Quantitative uptake in 99mTc-EDDA/HYNIC-TOC somatostatin receptor imaging – the effect of long-acting release somatostatin analogue therapy

Abstract

Purpose Withdrawal of long-acting release somatostatin analogue (LAR-SSA) treatment before somatostatin receptor imaging is based on empirical reasoning that it may block uptake at receptor sites. This study aims to quantify differences in uptake of 99mTc-EDDA/HYNIC-TOC between patients receiving LAR-SSA and those who were not. Methods Quantification of 177 patients (55 on LAR-SSA) imaged with 99mTc-EDDA/HYNIC-TOC was performed, with analysis of pathological tissue and organs with physiological uptake using thresholded volumes of interest. Standardised uptake values (SUVs) and tumour/background (T/B) ratios were calculated and compared between the two patient groups. Results SUVs were significantly lower for physiological organ uptake for patients on LAR-SSA (e.g. spleen: SUVmax 13.3 ± 5.9 versus 33.9 ± 9.0, P < 0.001); there was no significant difference for sites of pathological uptake (e.g. nodal metastases: SUVmax 19.2 ± 13.0 versus 17.4 ± 11.5, P = 0.552) apart from bone metastases (SUVmax 14.1 ± 13.5 versus 7.7 ± 8.0, P = 0.017) where it was significantly higher. Conclusion LAR-SSA has an effect only on physiological organ uptake of 99mTc-EDDA/HYNIC-TOC, reducing uptake. It has no significant effect on pathological uptake for most sites of primary and metastatic disease. This should be taken into account if making quantitative measurements, calculating T/B ratios or assigning Krenning Scores. There is the potential for improved dosimetric results in Peptide Receptor Radionuclide Therapy by maintaining patients on LAR-SSA.