What happens to the penumbra of a 60 Cobalt beam with increasing source diameter?

The penumbra refers to the transition zone between the fully irradiated area and the fully shielded area in radiation therapy. It is important for determining the dose distribution around the edges of the radiation field.

When the source diameter of a radiation beam, such as one produced by a 60 Cobalt unit, increases, the penumbra tends to increase as well. This happens due to the geometric unsharpness associated with larger source sizes. As the diameter of the source becomes larger, the rays emitted from it diverge more at the edges, leading to a broader region where the radiation dose transitions from high to low.

The penumbra is influenced by the source size because a larger source produces a wider spread of radiation beams that contribute to the dose in the penumbral region, causing it to be less defined and more extended. As a result, the increased uncertainty in where the dose is delivered increases the width of the penumbra, making the edges of the treatment field less sharp.

Understanding this principle is critical in medical dosimetry, as it impacts treatment planning and the precision of radiation delivery. An effective penumbra control is vital for ensuring that surrounding healthy tissues are spared from excessive radiation while still adequately treating the target