Radiation dose comparison of parathyroid planar scintigraphy, combined SPECT/CT, and multiphase 4DCT

Objectives Compare radiation doses for planar scintigraphy (P), SPECT/CT (S/CT), and 4DCT multiphase (4th=perfusion) in cases of difficult-to-detect parathyroid adenomas (PA) in pts with primary hyperparathyroidism (PHPT).

Methods 4DCT was performed in 5 pts where no lesion was detected by P or S/CT. Nonenhanced CT was followed by 75cc iohexol 300 IV with multiphase 4DCT imaging at 30, 60, and 90 sec (Philips Brilliance 64-slice scanner (120 kvP, 180-300 mAs, 2.0 mm). Dual-phase (15 min,2 hr) multiview P extended from the nose to below the aortic arch after IV Tc-99m-MIBI. S/CT (same region, timepoints, camera as P) was performed (Philips Precedence 6-slice scanner (120 kVp,20 mAs,3.3 mm)). Radiation doses were calculated using Stanford’s RADAR (P, S/CT) and BCM’s effective dose calculator (4DCT) which utilizes DLP (dose-length product) info.

Results P or S/CT evaluation gives the lowest radiation dose. Optimized 4DCT alone is similar if utilized as an initial study. Combined radiation dose is nearly doubled or tripled for P+S/CT+4DCT, and multiple localization studies in difficult cases even further increases total dose. Prior to 4DCT, Pt 1 had 3 nonlocalizing scintigraphic studies giving a total radiation dose of 35.7 mSv. Pt 5 had 3 nonlocalizing scintigraphic studies and 1 CT-neck prior to 4DCT with a total dose of 52.4 mSv. All pts were successfully localized and treated surgically. We are further optimizing our 4DCT protocol. (Further cases, data to be presented at meeting)

Conclusions For PA localization, P alone provides the lowest dose. Nonlocalized lesions may require S/CT and/or 4DCT. Given the implications of localization for surgical morbidity, the benefit of additional exams likely outweighs the added radiation dose.

Research Support Acknowledgement: Martin A. Lodge, PhD, Johns Hopkins Medicine, Dept of Radiology