Abstract: Provided is a lesion identification marker for use in radiation therapy that enables pure gold microparticles which absorb X-rays to be placed, with extremely low invasive potential, in any site in the body in an arbitrary amount appropriate for the type of radiation therapy and the therapeutic target site, and that enables the placement site to be identified over a long period of time by radiation therapy equipment. The lesion identification marker for use in radiation therapy includes a mixture of pure gold particles and a substance containing a calcium phosphate-based bone reinforcing material, or a mixture of pure gold particles, a mixing solution, and a substance containing a calcium phosphate-based bone reinforcing material. The volume mean diameter (MV) of the particles of the substance containing a calcium phosphate-based bone reinforcing material is in the range of 3-12 ?m.