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Evaluation of the possibility of using intraoperative gamma probe measurements for determining 177Lu-octreotate activity concentration in vivo

Conference contribution
Authors Viktor Sandblom
Ingun Ståhl
Roger Olofsson
Eva Forssell-Aronsson
Published in 27th Annual Congress on European Association of Nuclear Medicine, Gothenburg, Sweden, October 18-22, 2014.
Publication year 2014
Published at Institute of Clinical Sciences, Department of Radiation Physics
Institute of Clinical Sciences, Department of Surgery
Language en
Subject categories Cancer and Oncology, Radiological physics


INTRODUCTION Intraoperative gamma probes are routinely used for radio-guided surgery, primarily sentinel node mapping or localization of small tumors after injection of a diagnostic radiopharmaceutical, labelled with e.g. 99mTc or 111In. For therapy purposes, treatment of neuroendocrine (NE) tumors using the beta emitting radiopharmaceutical 177Lu-octreotate has shown promising results, with kidneys and red bone marrow as limiting organs. A new treatment method using 177Lu-octreotate for liver metastases is under development at our facility. For this development, a method to determine the activity concentration of 177Lu-octreotate in tumor and normal tissues in vivo during surgery is required. The aim of this study was therefore to evaluate whether an intraoperative gamma probe could be suitable for this purpose. METHODS Essential detector characteristics were acquired for two intraoperative gamma probes using different sources containing 177Lu. The range for linear response was determined using point sources of increasing activity. Spatial resolution (full width at half maximum, FWHM) and efficiency were determined, using a line source placed at different depths in a tissue equivalent plastic phantom, by moving each probe perpendicularly over the line source. To simulate a clinical situation, tumor phantoms were designed, consisting of agarose gel spheres (5–20 mm diameter) at the surface of a water background. The gel spheres, simulating tumors, contained a high activity concentration of 177Lu while the water, simulating normal tissue, contained a background activity concentration of 177Lu. Count rates were measured using the gamma probes in order to investigate the possibility to distinguish between normal and tumor tissue. RESULTS A linear response (within ±10 %) was seen for point source activities between 1–30 MBq, corresponding to count rates in the detectors of about 100–4,000 counts per second (cps). FWHM increased from about 10 to 70 mm when the depth of the line source increased from 0 to 80 mm. In the tumor phantom measurements, high ratios between count rates measured over the gel sphere and over the water background were acquired, indicating a good ability to distinguish between normal and tumor tissue. CONCLUSION An intraoperative gamma probe could be a useful tool for measurements of activity concentration of 177Lu-octreotate in tissues in vivo.

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