  We present MONAS (MicrOdosimetry-based modelliNg for relative biological
effectiveness (RBE) ASsessment) toolkit. MONAS is a TOPAS Monte Carlo
extension, that combines simulations of microdosimetric distributions with
radiobiological microdosimetry-based models for predicting cell survival curves
and dose-dependent RBE. MONAS expands TOPAS microdosimetric extension, by
including novel specific energy scorers. These spectra are used as physical
input to three different formulations of the Microdosimetric Kinetic Model
(MKM), and to the Generalized Stochastic Microdosimetric Model (GSM2), to
predict dose-dependent cell survival fraction and RBE. MONAS predictions are
then validated against experimental microdosimetric spectra and in vitro
survival fraction data. We present two different applications of the code: i)
the depth-RBE curve calculation from a passively scattered proton SOBP, and ii)
the calculation of the 3D RBE distribution on a real head and neck patient
geometry treated with protons. MONAS can estimate dose dependent RBE and cell
survival curves from experimentally validated microdosimetric spectra with four
clinically relevant radiobiological models. From the radiobiological
characterization of a proton SOBP field, we observe the well-known trend of
increasing RBE values at the distal edge of the radiation field. The 3D RBE map
calculated confirmed the trend observed in the analysis of the SOBP, with the
highest RBE values found in the distal edge of the target. MONAS extension
offers a comprehensive microdosimetry-based framework for assessing the
biological effects of particle radiation in both research and clinical
environments, contributing to bridging the gap between a microdosimetric
description of the radiation field and its application in proton therapy
treatment with variable RBE.
