Why is a high beam nonuniformity ratio undesirable in therapy?

• A. It has no effect on adverse effects
• B. It reduces dose to target tissue
• C. It improves dose uniformity
• D. It produces a less uniform beam, increasing risk of periosteal pain or hot spots

Answer: (D)

A uniform dose across the treatment field is essential in radiotherapy because you want every part of the target to receive the prescribed amount while minimizing exposure to surrounding tissues. A high beam nonuniformity ratio means the beam delivers varying intensities across the field, creating hot spots where the dose is higher than intended and sometimes cold spots where it is lower. Those hot spots increase the risk of tissue damage, especially near bone where the periosteum can be irritated, leading to periosteal pain and other radiation-induced effects. Because of these dose spikes, overall dose distribution becomes less predictable and less safe. This is why a high BNUR is undesirable: it produces a less uniform beam, increasing the risk of periosteal pain or hot spots.