Single-photon emission computed tomography (SPECT) and X-ray computed tomography (CT) is used to di-agnose and monitor the skeletal system. However, due to the combination of injected activity and CT radiation dose, its usage is linked to an increase in patient radiation exposure. This study aims to estimate the radiation dose received by patients while undergoing bone SPECT/CT and to establish its local diagnostic reference level (LDRL). In this retrospective study, data from 30 patients who had undergone bone scans were obtained from two hospital imaging devices. The effective doses (E) of the radiopharmaceutical agent and CT components were calculated. Furthermore, the patients' anteroposterior thickness at the midline and lateral widths were measured from their CT images to determine the size-specific dose estimate (SSDE). The mean age of the patients was 53.53 +/- 16 years. They were administered an injection activity of 740 MBq of 99mTc-Methyl diphosphonate for SPECT, yielding an E of 2.78 mSv. The mean CTDIvol, DLP, SSDE, and E values for the CT component were 8.76 +/- 3.25 mGy, 350 +/- 138 mGy cm, 13 +/- 4 mGy, and 4.90 +/- 1.94 mSv, respectively. Patients received a combined dose of 7.68 mSv from the radiopharmaceuticals and CT. The proposed LDRL for SPECT was comparable to international DRLs, but the CT LDRL was slightly higher. An SSDE-based LDRL is proposed. Moreover, the overall patient dose can be reduced even further by optimizing the injected activity and CT imaging factors.
