Relative contributions to patient effective dose in SPECT-CT hybrid imaging

Medical imaging using ionising radiation, while commonplace, has an associated risk of cancer induction. In trying to manage this risk it is beneficial to be able to quantify it through the use of effective dose. Estimates of effective dose resulting from radiopharmaceutical administration are well established in published data but the equivalent data for the computed tomography (CT) component of nuclear medicine hybrid imaging is less well-known. This study aims to estimate the absolute and relative contributions to effective dose from radiopharmaceutical and CT components of common single photon emission computed tomography (SPECT)-CT procedures. Using patient dose-audit data from three SPECT-CT systems we estimated the effective dose resulting from radiopharmaceutical administration by scaling the published effective dose data according to administered activity. The corresponding effective dose resulting from the CT component was estimated using the ImPACT CT dosimetry tool. Absolute effective dose from radiopharmaceutical administrations ranged from 0.2 (Tc-99m colloid head and neck sentinel lymph node imaging) to 14.2 mSv (dual-isotope Tc-99m sestamibi/I-123 sodium iodide parathyroid imaging). Effective dose from the CT components ranged from 0.3 (Tl-201 chloride brain scans - scan range covering the brain) to 4.4 mSv (Tc-99m HYNIC Ty3-Octreotide scans which use a scan range from the top of the brain to bottom of pelvis). Relative contributions (radiopharmaceutical/CT) ranged from 11/89 (Tc-99m colloid head and neck sentinel lymph node imaging) to 98/2% (Tl-201 chloride brain scans). There is a wide range of effective dose from radiopharmaceutical and CT components of hybrid imaging and both aspects can benefit from optimisation efforts, with the largest benefit coming from the CT components.