A patient receives 10 mGy of absorbed dose to the thyroid from X-rays and 2 mGy to the gonads from neutrons. If the radiation weighting factor for X-rays is 1 and for neutrons is 20, what are the respective equivalent doses?

• A 10 mSv to thyroid and 40 mSv to gonads ✓
• B 10 mSv to thyroid and 2 mSv to gonads
• C 20 mSv to thyroid and 2 mSv to gonads
• D 10 mSv to thyroid and 20 mSv to gonads

Explanation

Equivalent dose (H) = Absorbed dose (D) × Radiation weighting factor (Wr). For X-rays (and gamma rays), Wr = 1; for neutrons, Wr = 5–20 (typically 20 for fast neutrons). Thyroid from X-rays: 10 mGy × 1 = 10 mSv. Gonads from neutrons: 2 mGy × 20 = 40 mSv. The higher the Wr, the greater the biological effectiveness per unit absorbed dose. Alpha particles have Wr = 20, protons = 2, beta/electrons = 1. The unit of equivalent dose is the Sievert (Sv); effective dose incorporates tissue weighting factors (Wt) to account for radiosensitivity of different organs.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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