94 G. Yunwen: Radioprotection 2025, 60( 1), 91 – 98
4.2 Lutecium-177
Similar to 131 I, most of the 177 Lu injected into the human body is excreted through the patient’ s urine and other excreta( ICRP, 2004; Zaknun et al., 2013). According to available environmental impact assessment documents, hospitals planning to carry out 177 Lu treatments generally administer the medication to patients on a specific day of the week, with patients receiving the injection in the morning and leaving in the evening, roughly resulting in a hospital stay of 6∼9 hours. During their stay, the excreta must enter the decay tank.
The average dose administered to a single patient is denoted as A, and the number of patients is denoted as N. The proportion of 177 Lu excreted by the patient during their stay in the hospital is denoted as P. The value of P varies across studies. Levart et al. found that the retention rate of 177 Lu in the body 18.1 hours post-administration of 177 Lu-DOTATATE was approximately 36 %( Levart et al., 2019). Esser JP’ s team measured that approximately 71 % and 81 % of the administered dose of 177 Lu-DOTATATE or 177 Lu-DOTATOC, respectively, was excreted in urine within 24 hours( Esseret al., 2006). Kwekkeboom et al. indicated that about 64 % of 177 Lu was excreted in urine within the first 24 hours following the administration of 177 Lu-DOTATATE( Kwekkeboom et al., 2001). Referring to the above results and the conservative principle, it is assumed that 50 % of 177 Lu is excreted during the patient’ s stay, i. e., P = 0.5.
Assuming the decay tank fills in T = 7n þ 1 days( the excreta of a new batch of patients enters the decay tank, resulting in higher activity levels than any other day in the 7- day cycle). Under this assumption, the total activity of 177 Lu in the decay tank when it is full, denoted as D, is:
D ¼ Xn j¼1
PNAexp ½ �lð7j � 6ÞŠþPNA
¼ Pel e 7l � 1 NAð1 � e�lT ÞþPNA ¼ kNA
¼ 1:02NAð1 � e �lT Þ ð10Þ where l is the decay constant of 177 Lu.
In most cases, T ≥ 81 days, and 1 – exp(– lT) can be approximated as 1. Therefore, the total activity of 177 Lu in the tank can also be simplified to kNA. When T = 7n þ 1, the value of k is 1.02. If T = 7n þ 1 þ g, the total activity of 177 Lu in the tank can be considered as the activity at 7n þ 1 days, further decayed by g days, that is kNA exp(– lg).
5 Theoretical minimum storage days and required volume of decay tank
According to China’ s regulatory requirements, the hospital needs to conduct an EIA and obtain approval from regulatory authorities before constructing a nuclear medicine department. The EIA document must determine the volume of the decay tank. The volume varies significantly, influenced by factors such as patient load, location, and notably, the year of the hospital’ s establishment. Generally, hospitals founded prior to 2021 are equipped with 2 to 4 decay tanks, with individual tank volumes varying between 20 to 80 cubic meters. After the Radiation protection and safety requirements for nuclear medicine( HJ 1188)( Ministry of Ecology and Environment, 2021) was issued in 2021, the newly-built hospitals usually determine the volume of tanks based on a storage period of 180 days. As a result, the volume of decay tanks increases significantly. The hospitals typically possess 3 to 5 decay tanks, with tank volumes ranging from 60 to 120 cubic meters.
Utilizing the radioactivity levels within the decay tank as a foundation, this chapter presents a theoretical framework for calculating the minimum storage period and the requisite tank volume. Using the new framework, the required volume of the decay tanks can be significantly reduced while ensuring compliance with regulatory requirements.
5.1 Theoretical minimum decay days for 131 I
Since the total activity specified in Section 8.6.2 of GB 18871 is the practical restriction for the discharge of 131 I, Let E m represent the monthly allowable discharged activity( 9 10 6 Bq) and T min represents the theoretical minimum decay days for 131 I. T min should satisfy:
Thus
De �lT min
¼ E m
T min ¼ lnðD = E mÞ l
The volume of the decay tank, V, should satisfy: V ≥ W 1T min
M � 1 ð11Þ
ð12Þ ð13Þ where M represents the number of decay tanks, W 1 represents the daily liquid effluent volume discharged into a decay tank. When equation( 13) is taken as equality, the volume V obtained is the minimum required volume of the decay tanks. Table 1 shows the minimum required volume of the decay tank for different numbers of patients, assuming an average dose of 7.4 GBq per patient, a hospitalization cycle of 3 days, and 3 decay tanks.
5.2 Theoretical minimum decay days for Lutecium- 177
Since the specific activity of 177 Lu at the outlet of the decay tank must not exceed 10 Bq / L, Similarly, let T min represent the theoretical minimum decay days for 177 Lu. T min should satisfy:
kNA V e�lT min
¼ 10Bq = L ¼ 10 3 Bq = m 3 ð14Þ
where kNA represents the activity when the decay tank is just filled, V is the volume of the decay tank. kNA / V is the specific activity of 177 Lu. Equation( 14) means that if the liquid effluent has been stored for T min, the specific activity of 177 Lu complies with discharge criteria.