If the distance to the patient is tripled, by how much does the dose reduce?

If the distance to the patient is tripled, by how much does the dose reduce?

• A. 2 times
• B. 3 times
• C. 9 times
• D. 27 times

Answer: (C)

The distance-dose relationship follows the inverse square law: dose is proportional to 1/d^2. If you triple the distance, the dose becomes 1/(3^2) = 1/9 of its original value. So the dose is reduced by a factor of nine, i.e., it becomes nine times smaller. The other numbers would come from different relationships (for example, doubling distance would give a fourfold reduction; a cubic relationship would give a 27-fold change), which aren’t applicable here.

The distance-dose relationship follows the inverse square law: dose is proportional to 1/d^2. If you triple the distance, the dose becomes 1/(3^2) = 1/9 of its original value. So the dose is reduced by a factor of nine, i.e., it becomes nine times smaller. The other numbers would come from different relationships (for example, doubling distance would give a fourfold reduction; a cubic relationship would give a 27-fold change), which aren’t applicable here.