A new tissue-equivalent, reusable material for dosimetric verification of radiotherapy is developed and characterized. The material comprises a hydrogel matrix containing copper-doped lithium fluoride nanoparticles that exhibit optically stimulated luminescence (OSL). Using a laser light sheet and a sensitive camera equipped with appropriate filters, the OSL signal is read out layer by layer thus constructing the 3D dose distribution. The good tissue equivalence allows the new material to register the dose from a prescribed treatment plan as if delivered to a patient. Cubic dosimeters measuring 50 × 50 × 50 mm³ are developed and used to measure 3D dose distributions (from voxels measuring 0.8 × 0.8 × 1.0 mm³) with a statistical dose precision of 5% at 100 Gy dose levels. The spatial precision of offsets in dose gradients between planned and measured dose distributions is below the voxel size. A 3% / 3 mm gamma analysis between a prescribed treatment plan and the retrieved and corrected readout yields a pass rate of 89.5%. Light scattering within the dosimetric volume affects the dosimeter performance, indicating the potential for future improvements of spatial and dose precision. In conclusion, the dosimeter system shows high promise for future clinical validation of radiotherapy.