# Silicon 3D Microdosimeters for Advanced Quality Assurance in Particle Therapy
## 1 Introduction  
In contrast to conventional X-ray therapy, particle therapy delivered with protons and heavy ion beams allows extremely conformal dose painting and an improved therapeutic ratio due to the Bragg peak phenomena. The dose prescription process in proton and heavy ion therapy includes a radiobiological concept called the relative biological effectiveness (RBE). RBE is the ratio of the absorbed doses that produce the same biological effect between a proton or ion irradiation and a reference radiation. RBE depends on linear energy transfer (LET), dose and biological endpoint as well as the intrinsic radiosensitivity of the tissue [1]. Currently, an RBE value of 1.1 is used worldwide in proton therapy as it was anticipated, from short-term clinical outcome data, that variation in proton RBEwith tissue type, LET, dose per fraction and study endpoints would be small [2]. Despite this, a large amount of data is available [3] showing large variation and considerable uncertainties in proton RBE values. Protons exhibit a stronger biological effect on healthy tissues surrounding the treated region compared to photon therapy. The RBE-weighted dose (RBED) concept was first introduced for protons and heavy ions, for which the RBE varies significantly throughout the treatment volume. RBED planning requires RBE verification along with traditional absorbed dose quality assurance to guarantee accurate treatment. While the absorbed dose is routinely measured with an ionisation chamber, microdosimetric quantities, such as the dose-averaged lineal energy (\(\overline{y_{D}}\)) or dose-averaged lineal energy transfer (\(\text{LET}_{\text{d}}\)), need to be measured with high spatial resolution for RBE prediction. The current gold standard detector for microdosimetry is the tissue equivalent proportional counter (TEPC); however, conventional TEPCs are not ideal for routine clinical microdosimetry due to pile up in high-dose rate clinical therapeutic beams. A new mini-TEPC with a smaller sensitive volume was developed at Legnaro National Laboratory to overcome this issue and it was successfully tested using the CATANA 62 MeV proton beam [4]. Nevertheless, both types of TEPCs still require high voltage operation and gas supply that could limit routine application in the clinical setting. The Centre for Medical Radiation Physics (CMRP) introduced the concept of SOI microdosimeters that are based on micron-sized cylindrical sensitive volumes (SV) in an array. These microdosimeters mimic the dimensions of cell nuclei and are fabricated with both planar and Micro-Electro-Mechanical Systems (MEMS) 3D detector technology at SINTEF MiNaLab, Norway. The microdosimeter requires less than 10 V bias supply. The thickness of the SOI microdosimeter active region is 2, 5 or 10 \(\upmu\)m depending on the application such as proton, heavy ion or boron neutron capture therapy (BNCT). Previous generations of CMRP SOI microdosimeters were fabricated using planar technology. Through this technology, a p-n junction is formed by implementing an oxide-passivated surface and using the ion implantation process to create doping impurities at the surface of a silicon detector. Planar technology microdosimeters were developed and successfully tested [5] but had limited 3D SV definition due to the silicon not being able to be patterned any more than 1-2 \(\upmu\)m under the surface of the 10 \(\upmu\)m thick active SOI layer, leading to lateral diffusion of the charge outside the SV [6; 7; 8; 9; 10; 11]. In order to overcome these drawbacks in planar technology, 3D detector technology has been used to fabricate 3D SOI microdosimeters presented in this review (often referred to as the \"Mushroom\" microdosimeter). The 3D detector concept was first proposed by [PERSON] in 1995 [12], and the fabrication of 3D detectors was made possible by the advanced developments in MEMS technology [13].