How is the committed dose to an organ calculated from a radionuclide intake?

• A. Sum of activities in all organs.
• B. Measure instantaneous dose rate to the organ.
• C. Use biokinetic models to compute time-integrated organ dose (H_T) and sum across tissues with tissue weighting factors for effective dose.
• D. Multiply intake by energy per decay.

Answer: (C)

When a radionuclide is taken in, its distribution and how quickly it clears from each organ determine how much energy is deposited over time. To get the committed dose to a particular organ, you first use biokinetic models to describe the time course of activity in that organ after intake. From this, you calculate the time-integrated activity (the total number of decays that will occur in that organ over time). That accumulated activity is then converted into dose using tissue-specific dose factors (S values), giving the organ’s committed dose (H_T). If you’re assessing overall risk, you sum these organ doses across tissues with the appropriate tissue weighting factors to obtain the committed effective dose.

Why this approach matters: simply summing activities across organs or using only an instantaneous dose rate ignores how dose builds up over time and how energy from decays is actually deposited in tissues. And multiplying intake by energy per decay skips how much energy actually reaches and stays in a given organ due to distribution and biological clearance. Using biokinetic models with time-integrated activity and S values provides a time-aware, organ-specific dose that can be combined into an overall effective dose.