Quantitative uptake in 99m Tc-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 99m Tc-EDDA/HYNIC-TOC between patients receiving LAR-SSA and those who were not. METHODS: Quantification of 177 patients (55 on LAR-SSA) imaged with 99m Tc-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 SUV max 13.3 ±â€…5.9 versus 33.9 ±â€…9.0, P  < 0.001); there was no significant difference for sites of pathological uptake (e.g. nodal metastases SUV max 19.2 ±â€…13.0 versus 17.4 ±â€…11.5, P  = 0.552) apart from bone metastases (SUV max 14.1 ±â€…13.5 versus 7.7 ±â€…8.0, P  = 0.017) where it was significantly higher. CONCLUSION: LAR-SSA has an effect only on physiological organ uptake of 99m Tc-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.