electron emitting radionuclides have been in existence for several years. These calculational techniques have been applied to a number of studies – theoretical, in vitro and to a lesser amount, in vivo. Two general calculational approaches have been used. Analytical methods are based on equations that parameterize the energy deposited from individual source emissions.20, 51, 52, 88 These calculations are fast

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