A patient received an absorbed dose of 10 milligray (mGy) to the lungs with a tissue-weighting factor of 0.12. What is the resulting effective dose (EfD)?

• A. 0.12 millisieverts (mSv)
• B. 1.0 millisieverts (mSv)
• C. 83.3 millisieverts (mSv)
• D. 1.2 millisieverts (mSv)

Answer: (D)

The key idea is that the effective dose accounts for how sensitive a particular tissue is to radiation by weighting the absorbed dose with a tissue-weighting factor. For diagnosing X-ray exposure to the lungs, the tissue-weighting factor is 0.12. The absorbed dose is 10 mGy. Since for X-rays the radiation weighting factor is 1, the equivalent dose to the lung tissue is still 10 mGy. Multiply by the tissue-weighting factor: 10 mGy × 0.12 = 1.2 mGy-equivalents. Converting to sieverts (and thus to millisieverts for this small dose) uses the fact that for photons 1 Gy equals 1 Sv, so 1.2 mGy equals 1.2 mSv. Therefore the resulting EfD is 1.2 mSv.