Local Diagnostic Reference Levels for Adult Bone SPECT/CT Imaging: A Multicenter Study in South Africa

Abstract

Hybrid SPECT/CT imaging improves diagnostic accuracy by combining functional and anatomical information. However, it contributes additional radiation exposure from both radiopharmaceutical administration and CT acquisition. To mitigate such risks, the ICRP proposed the implementation of diagnostic reference levels (DRLs). However, published hybrid SPECT/CT DRL data remain limited, particularly in low- and middle-income countries (LMICs), including South Africa. This study derives preliminary multicenter local DRL estimates for adult bone SPECT/CT at selected South African nuclear medicine facilities. Given that proposed DRL values are based on four facility medians, these benchmarks should be interpreted as provisional optimization references rather than statistically robust national standards.

A retrospective multicenter study was conducted across four nuclear medicine facilities in South Africa using five hybrid SPECT/CT systems (GE NM/CT 670 Pro, GE NM/CT 860, GE Optima NM/CT 640, Siemens Symbia Intevo 6, and Mediso AnyScan SPECT/CT). Adult bone SPECT/CT examinations performed between 2021 and 2024 were reviewed (n = 665; mean age: 64 years; 53% male). Administered activity, CTDIvol, and DLP were extracted from scanner dose reports and archived patient files. CT acquisitions were stratified by anatomical acquisition coverage. LDRLs were derived as the 75th percentiles of facility median values in accordance with ICRP Publication 135 methodology.

Proposed 99mTc-MDP-administered activity LDRLs were 900 MBq (1–2 acquisition coverages), 820 MBq (3 acquisition coverages), and 740 MBq (4–5 acquisition coverages). CTDIvol LDRLs ranged from 3.0 to 5.5 mGy across acquisition coverage categories, with corresponding DLP values of 176 to 710 mGy•cm; the highest values were associated with chest/abdomen protocols where scan length was greatest. Facilities equipped with automatic-exposure control demonstrated lower CT output variability compared with older fixed-tube current systems, with DLP variation primarily driven by acquisition coverage rather than tube output.

This multicenter study establishes preliminary LDRLs for adult bone SPECT/CT at South African nuclear medicine facilities. The proposed values demonstrate broad alignment with international benchmarks while revealing that scanner technology and acquisition coverage are the principal determinants of CT dose variability in this context. These findings support technology-aware protocol optimization and provide a multicenter baseline for future nationally representative DRL development and weight-adjusted dose frameworks in South Africa and comparable LMIC settings.