Microdosimetry: experimental methods and medical applications

Introduction: Microdosimetry is a fundamental method that studies the nature of energy transfer in micron volumes in the particular biological cells. In a biological target, the amount of ionization does not indicate the magnitude of biological radiation-induced damage. However, the severity of biological harm depends strongly on the amount of the linear energy transfer along the direct path of the ionization particle, LET. Generally, the main objective of the microdosimetry is to measure the two fundamental parameters including lineal energy transfer (y) and specific energy (z).   Materials and Methods: By scientific search engines Pubmed, Scopus, Elsevier the keywords "Microdosimetry", "Lineal energy transfer", "Specific energy" were searched and extraction data were analyzed qualitatively.   Results: Experimental methods in microdosimetry are based on the theory of Bragg-Gray cavity. The low-pressure gaseous proportional counter is the main tool in the microdosimetry. The material and thickness of the counter wall should be such that it generates “Charge Particle Equilibrium”. The standard plastics formulation of the wall which was introduced by Shonka is a combination of calcium fluoride, polyethylene, nylon and carbon. The most common gases for sensitive volume of counter are based on the composition of methane and propane. Other experimental methods in microdosimetry are: Autoradiography technique, Wilson cloud chamber, micro strip gas counter (MSGC) and gas electron multiplier (GME) technique with sensitive volume of less than 1mm3 and optical ionization chamber.   Conclusion: Microdosimetry has applications in radiation protection, radiobiology and radiotherapy. The biological effect of the radiation depends on the weight factor of the beam, which is obtained by multiplying it in the absorbed dose by the equivalent dose amount. In radiobiology, the RBE ratio is more accurately obtained by microdosimetry. In high LET radiation, the differences between LET and RBE is not negligible; therefore, in the new techniques of radiation therapy with high LET radiation such as neutron and heavy particles, microdisimetry can accurately measure dose.