There is a special need to develop a dosimetry technique with a large-dynamic range and high-spatial resolution to characterize the microstructured X-ray beams used in microbeam radiation therapy (MRT) for cancer. We report the synthesis and characterization of oxyfluoride glass-ceramic (SiO2–Al2O3–CaF2–CaO–SmF3) plates, which contain trivalent-samarium-doped calcium fluoride (CaF2:Sm3+) nanocrystals, for use as a dosimetric detector material, particularly for MRT applications. Our approach utilizes the extent of Sm3+→Sm2+ valence reduction caused by X-ray irradiation as a probe of the X-ray dose delivered; and confocal fluorescent microscopy is used to read out the distribution of valence reduction through the photoluminescence (PL) signal. Our study showed that the Sm3+→Sm2+ valence reduction takes place in CaF2 nanocrystals, but not in the glass matrix. The Sm2+ shows PL emission predominantly due to the fast 4f55d1→7F0 transition, which allows us to read out the detector plate at a high scanning speed. Further, our experiments showed that the detection dose range reaches several thousands of grays, and X-ray dose distribution is detected at a micrometer scale. In addition, the Sm2+ signal can be erased either by heating the irradiated sample at a suitable high temperature or by exposing it to UV light; and the erased glass-ceramic plate is reusable. The new Sm-doped oxyfluoride glass-ceramic with CaF2 nanocrystals reported in this work shows potential for practical use in high-dose and high-resolution dosimetry for MRT.