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Determination of 177Lu-octreotate activity concentration in vivo using intraoperative gamma probes - a comparative study

Conference contribution
Authors Viktor Sandblom
Ingun Ståhl
Roger Olofsson
Eva Forssell-Aronsson
Published in Cancerfondens planeringsgrupp för onkologisk radionuklidterapis vintermöte 2015, 22-23 januari, Umeå, Sverige.
Publication year 2015
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

Abstract

INTRODUCTION Radionuclide therapy using 177Lu-octreotate has shown promising results for patients with metastasised neuroendocrine (NE) tumours. However, complete remission is achieved only in a few patients due to healthy limiting organs, e.g. kidneys and bone marrow. A new treatment method of NE liver metastases is under development at our facility, based on local administration of 177Lu-octreotate to the liver. However, this method requires the possibility to quantify activity concentration in vivo. Intraoperative gamma probes are routinely used for radio-guided surgery and localisation of diagnostic radionuclides. The aim of this study was to evaluate whether intraoperative gamma probes could be used to determine 177Lu activity concentration in vivo. METHODS Important performance parameters were acquired for two intraoperative gamma probes using different sources of 177Lu, in order to compare the characteristics of the probes. Point sources were used for determination of the range for linear response, and the spatial resolution (FWHM) was evaluated by measuring the line profile over a line source at different depths in a phantom of tissue equivalent plastic. To investigate the ability of the probes to distinguish between normal and tumour tissue, a clinical situation was simulated by creating an agarose gel phantom. This phantom contained a homogenous distribution of 177Lu of a low activity concentration, simulating normal tissue. To simulate metastases, hot gel spheres (5-20 mm), containing a higher activity concentration, were added to the phantom. RESULTS The point source measurements demonstrated great differences between the two probes; one probe showed saturation of the signal at much lower activities than the other. The spatial resolution (FWHM) increased from about 10 to 100 mm when the depth of the line source was increased from 0 to 80 mm. One probe showed lower values of FWHM than the other, for all depths. The measurements using the agarose gel phantom showed high ratios between count rates measured over the hot gel spheres and over the background, for both probes. These results indicated a good ability to distinguish between normal and tumour tissue. CONCLUSION The two evaluated gamma probes showed differences in technical performance. However, satisfying results were obtained for both probes, indicating that intraoperative gamma probe measurements are feasible for determination of 177Lu activity concentration in vivo.

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