Derived CDG for bioassay – a tool for screening people in a radiological emergency

The decision on initiating clinical treatment for a person who has been internally contaminated by radionuclides is not trivial. NCRP Publication 161 presented an operational quantity named Clinical Decision Guide - CDG to help in this decision. The bases for establishing the CDG values are dose values related to the risks of stochastic and deterministic effects to body tissues. This work shows a computational system -  Derived CDG -  which calculates CDG and derived CDG applied to in vivo and in vitro radiobioassay for several age groups and for several times after intake by inhalation and ingestion. It is a tool to be used to screen people with internal contamination who need medical follow-up. The system enables the users to change parameters used in calculation for different simulated scenarios.


INTRODUCTION
Internal contamination of people may occur in a radiological emergency. In such situation, internally contaminated people receive a committed effective dose which is directly related to the time that incorporated radioactive material remains in the body. One possibility is to reduce the committed effective dose by carrying out decorporation therapies whose objective is to accelerate the excretion of incorporated radioactive material. Another possibility is the medical follow-up of the person who has been contaminated in order to observe the appearance of an effect so that the treatment can start immediately.
An issue that has been discussed for a long time is regarding dose threshold or intake level to The NCRP 161 [1] presented a new operational quantity called Clinical Decision Guide (CDG) that represents the activity incorporated (intake) by an adult individual that corresponds to threshold dose to which a clinical follow-up is recommended. It was also highlighted that the threshold dose could be modified according to the emergency conditions as number of people affected, treatment capability, etc.
Decorporation therapies enhance the excretion of radioactive material deposited into the body and can be recommended for people with a certain level of internal contamination. Thus, according to NCRP [1], decorporation therapies are recommended for incorporations of 1 to 2 CDGs; certainly carried out for incorporations greater than 2 CDGs; and, unusual for incorporations of ~ 0.2 CDGs [1].
The decorporation efficacy is greatest if treatment is immediately initiated after intake. So, tools for making decisions on decorporation therapies to be used in emergency situations should be developed in the response preparation phases [4].
Also, to be operational, the CDG calculated could be derived for different bioanalysis techniques applied for monitoring internally contaminated people, which is useful in emergency situations. The derived CDG is the activity concentration in excrete or organs of the body that corresponds to the dose threshold adopted to initiate clinical follow-up.
The objective of this work was to develop a computational application to calculate CDG and derived CDG for bioanalysis for individuals in different age groups: adults, 15 years of age, 10 years of age, 5 years of age, 1 year of age and 3 months of age.
This application allows the user to change parameters used in the calculation to simulate different scenarios.

MATERIALS AND METHODS
The theoretical base used in this work is the publication NCRP 161 [1] which presented the operational quantity CDG -Clinical Decision Guideto assist in the decision on clinical treatment of a person who has incorporated radioactive material. According to Equation 1, the CDG for adults is the minimum intake satisfying these dose constraints for both stochastic and deterministic effects, where:  [5,6].
A study to evaluate radionuclide concentration in excrete corresponding to CDG pointed out that the adoption of one fifth of adult value for other age groups, rises the dose values up to 3.5 for children in relation to the value defined for adults, mainly if ingestion is the incorporation pathway.
Therefore, it was suggested that CDG can be calculated for each age group specifically [5].
The adoption of one fifth of the adult CDG value for other age groups was also discussed in a study in which the objective was to calculate the concentration of activities in excreta for different possible scenarios involving radiological emergencies. As it was also verified that the dose for children would be much higher than for adults, the authors did not adopt for children one fifth of the CDG value of adults. However, they considered as reference the one fifth of the adult dose as a limit for calculating the CDG for other age groups [6].
According to the studies cited before, the system presented here considered for no adult individuals, the values of dose to calculate CDG as follows: • 0.05 Sv is the value of effective dose in 50 years for consideration of stochastic effects;  (2)

Application development
For each radionuclide of interest, the CDG value was calculated for age groups defined by ICRP Publication 72 [7]. The dose coefficients used to calculate the CDGs are those presented by the ICRP CD-1 [8]. In some situations, to change the value of the parameters used in the calculation of the CDG could be necessary. A specific form for changing parameter values allows the user to simulate different scenarios.

Results verification
To verify the results generated by this system, the CDG values presented in other publications [1,5] was calculated using the same parameters for each simulated scenario described.   a specific screen will be displayed for the user to make the desired changes and save them by clicking the Save Data button.

RESULTS AND DISCUSSION
After this step, click the Calculate CDG button to perform calculations. If the user has changed the value of the parameters, the system will require a name to save a file with the results.
To view the results, click the Result button and a screen with the results will be displayed, as shown in Figure 2. The CDG values are presented by age groups with the limiter, which is the smallest argument in the Equation 1 and 2. Figure 2 illustrates the calculation of CDG for an incorporation of Y-90 by inhalation, type M and AMAD of 1 m, which is the scenario: Y-90 / Inhalation / Type M / AMAD 1 m. It can be seen that the limiting factor for adults, 15 years of age and 10 years of age, is the RBE weighted absorbed dose in the lung; for other age groups, the committed effective dose is the limiting factor.  The Report presented in Figure 3 contains information on the parameters used by the system in the calculation of CDG and derived CDGs for bioanalysis.
In the age groups tables, the values of derived CDGs for bioanalysis are presented as a function of the time elapsed after the incorporation until 70 years of age. Figure 4 shows the Adult  The reliability of the calculations was assessed using the system presented here to reproduce CDG values found in other publications. Simulations were carried out to calculate the CDG values presented in two publications [1,5]. The comparison between the values generated by the system and those presented by the publications did not show significant variation, as can be seen in annexes I and II of this article.

CONCLUSION
The operational quantity CDG is an important contribution to the screening of people internally contaminated with radioactive material that need clinical monitoring. The dose levels established by NCRP 161 as a basis for calculating the CDG are the threshold from which the risk must be reduced through clinical treatment. However, it was recognized that depending on the characteristics of the emergency, for example, the number of people involved, these dose values may change.
The system for calculating derived CDG for bioanalysis described in this article presents the concentration of activity in each compartment of the biokinetic model or in the excreta that can be monitored through bioassay. The activity concentrations are presented in electronic spreadsheets, by age group according to the time elapsed after the incorporation. The purpose of generating these worksheets is that they are simple to consult in an emergency situation, since the time interval before the start of treatment influences the effectiveness of decorporation therapies.
In this way, the application to derived CDG calculation is a tool to be used for screening internally contaminated people in situations of radiological emergency, which allows the user to change the values of the parameters to simulate scenarios of interest.

ACKNOWLEDGMENT
The author is grateful to Dr. Luiz Bertelli for the discussions and suggestions that contributed for the development of the computational application presented in this article.