Abstract
PURPOSE: Recent reports personalizing the administered activity (AA) of each cycle of peptide receptor radionuclide therapy based on the predicted absorbed dose (AD) to the kidneys (dose-limiting organ) have been promising. Assuming identical renal pharmacokinetics for each cycle is pragmatic, however it may lead to over- or under-estimation of the optimal AA. Here, we investigate the influence that earlier cycles of [(177)Lu]Lu-DOTATATE had on the biokinetics and AD of subsequent cycles in a recent clinical trial that evaluated the safety and activity of [(177)Lu]Lu-DOTATATE in pediatric neuroblastoma (NBL). We investigated whether predictions based on an assumption of unchanging AD per unit AA (Gy/GBq) prove robust to cyclical changes in biokinetics. METHODS: A simulation study, based on dosimetry data from six children with NBL who received four-cycles of [(177)Lu]Lu-DOTATATE in the LuDO trial (ISRCTN98918118), was performed to explore the effect of variable biokinetics on AD. In the LuDO trial, AA was adapted to the patient's weight and SPECT/CT-based dosimetry was performed for the kidneys and tumour after each cycle. The largest tumour mass was selected for dosimetric analysis in each case. RESULTS: The median tumour AD per cycle was found to decrease from 15.6 Gy (range 8.12-26.4) in cycle 1 to 11.4 Gy (range 9.67-28.8), 11.3 Gy (range 2.73-32.9) and 4.3 Gy (range 0.72-20.1) in cycles 2, 3 and 4, respectively. By the fourth cycle, the median of the ratios of the delivered AD (AD(D)) and the predicted (or "expected") AD (AD(E)) (which was based on an assumption of stable biokinetics from the first cycle onwards) were 0.16 (range 0.02-0.92, p = 0.013) for the tumour and 1.08 (range 0.84-1.76, p > 0.05) for kidney. None of the patients had an objective response at 1 month follow up. CONCLUSION: This study demonstrates variability in Gy/GBq and tumour AD per cycle in children receiving four administrations of [(177)Lu]Lu-DOTATATE treatment for NBL. NBL is deemed a radiation sensitive tumour; therefore, dose-adaptive treatment planning schemes may be appropriate for some patients to compensate for decreasing tumour uptake as treatment progresses. Trial registration ISRCTN ISRCTN98918118. Registered 20 December 2013 (retrospectively registered).