Kinetics of ^99mTc-HYNIC TOC uptake in patients with NET

^99mTc-labeled octreotide analog, HYNIC-TOC, is a new addition to radio-pharmaceuticals for imaging and screening patients with neural crest tumors. ^99mTc-HYNIC-TOC images are increasingly being used to identify need of ¹⁷⁷Lu-DOTA-NOC/DOTA-TATE therapy in selected patient. Universally, a protocol of whole-body imaging at about 3-4 h postinjection of ^99mTc-HYNIC-TOC is used. Aims and Objectives: This work is aimed to study the time-dependent changes in uptake of ^99mTc-HYNIC-TOC agent in tumors as well as some organs of interest. Whole-body sweep scans were acquired in nine patients at about 1, 2.5, and 4 h postinjection of 8-12 mCi of ^99mTc-HYNIC-TOC. Time-normalized and pixel-normalized counts in selected regions such as whole liver (and also selected tumors in liver and apparently normal areas within the same liver), spleen, thyroid, heart, apical region of lungs, and thigh (for muscle uptake) were obtained. Normalized count ratios of liver to heart, liver to spleen, liver to thigh, heart to thigh, and spleen to heart were analyzed for these time points. These data were analyzed to study the pattern of uptake in these sites for at least three time points. All patients selected in the study group had received ¹⁷⁷Lu-DOTA-TATE therapy in one or more sittings and had biopsy-proven NET. On an average, sweep scans were performed at 60, 153 (±23), and 252 (±8) min after ^99mTc-HYNIC-TOC injection. At about 4 h, counts of heart, thigh, and apical lung region and thyroid fell to a range of 64%-76% of that at 1 h, while that in the liver were 101% (±10%) and Spleen were 106% (±9%) of that at 1 h. Within these, gross liver counts changed marginally in 8 out of 9 patients but increased by about 20% in one. However, the tumors in liver showed disparate behaviors as compared to apparently normal areas. It was observed that in about 50% of liver tumors, uptake of ^99mTc-HYNIC-TOC peaked at 2.5 h and reduced subsequently, but in the remaining, the uptake increased up to 4 h of observation. Counts in the spleen were increased by about 10% in three out of nine patients at the end of 4 h. This study highlights the varied uptake patterns of whole liver and tumors within liver with respect to time. It signifies that the metabolic activity within some tumor type is different and may be of use in prognosticating either the success of therapy or the growth kinetics of tumors subtypes. (author)