How can you reduce the geometric penumbra of a 60 Co unit?

Using a source of smaller diameter effectively reduces the geometric penumbra. The penumbra refers to the transition zone between full radiation exposure and the area that receives no radiation. In radiotherapy, a smaller source diameter results in a sharper dose gradient around the edges of the beam, thereby minimizing the width of the penumbra. This sharper gradient is advantageous because it improves the precision of dose delivery to the target volume while sparing surrounding healthy tissue.

When the source is smaller, the rays emitted from the source diverge over a shorter distance before reaching the treatment area, leading to a better-defined beam edge. This effect is crucial in treatments where precision is paramount, such as in targeting tumors that are closely adjacent to critical structures.

In contrast, using a larger source would create a broader penumbra, making it more challenging to confine the radiation dose to the target and increasing the risk of exposing surrounding healthy tissue to unnecessary radiation.