A modular prosthesis system for replacement of a head portion of a proximal radius includes a monolithic stem component, a head component, and a locking mechanism formed by the stem and head components. The stem component defines a stem anchoring portion having a longitudinal axis and configured to couple to the proximal radius, and a dovetail-shaped first mounting portion on a first end face that extends in a first direction transverse to the longitudinal axis. The head component has a dovetail-shaped second mounting portion on a second end face opposite the first end face slidably engaged with the first mounting portion along the first direction. One of the first and second mounting portions intersects the longitudinal axis. The locking mechanism is formed at an interface between the stem and head components and is engaged through relative translational movement between the stem and head components. A related method is provided.