The output of a kilovoltage therapy unit is roughly proportional to which of the following?

The output of a kilovoltage therapy unit is roughly proportional to the square of the tube voltage (kV). This relationship stems from the physics of x-ray production, where the energy of the x-ray photons generated is directly influenced by the accelerating voltage applied across the x-ray tube.

When the kV is increased, the energy of the electrons being accelerated towards the target material also increases. Consequently, when these high-energy electrons collide with the target, they produce x-rays, and the quantity of x-rays (output) generated is proportional to the square of the accelerating voltage due to the increased energy available for photon production. Essentially, doubling the kV increases the energy of the x-ray photons and boosts the output significantly.

In considering the other options, the cubic relationship and linear relationships involving kV and atomic number (Z) do not reflect the fundamental principles governing x-ray production in kilovoltage therapy. The output does not scale directly with kV raised to the power of three nor is it directly proportional to the product of kV and the atomic number of the target material. Although the atomic number can influence the efficiency of x-ray production, it is typically the square of the voltage that primarily determines the output level in kilovoltage therapy units