This work investigates the change in shape of optically stimulated luminescence (OSL) curves from carbon-doped aluminum oxide (Al2O3:C) dosimeters as a function of ionization density. The dosimeters were exposed to Ne 371.9MeV/u and Fe 120.4MeV/u heavy charged particle (HCP) beams, and 90 Sr/ 90 Y beta radiation in the interval of absorbed dose to water from 2mGy up to 8.5kGy. The change in the shape of the OSL curves occurs for absorbed doses higher than 10Gy for beta radiation and higher than 50Gy for the HCP beams. Based on the HCP fluences corresponding to the absorbed dose of 50Gy and analytical models of radial dose distribution, we estimated that the dose level in the overlapping track region is comparable to the beta dose in which the OSL curve start to change. This is an indication that track overlap is responsible for changes in shape of the OSL curves of Al 2 O 3 :C Luxel™ dosimeters as the HCP fluence increases.