Explain the inverse square law and its practical implication for patient dose in radiography.

• A. Radiation intensity increases with distance; moving closer increases dose.
• B. Radiation intensity is constant with distance.
• C. Radiation intensity decreases with the square of the distance from the source; increasing distance reduces dose dramatically.
• D. Distance does not affect dose when using shielding.

Answer: (C)

The inverse square law shows that radiation intensity from a point source falls off with the square of the distance from the source. In radiography, the X-ray tube acts as that source, so the dose at the patient’s skin is tied to how far the skin is from the tube. Because the beam spreads as it travels, increasing the source-to-skin distance dramatically reduces the dose: doubling the distance drops the dose to about a quarter, tripling to about one-ninth, and so on. This is why using a longer distance is a powerful way to lower patient exposure, provided image quality can still be maintained with the adjusted technique. In practice, it’s an idealized relationship—the real setup includes finite focal spot size, scattering, and tissue attenuation—but the key idea remains: more distance means much less dose; less distance means dose increases quickly.