Critical Organ Dosimetry for Radiopeptide Therapy of Neuroendocrine Tumours with 177Lutetium-DOTA-Tyr3-Octreotate Utilising Serial Quantitative SPECT/CT and Planar Scintigraphy
Patients undergoing treatment for neuroendocrine tumours with the radiolabelled somatostatin analogue 177Lutetium-DOTA-Tyr3-octreotate (177Lu-Octreotate) at Fremantle Hospital using the CLEMENT (Capecitabine-radiosensitising Lutetium-177-octreotate Endoradiotherapy Management of Endocrine Neurogenic Tumours) protocol have been serially imaged using wholebody planar scintigraphy, quantitative SPECT imaging and low-dose CT attenuation correction. Four timepoints were used for each patient and the resulting scintigraphic images analysed using region of interest (ROI) software tools to generate count densities in the critical organs of kidney, liver and bone marrow. Conversion between count density and absorbed activity within the organs was then achieved by comparison with standard count densities derived from serial imaging of 177Lu phantoms on the same gamma camera. The calculated absorbed activities were processed using the dosimetry software Olinda (MIRDOSE scheme) to produce estimates of radiation absorbed dose in Gray (Gy) to critical organs. Initial analysis concentrated on renal dose, with delivered doses in the range ~ 4-8 Gy per treatment. Liver dose was estimated at under 30 Gy and dose to bone marrow was less than 2 Gy. These calculated doses are below the threshold of significant radiation toxicity in these relevant critical organs. Generation of a pre-treatment tracer imaging protocol to individualise therapy activities administered to patients will be based upon this dosimetry methodology. A Monte-Carlo dosimetry simulation using both the CT and SPECT data is in progress to improve accuracy in calculation of delivered dose to critical normal organs and, in particular, to estimate the therapeutic radiation absorbed dose to tumours.