Advanced radiotherapy techniques, which plan and deliver a treatment in complicated 3D geometries with steep dose gradients, push 3D dosimetry with correspondingly high spatial resolution to the top of scientific and clinical agendas. This paper presents the first steps taken towards an inexpensive and reusable material for 3D dosimetry based on optically stimulated luminescence (OSL). Carbon-doped alumina (Al ₂O ₃:C) nanoparticles were synthesized using supercritical flow synthesis, in which product properties can be finely controlled. The particles were characterized using electron microscopy and powder x-ray diffraction. C-doping did not alter the crystallographic structure appreciably, and a high elemental signal from C could be measured. Nanoparticles of amorphous γγ-Al ₂O ₃:C were achieved, however calcining these to produce the OSL-relevant αα-phase yielded microparticles. Future work will aim at producing phase-pure αα-Al ₂O ₃:C nanoparticles with a narrow size distribution below 10 nm, and controllable C-concentration and O-deficiency.