Radiation Safety Considerations of Household Waste Disposal After Release of Patients Who Have Received [¹⁷⁷Lu]Lu-PSMA-617

OPTION A: DECAY IN STORAGE

For the scenario in which the patient is instructed to retain waste in the home for decay in storage, the maximally exposed member of the public is likely to be a member of the patient’s household. Cumulative radiation exposure (D) to a household member can be estimated as follows: Eq. 1where Γ is the exposure rate constant for ¹⁷⁷Lu (7.6 μSv m²/GBq h [0.028 mrem m²/mCi h]); is the activity in the patient as a function of time; is the occupancy factor of an individual relative to the patient, that is, the fraction of time spent near the patient; is the distance (m) between the household member and the patient; is waste activity stored in the house as a function time; is occupancy factor relative to the waste, that is, the fraction of time spent near the waste; and is the distance between the household member and the waste.

is approximated by , where is the administered activity (typically 7.4 GBq] 200 mCi]), and are the fractions of activity administered in the early and late elimination phases ( and ), and and are the elimination rate constants ( h⁻¹ and h⁻¹) (2).

Evaluation of the integral in Equation 1 therefore yields the following: Eq. 2where W is the fraction of activity excreted in the urine that ends up as contaminated solid waste, is the total activity excreted by the patient, and is the physical decay constant for ¹⁷⁷Lu (0.004345 h⁻¹).

It can be shown that is calculated as follows: Eq. 3

This formulation implies that 92.3% of the injected activity is excreted, on average, and 7.7% decays in vivo.

Substituting the result of Equation 3 into Equation 2, and using some reasonable assumptions for occupancy and distance factors (;  = 1 m;  = 1.00;  = 3 m), the estimated exposure to a household member after release of a patient treated with 7.4 GBq (200 mCi) of [¹⁷⁷Lu]Lu-PSMA-617 is 249 μSv (24.9 mrem) from activity in the patient and 330 μSv (33.0 mrem) from activity in the waste, resulting in a total estimated exposure of 579 μSv (57.9 mrem) per treatment.

OPTION B: IMMEDIATE WASTE DISPOSAL

For the situation in which a patient with urinary incontinence is instructed to dispose of solid radioactive waste in the ordinary household waste stream with no delay, the radiation exposure to the sanitation worker who collects and transports the waste is of primary relevance. If one assumes that there is, on average, 24 h between waste creation and waste collection, the portion of administered activity that is collected by the sanitation worker is given by , or approximately 83.1% of the administered activity (). Conservatively assuming that the household member is the individual who transports the waste outside the home for collection, taking 1 min to do so, and keeping all other considerations the same as above, the household member is expected to receive 36 μSv (3.6 mrem) from activity in the waste per treatment.

Assuming negligible radioactive decay during waste transport and an occupancy factor of 1, the radiation exposure to the sanitation worker can be expressed as Eq. 4where is the time (h) required to manually collect and empty the waste container into the sanitation truck, is the distance (m) to the waste during container collection and emptying, is the time (h) required to transport the waste to a waste facility, is the distance (m) to the waste during transport, and α is the radiation transmission factor through the adjacent waste and the truck during transport.

The radiation transmission factor (α) can be conservatively estimated using Equation 5 for the approximate steel thickness of a sanitation truck wall (x = ∼0.476 cm [³/₁₆ in]), the mass attenuation coefficient of elemental iron for 208-keV photons ( = 0.143 cm²/g) (3), and the appropriate scatter build-up factor for the relevant energy and number of mean free pathlengths (B = 1.28) (4). Eq. 5

This transmission estimate is conservative, as it ignores obliquity with respect to the ¹⁷⁷Lu γ-rays striking the truck wall, attenuation of lower-energy ¹⁷⁷Lu emissions (i.e., 113 keV), and attenuation by waste within the truck.

Some assumptions may be made regarding the time required to collect and transport the waste to a local municipal facility. Collection and transport times ( and ) may be estimated as 30 s (0.0083 h) and 4 h, respectively. Distances from the waste during collection and transport ( and ) may be estimated as 0.25 m and 2.0 m, respectively. Evaluating Equation 4 using these values yields a sanitation worker exposure estimate of 10.3 μSv (1.03 mrem) per patient treatment (assuming  = 7.4 GBq [200 mCi]) or approximately 61.7 μSv (6.2 mrem) for a total of 6 treatments (A_total = 44.4 GBq [1,200 mCi]).

