Does the timing of somatostatin-analog therapy affect the imaging properties of DOTA-TOC PET?

Introduction: The discontinuation of somatostatin-analogs (SSAs) prior to somatostatin-receptor based SPECT and PET imaging has long been advocated by professional guidelines due to the belief that therapeutic doses of SSAs may saturate a patient’s somatostatin receptors and adversely affect the diagnostic performance of somatostatin-receptor imaging. Withholding SSAs can aggravate symptoms and complicate the scheduling of nuclear imaging. There is a small but emerging body of literature suggesting that imaging properties of DOTA agents are independent of SSA timing

Methods: We retrospectively reviewed DOTA-TOC PETs performed at our institution between July 27^th, 2018 and November 4^th, 2022. Time between last SSA prior to radiopharmaceutical injection was recorded, as was the type of SSA therapy. For each patient, SUV_Max of the most radiotracer-avid lesion (SUV_Lesion), SUV_Max of the spleen (SUV_Spleen), and their ratio (SUV_Lesion/ SUV_Spleen) was recorded as measures of tumor uptake, background uptake, and signal-to-background, respectively. Patients were divided into three groups based on the time interval between therapy and imaging (0-10 days, 11-20 days, 21+ days). SUV_Lesion, SUV_Spleen, and SUV_Lesion/ SUV_Spleen for each group was compared graphically and by one way ANOVA. Due to the different biological half-lives of octreotide LAR and lanreotide, this analysis was repeated in those subgroups of patients.

Results: After excluding negative studies, patients with incomplete records, and those not receiving SSA therapy, a total of 107 patients were identified (40 lanreotide, 67 octreotide LAR). 18 patients were imaged within 10 days of therapy, 27 between 11-20 days, and 62 after 21 or more days. No difference between time from therapy and mean SUV_Lesion, SUV_Spleen, or SUV_Lesion/ SUV_Spleen was found. ANOVA analysis did not identify a statistically significant difference between the three groups. We also found no differences in the relationship between time from therapy and imaging parameters assessed when looking at lanreotide (SUV_Lesion p=0.79, SUV_Spleen p=0.09, or SUV_Lesion/ SUV_Spleen p=0.59₎ and octreotide LAR (SUV_Lesion p=0.69, SUV_Spleen p=0.15, or SUV_Lesion/ SUV_Spleen p=0.51) subgroups independently.

Conclusions: Time since SSA did not impact imaging properties of DOTA-TOC PET. The current results suggest that patients undergoing DOTA-TOC PET may not need to adjust the timing of their SSA or delay imaging due to recent therapy.

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