The TOol for PArticle Simulation (TOPAS) is a Geant4 based user-friendly Monte Carlo (MC) code to simulate particle transportation for research and clinical Physicists.  TOPAS has an extension system that allows to add new functions for their own application such as TOPAS-nBio. Based on the TOPAS, the current study aims to develop the TOPAS_MRCP extension dedicated to calculate organ doses originating from internal and external radiation for effective dose evaluation based on patient dose distributions in radiation therapy. In this study, the mesh-type reference computational human phantom (MRCP) recently developed by the international commission on radiological protection (ICRP) was used and implemented as computational human phantom into TOPAS. TOPAS_MRCP is written in C++ based on an original Geant4 code and consists of two components; one for geometry plotting and one for dose scoring in each organ. After successfully implementation of MRCP as a TOPAS extension, we calculated organ dose conversion coefficients (DCC) for validation. The result shows a similar tendency at each organ between TOPAS_MRCP, Geant4_MRCP, and ICRP publication 106 reference data with all values being identical within 5%. TOPAS users can easily import MRCP through parameter control files and the user can use various beam conditions provided by the TOPAS macro command. We believe TOPAS_MRCP will be used by many TOPAS users to calculate organ doses from internal and external radiation for radiation therapy studies.