Which expression correctly represents the equivalent dose to a tissue?

• A. E = Σ w_T H_T
• B. H_T = E × w_R
• C. H_T = D / w_R
• D. H_T = D × w_R

Answer: (D)

Biologically meaningful dose to tissue comes from adjusting the energy actually absorbed by the tissue (the absorbed dose) with a radiation-specific factor that reflects how damaging that type of radiation is. This weighting factor, w_R, scales the absorbed dose D to give the equivalent dose H_T. So the correct relation is H_T = D × w_R.

If the radiation is X-rays or gamma rays, w_R is 1, so the equivalent dose equals the absorbed dose. For more damaging radiation like alpha particles, w_R is much larger (around 20), making the equivalent dose much bigger than the absorbed dose and signaling greater biological effect. In a mixed radiation field, you’d sum contributions from each radiation type: H_T = Σ_R D_R × w_R, but the core idea remains multiplying the absorbed dose by the appropriate weighting factor.

The other expressions mix in effective dose or divide by the weighting factor, which doesn’t reflect how the biological effect scales with radiation type.