Why is a trimmer bar attached to the collimator?

A trimmer bar is attached to the collimator primarily to reduce the effect of penumbra, which is the region at the edge of the radiation beam where the dose falls off rapidly. The penumbra effect can create uncertainties in the dose delivered to the target and surrounding tissues, particularly in treatments where precise dose allocation is critical, such as in tumor irradiation.

By utilizing a trimmer bar, which can adjust the beam margins more precisely, the transition between the therapeutic radiation dose and the non-irradiated area is managed more effectively. This leads to a sharper dose gradient at the edges of the treatment field, helping to improve the uniformity of the absorbed dose within the treated volume and preserving surrounding healthy tissue from unintended radiation exposure.

In summary, the correct answer highlights the importance of the trimmer bar in enhancing the precision of radiation delivery, significantly mitigating the penumbra effect that could compromise treatment efficacy and safety.