99mTc-EDDA/HYNIC-TOC is an easily available and cheaper radionuclide that could be used for somatostatin-receptor-based imaging of neuroendocrine tumours (NETs). We aimed to evaluate the diagnostic performance of 99mTc-EDDA/HYNIC-TOC compared to111In-DTPA-octreotide in patients (pts) with NETs. We performed a prospective diagnostic study including pts with biopsy-confirmed NET and at least one visible lesion at conventional imaging. Two independent nuclear medicine physicians evaluated pts who underwent 99mTc and 111In scans and images. The primary outcome was comparative diagnostic accuracy of 99mTc and 111In. Secondary outcomes include safety.

Nine pts were included and performed 14 paired scans. Overall, 126 lesions were identified. 99mTc demonstrated superior sensitivity both when all images were analysed (93.7, 95% CI 88.1% – 96.8% versus 74.8%, 95% CI 66.6 – 81.6%, p < 0.001) and when liver-specific images were analysed (97.8%, 95% CI 92.7% – 99.5% versus 85.1%, 95% CI 76.6% – 91.0%, p < 0.001). 99mTc was also associated with a lower negative likelihood ratio (LR) (0.002, 95% CI 0.009 – 0.1 versus 0.19, 95% CI 0.12 – 0.42, p = 0.009) when evaluating hepatic lesions. Adverse events happened in 3 pts after 111In and in 2 pts after 99mTc, all grade 1. The 99mTc demonstrated a higher sensitivity overall and a better negative LR in liver-specific images compared to 111In in pts with NETs. Our findings suggest that 99mTc is an alternative to 111In and is especially useful in ruling out liver metastases. NCT02691078.