We present a new Electron Paramagnetic Resonance (EPR) X-ray sensor made of hydroxyapatite alginate composite material (HApA). This is a biocompatible, porous material based on sodium alginate polymer and hydroxyapatite (HAp) crystallites, which was proposed in the literature as bone scaffold. HApA scaffolds reproduce, by similar morphology, the mass density values of the healthy (0.3 g/cm3) or pathologic lung tissue (e.g., 0.1 g/cm3 and 0.6 g/cm3 for the emphysematous and fibrotic lung, respectively). We have demonstrated that HApA after irradiation shows a strong radical signal suitable for the purpose of physical dosimetry of ionizing radiation. The quantitative detection of the radioinduced signal was performed by means of EPR spectroscopy. Dose values approaching the therapeutic range (180 Gy) were delivered with photon beams at 100 kVp from an X-ray irradiator. The dose was measured by lithium formate (LiFo) EPR dosimetry. LiFo pellet dosimeters were irradiated next to the HApA scaffolds. The EPR signal of HApA, absent in unexposed HApA and in its HAp and sodium alginate components, resembles that of irradiated pure HAp, suggesting that the signal is associated to the HAp crystals within the polymeric matrix (prevalently radioinduced CO2-* defects of the carbonate impurities in the HAp lattice). The differences between the signals of HApA and that of pure HAp are explained by the presence of unstable radical species. These contribute a signal which overlaps to the main stable CO2-* radical signal, coming from CO33-* and CO-*. Based on the time evolution of the EPR main peak, two decay times were determined, of about 10 days and 15 months, respectively. This suggests that the unstable radicals are strongly stabilized in HApA by two different microenvironments. These are possibly originated, during HApA gelation, at the interface layer between the inorganic (HAp) and the organic (sodium alginate) component. The EPR signal of HApA is comparable to that of LiFo irradiated under the same conditions, and several times larger than that expected in an equivalent mass of pure HAp. The results suggest HApA as a possible EPR dosimetric material for sensing X-ray irradiation of low-density organs.