Recent Trends in Internal Exposure Dose Calculation Methods in Nuclear Medicine Therapy*—Comprehensive Review of Monte Carlo, MIRD, Imaging, and Software—

Internal exposure dose estimation in nuclear medicine has a long history, but it is currently attracting unprecedented attention due to the rapid global development of new radiopharmaceuticals for theranostics. This paper integrates recent trends in internal dosimetry methods in Japan and globally. We review the fundamental principles and latest developments in four key areas: (1) Monte Carlo methods, focusing on the RT-PHITS system for precise individual dosimetry; (2) the MIRD method, detailing its basic concepts and the new “MIRDcalc” software; (3) quantitative imaging and Time-Integrated Activity (TIA) acquisition essential for accurate dosimetry; and (4) the current status of commercial dosimetry software supporting clinical workflows. This comprehensive review aims to provide a perspective on the new era of high-precision internal dosimetry.