An innovative alternative in the management of PRRT radioactive waste.

Introduction: Efficiency of Peptide Receptor Radionuclide Therapy (PRRT) with ¹⁷⁷Lu-DOTATATE is now clearly established and Lutathera^® is now registered in both Europe and the USA. However, direct radionuclide synthesis leads to production of ¹⁷⁷Lu (T_1⁄2 : 6,7 days) but also of ^177mLu (T_1⁄2 : 160,4 days), which can generate many problems of biological waste management. In Belgium, legal release limits are 1.9 KBq/L and 0.590 KBq/L for ¹⁷⁷Lu and ^177mLu, respectively. We report on the initial experience with a prototype of toilet using an innovative technique of filtration to significantly decrease radioactivity levels in collected urines/feces.

Methods: From 07/2021 to 12/2021, we performed in two university hospitals a total of 14 cycles of PRRT in 6 different patients (4 women, 2 males) with metastatic neuroendocrine tumors (4 ileal, 2 pancreatic). The toilet system was developed by BKH Water/ ZerEau (NL). It works as a normal toilet from the perspective of the patient. After flushing the toilet bowl, a filter process starts, which consists of several steps: First a pre-filtration of all solids and fines, e.g. toilet paper and feces by means of micro-filtration, which is then rinsed and dewatered by vacuum and plunger technology to reduce volume. Second, an absorption filtration with specific active absorbents and flow design reduces radio-active components in the wastewater. A sample of 250mL of this water is then collected after the filtration cycle. Using a multi-channel analyser with NaI cristal, these measurements were validated by an external laboratory. Used filters need to be stored and lead-shielding of cartridges can be reused.

Results: Due to technical issues with the prototype, two treatments could not be considered. A mean ¹⁷⁷Lu-DOTATATE activity of 7521±113MBq was injected in the other 12 cycles, including on average 0.015% of ^177mLu. All treatments were given in the morning, the patients stayed overnight, and were discharged after 24 hours. The activity measured in the wastewater was 72±86KBq/L (median: 28KBq/L) of ¹⁷⁷Lu and 10±11Bq/L (median: 4Bq/L) of ^177mLu, corresponding to a log3.4±0.4 decrease of expected values. Moreover, following a learning curve and technical adjustments (filter pore size, system flush flow rate, etc) lead to further decrease as the activity was 83±99KBq/L for the 8 first cycles and 50±59KBq/L for the 4 last ones. This system entailed a storage time to 31±11 days (median: 26 days), secondary to the presence of the only ¹⁷⁷Lu. In all cases, ^177mLu residual activity was below the legal release limits. Of course, solid and liquid filter cartridges that contain the ^177mLu need to be stored for much longer, but the storage is greatly facilitated by the solid state and low volume of the waste.

Conclusions: This innovative filtration process results in filtered water that can be discharged within a reasonable period of storage time. Further improvements are being made to this first prototype. Additional studies are under way, but the initial results tend to indicate that this system may greatly increase the availability of the treatment to all patients.

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