Optimization of the number of post-therapeutic planar imaging time points for the most reliable organ and tumour dosimetry in peptide receptor radionuclide therapy

177Lu-DOTATATE therapy is an effective treatment strategy for the treatment of neuroendocrine tumours. However, it could cause severe side effects in major critical organs. Therefore, individual dosimetry is essential for the safety and treatment of patients. Multiple imaging time points (4–6 time points) post administration of radioactivity can be used to obtain the best fit for dosimetric calculations, however, this is a tedious and time-consuming process. In this study, we aimed to reduce the number of imaging time points to three and optimized them for less cumbersome organ dosimetry. In this retrospective study, the images from 40 patients with neuroendocrine tumors were analysed. Sequential planar whole-body scans were acquired at five time points (0.5, 4, 24, 72/96 and 160 h) post administration of 177Lu-DOTATATE, which was considered the standard data set for dosimetry. Subsequently, seven subsets of three different time points (T1-0.5,4,24 h; T2-4,24,72/96 h; T3-0.5,4,160 h; T4-4,24,160 h; T5- 24,72/96,160 h; T6- 4,72/96,160 h; T7-0.5,4,72/96 h) were generated using standard data set. For each of these, normalized cumulative activity (NCA) and absorbed dose (AD) for kidneys, liver, spleen, lung, and tumours were estimated using Dosimetry Toolkit (GE Healthcare, Chicago, IL) and OLINDA EXM v.2 software respectively. The best three correlation coefficients of AD (mGy/MBq) of the standard data set with subsets were 0.74 (T2), 0.71(T6), 0.94 (T7) respectively for kidneys; 0.80(T3), 0.64(T6), 0.77(T7) respectively for spleen; 0.99(T3), 0.98(T6), 0.99(T7) respectively for liver; 0.63(T3), 0.62(T4), 0.75(T7) for lung and 0.92(T3), 0.90(T6), 0.96(T7) for tumours respectively. It was demonstrated that organ dosimetry with three-time point imaging can be as accurate as standard imaging when the imaging three time points are chosen optimally. The imaging subset of T7 (0.5, 4 and 72/96 h) produced the best results for organs and tumour lesions compared to other subsets.