Comparison of LEM and MKM based RBE-weighted doses from microdosimetric measurements with silicon 3D microdosimeter

Introduction: In carbon ion radiation therapy, the Local Effect Model (LEM) predicts higher value of Relative Biological Effectiveness (RBE) with respect to the Microdosimetric Kinetic Model (MKM) in most clinical situations. This work aims at comparing the MKM-based RBE-weighted dose calculated from experimental microdosimetric quantities against the RBE-weighted dose computed by a Treatment Planning System (TPS) using the LEM version I. Methods: The Silicon-On-Insulator (SOI) 3D mushroom microdosimeter, developed by the Centre for Medical Radiation Physics (CMRP), Wollongong (Australia), was placed in an RW3 phantom and was irradiated with carbon ions with different plans at the Centro Nazionale di Adroterapia Oncologica (CNAO), Pavia (Italy), acquiring microdosimetric spectra along the beam direction and calculating the corresponding RBE10 with the MKM model applied for Human Salivary Gland (HSG) cells. Results: A good agreement between experiment and Monte Carlo simulation was found, obtaining RBE10 values ranging between 1.2 and 2.8. The prescribed LEM I-based RBE-weighted dose of 3.0 GyE in a cubic SOBP showed to be 33 % larger than the MKM-based RBE-weighted dose, consistently with other results found in literature. Conclusion: This work confirms the reliable use of the 3D SOI microdosimeters as a quality assurance tool for RBE prediction in particle therapy. In addition, the presented outcomes represents a validation through experimental measurements of previous works found in literature on the MKM and LEM RBE-weighted dose comparison based on TPS calculations and Monte Carlo simulations.