Feasibility and accuracy of single time point imaging for renal dosimetry following 177Lu-DOTATATE (Lutate') therapy

BackgroundThis study aims to assess both feasibility and accuracy of renal dosimetry imaging protocols in patients receiving Lutate therapy for neuroendocrine tumours (NETs), when data acquisition over multiple days is not possible on all cycles.MethodPatients who had received a full 4cycles of Lutate therapy with complete imaging at each cycle were included. Imaging consisted of quantitative SPECT/CT of the kidneys at 4, 24 and 96-120h post injection. Renal absorbed dose was calculated for each data set, and in addition, five alternative methods were explored for comparison. Method 1: a patient average clearance time (t(1/2) average) derived from the first half of contributing patient data was used to estimate absorbed dose for subsequent patients based on 4h imaging alone; method 2: t(1/2) average was applied to subsequent patients on 24h imaging alone; method 3: a patient-specific clearance rate (t(1/2) patient) was determined from complete image data of cycle 1 and applied subsequently to remaining cycles using 4h image data alone; method 4: t(1/2) patient was applied to 24h imaging alone in subsequent cycles; method 5: the 120h data was estimated on subsequent cycles based on the cycle 1 fraction of injected activity (%IA) at 24 and 120h.ResultsTwenty treatments from 18 patients, resulting in 80cycles of therapy, were analysed. The measured average renal absorbed dose per cycle of treatment was 0.380.19Gy/GBq when derived from full imaging data. The use of t(1/2) average applied to a single time point led to large deviations of dose estimates from true values (on average 59% and 30%, when using 4h data and 24h data, respectively). The use of complete image data on cycle 1 and the derivation of t(1/2) patient led to improved dose estimates, with an average deviation from true values of 13% and 2% when using 4h data only and 24h data only, respectively. The use of a 120h %IA derived from cycle 1 led to an average deviation from true dose estimates of 14%.Conclusion In instances where demands on both patients and facilities make multiple time point data acquisition impractical, renal dosimetry is best derived through complete imaging at cycle 1 only followed by a single 24h imaging time point on subsequent cycles, assuming no significant changes in renal function during the time course of therapy.