It is possible that a sanitation worker may provide services to multiple households that have patients undergoing treatment with [¹⁷⁷Lu]Lu-PSMA-617. According to the U.S. Bureau of Labor Statistics, approximately 138,700 “refuse and recyclable material collectors” provided services to the approximately 122,354,219 households in the United States in 2021. This implies that each sanitation worker may provide services to 882 households; however, there may be overlap (i.e., waste collection and recycling collection may be provided separately for each household), therefore, to be conservative we can assume that each sanitation worker provides services to 3,000 households.

There are approximately 268,490 new cases of prostate cancer each year in the United States, approximately 5.6% of which (15,035) will present as, or progress to the point of being, metastatic prostate cancer (5). If we assume that all individuals who develop metastatic prostate cancer receive [¹⁷⁷Lu]Lu-PSMA-617 and that 24% have urinary incontinence (weighted average from Daugherty et al. (1)), this amounts to a total of 3,608 patients with incontinence from whom sanitation workers might collect waste annually. The per-household probability of having a patient undergoing treatment is therefore 7,818 of 122,354,219, or roughly P = 0.0029% per year. Based on the assumption that individual sanitation workers service 3,000 homes, the probability of encountering N patients in a given year is given by the following binomial probability: Eq. 6

A conservative estimate of the probability that a sanitation worker will provide services to N patients is therefore as follows: P(0) = 91.7%, P(1) = 7.98%, P(2) = 0.347%, P(3) = 0.010%, P(4) = 0.0002%, and P(5) = 0.000004%.

Based on the calculation result from Equation 4, to exceed the typical regulatory limit of 1 mSv (100 mrem) a sanitation worker would need to provide services to at least 16 homes containing patients receiving [¹⁷⁷Lu]Lu-PSMA-617 in a given year. The probability of this occurring is approximately given by Eq. 7

The number of sanitation workers who would be expected to exceed 100 mrem is therefore .

SUMMARY AND CONCLUSIONS

When a [¹⁷⁷Lu]Lu-PSMA-617 patient is incontinent, the expected excess effective dose to a member of the household is expected to be approximately 330 μSv (33 mrem) per 7.4 GBq (200 mCi) of treatment if waste is retained for decay within the household. By comparison, if the waste is disposed of in the normal household waste stream, the maximally exposed sanitation worker is expected to receive approximately 10.3 μSv (1.03 mrem) per 7.4 GBq (200 mCi) of treatment, and the household member exposure is reduced to 36 μSv (3.6 mrem). Therefore, disposal of solid contaminated waste in the normal waste stream results in approximately a 10-fold reduction in estimated household member exposure, with respect to the waste, with only a marginal increase in sanitation worker exposure relative to natural background radiation (∼8 μSv/d [∼0.8 mrem/d]).

The estimated exposures presented here are conservative, and true exposures are likely to be lower, both for decay in storage and for immediate household waste disposal. Household members are unlikely to spend 100% of their time at a distance of 3 m from the radioactive waste. A more conservative estimate might be 50% occupancy at a distance of 5 m, which would reduce exposure by a factor of approximately 5. In the case of a sanitation worker collecting waste, waste receptacle emptying is often automated (not performed by hand), and attenuation within the surrounding waste may be significant, likely more than 1 m of compacted waste, with a density of up to approximately 600 kg/m³. These factors have the potential to decrease sanitation worker exposure by a factor of more than approximately 6. Additionally, the calculations provided in this paper assume immediate release from medical care after [¹⁷⁷Lu]Lu-PSMA-617 administration, whereas many patients will void before release, thereby reducing the exposure estimates in both scenarios A and B by approximately 30%. (2) Although not considered in this work, it is also possible that the patient could be catheterized for several days after administration, allowing for urine discharge into the sewage system. Although feasible, and some practices may consider this option, catheterization increases infection risk and reduces patient comfort, thereby potentially reducing the overall quality of care.

Regardless of the conservative nature of these calculations, it seems clear that the pragmatic approach to [¹⁷⁷Lu]Lu-PSMA-617 solid-waste management is to instruct patients to contain the waste in sanitary trash bags and to dispose of contaminated waste in the standard household waste stream. This approach is expected to minimize radiation exposure to members of the public, and cumulative exposures are expected to be well below regulatory limits.

DISCLOSURE

No potential conflict of interest relevant to this article was reported.