What is the approximate focal spot size of a KV x-ray therapy tube?

The approximate focal spot size of a kilovolt (kV) x-ray therapy tube is generally around 3 mm. This measurement is significant as it impacts the resolution and detail of the images produced in radiotherapy and diagnostic imaging. A larger focal spot can lead to a decrease in image sharpness and increased penumbra, which is the zone of partial opacity around the edges of a radiographic image. Therefore, the 3 mm size balances the need for sufficient penetration and dose delivery, while still allowing for adequate image quality. Understanding this focal spot size is crucial for medical dosimetrists, as it affects treatment planning and the precision of radiation delivery to tumors while sparing surrounding healthy tissue.