In the present work, a newly-developed MC-based treatment planning (MCTP) tool for proton therapy is proposed to support treatment planning studies and research applications.It allows for single-field and simultaneous multiple-fields optimization in realistic treatment scenarios and is based on the MC code FLUKA.Relative biological effectiveness (RBE)-weighted dose is optimized either with the common approach using a constant RBE of 1.1 or using a variable RBE according to radiobiological input tables.An example of treatment plan in a patient-CT geometry is presented for clinical treatment parameters as used at the Italian National Center for Oncological Hadron Therapy (CNAO).