Correlation Between External Exposure and Quantified Activity for Patients Undergoing NeT-177LU Therapy

Abstract

Abstract Background: To assess the absorbed dose in red marrow as a critical organ of treatment, the total uptake of 177Lu activity for patients undergoing neuroendocrine tumor (NET) with LUTATERA_177_Lu_therapy was determined from routine external dose survey measurements and total quantification from SPECT-CT images. Aims: Study the correlation of body activity obtained with image with the activity obtained by means of an external exposure monitor. Optimize the number of measurements necessary to be able to assess the evolution of body activity of each patient with the least discomfort. Methods: The data of 4 images per cycle for 4 cycles of 3 patients were analyzed. The temporal evolution of Aima(t) whole body activity was estimated by means of a series of image acquisitions with SPECT-CT on days 1, 2, 5 and 7 of each cycle. The external dose at 1 m from the patient was determined with a radiation survey meter, measuring the ambient dose equivalent H* at the same moments as with the SPECT-CT images to determine the evolution of whole body activity Amon(t) determined with a conversion factor from external exposure to activity obtained before the patient cleared activity, that is, in the first minutes after administration, t~0. The correlation between Aima(t) vs Amon(t) was studied by means of a model that estimates the external exposure produced by an activity point equivalent and the study of the distribution pattern of activity of the organs and tissues with the highest uptake. Red marrow doses produced by body activity are estimated using the OLINDA software. Results: The value of Amon(t) obtained from the administered activity (t~0) is 53%, 57% and 75% of that obtained by quantifying the activity with imaging Aima(t) for patients 1, 2 and 3, respectively. The distribution of activity in the first moments after administration presents a more superficial uptake pattern than that which occurs in the days following this administration, formed mostly in very internal organs such as the liver, kidneys and spleen. The red marrow doses obtained for the 3 patients are 0.92, 1.29 (patient with only one kidney) and 0.58 Gy, respectively. Conclusion: The red marrow dose of patients undergoing NET with Lu-177 can be estimated from external measurements with radiation survey meter provided that at least, (to reduce patient discomfort to a minumum), one SPECT-CT image is available, acquired over days 1 to 5 after administration.