Design concept for a novel SQUID-based microdosemeter

Experimental microdosimetry: history, applications and recent technical advances

The invention of tissue-equivalent proportional counters simulating micrometre diameter volumes, intended to measure the linear energy transfer of a radiation field, resulted in a practical demonstration of the stochastic nature of energy deposition in small volumes. Besides contributing to a better understanding of the interactions between ionising radiation and biological systems, these detectors have had a significant impact on applied radiation dosimetry. The initial instruments were elegant but suitable only for laboratory experiments because of their sensitivity to environmental conditions and the complex support systems they required. However, their ability to separate the dose due to neutrons from that delivered by photons motivated detector design modifications that eventually resulted in robust detectors suitable for use as radiation survey instruments. Proportional counters simulating micrometre tissue volumes turned out to be the ideal detectors for monitoring the complex radiation environments, including on the space shuttle and International Space Station, and have served as the primary active dosimeters in space for nearly two decades. The need for more sophisticated measurements has led to further improvements in detector design, and the need for smaller and lighter dosimeters is motivating further developments in both detectors and data processing systems.

Future development of biologically relevant dosimetry

Proton and ion beams are radiotherapy modalities of increasing importance and interest. Because of the different biological dose response of these radiations as compared with high-energy photon beams, the current approach of treatment prescription is based on the product of the absorbed dose to water and a biological weighting factor, but this is found to be insufficient for providing a generic method to quantify the biological outcome of radiation. It is therefore suggested to define new dosimetric quantities that allow a transparent separation of the physical processes from the biological ones. Given the complexity of the initiation and occurrence of biological processes on various time and length scales, and given that neither microdosimetry nor nanodosimetry on their own can fully describe the biological effects as a function of the distribution of energy deposition or ionization, a multiscale approach is needed to lay the foundation for the aforementioned new physical quantities relating track structure to relative biological effectiveness in proton and ion beam therapy. This article reviews the state-of-the-art microdosimetry, nanodosimetry, track structure simulations, quantification of reactive species, reference radiobiological data, cross-section data and multiscale models of biological response in the context of realizing the new quantities. It also introduces the European metrology project, Biologically Weighted Quantities in Radiotherapy, which aims to investigate the feasibility of establishing a multiscale model as the basis of the new quantities. A tentative generic expression of how the weighting of physical quantities at different length scales could be carried out is presented.