G. Yunwen: Radioprotection 2025, 60( 1), 91 – 98 95
Table 1. Minimum storage time and volume of decay tank required for 131 I. Number of patients Total activity stored( GBq) Minimum storage time( days) Volume of decay tank required( m 3)
5 |
116.9 |
109.3 |
30.1 |
6 |
140.3 |
111.4 |
36.8 |
7 |
163.7 |
113.2 |
43.6 |
8 |
187.1 |
114.8 |
50.5 |
9 |
210.5 |
116.1 |
57.5 |
10 |
233.8 |
117.3 |
64.5 |
15 |
350.8 |
122.0 |
100.7 |
Table 2. Minimum storage time and volume of decay tank required for 177 Lu.
Number of patients
Assuming the number of decay tanks is M and the daily liquid effluent volume discharged into a decay tank is W 1, the volume V of a single decay tank should satisfy:
V ≥ W 1T min M � 1 ð15Þ
When equation( 15) is taken as equality, V obtained is the minimum required volume of the decay tank. W 1 is roughly proportional to the number of patients, so W 1 can be expressed as W 0 N, where W 0 represents the daily water consumption per patient, and N is the number of patients per batch. Equation( 15) can be rewritten as:
This leads to:
Hospitalization time( hours)
Minimum storage time( days)
5 |
|
|
31.3 |
6 |
|
|
37.5 |
7 |
|
|
43.8 |
8 |
6∼9 |
113.6 |
50.0 |
9 |
|
|
56.3 |
10 |
|
|
62.5 |
15 |
|
|
93.8 |
V ¼ W 1T min M � 1 ¼ W 0NT min
M � 1
T min ¼ ðM � 1ÞV W 0 N
Substituting equation( 17) into equation( 14):
kNA V exp �l ðM � 1ÞV
¼ 10000
W 0 N
Let X = V / N, and substitute it into equation( 18): kA lX M � 1 exp � ð Þ
¼ 10000 X
W 0
Volume of decay tank required( m 3)
ð16Þ
ð17Þ
ð18Þ
ð19Þ
Assuming W 0 is 0.11 m 3 and the number of decay tanks M = 3, solving equation( 19) gives X = V / N = 6.25 m 3 / person, meaning the volume of the decay tank required per patient is 6.25 m 3; the corresponding minimum storage time is 113.6 days. Table 2 shows the required volume of the decay tank for different numbers of patients.
5.3 Minimum storage time considering both 177 Lu and 131 I exists in the decay tank
If a hospital uses both 131 I and 177 Lu for treatment, the storage time for liquid effluent should be the larger value between the minimum storage time for 131 I and 177 Lu. From Tables 1 and 2, it can be seen that under the assumed circumstance, the minimum storage time required for 177 Lu is slightly larger when the number of patients is 9. When the number of patients is ≥10, the volume required for 131 I exceeds that for 177 Lu.
5.4 Actual measurement results
Currently, nuclear medicine departments in China’ s hospitals mainly conduct 131 I treatment, while 177 Lu is still in clinical trial stages. Therefore, it is difficult to obtain monitoring data for 177 Lu, so the actual measurement results mainly focus on 131 I.
Four hospitals were selected, and the activity concentration of 131 I in decay tanks when full was monitored. The results are shown in Table 3. From Table 3, it can be seen that:
1 The actual activity of 131 I in the decay tanks when full is lower than the theoretically calculated value;
2 In the case of operation at full capacity( hospital No. 3), the actual measured value is close to the theoretically calculated value;
3 Based on the methods presented in this article, the monthly discharged activity of 131 I from the four hospitals is below the discharge limit, complying with regulatory requirements.
6 Discussion and analysis
According to the theoretical activity formula for 131 I and 177 Lu in a full decay tank, the total activity is proportional to the number of patients and the dosage per patient.