The beam output (cGy/min) for a 60Co unit increases with larger field sizes primarily due to what?

The increase in beam output for a 60Co unit with larger field sizes is primarily due to collimator scatter. As the field size increases, there is a greater volume of the beam interacting with the collimator, which can scatter radiation back into the treatment field. This scattering effect enhances the dose delivered to the patient in the treatment area.

Collimator scatter becomes significant because it contributes additional radiation from the collimator structure itself, which is not counted in the primary beam but still contributes to the overall dose received by the patient. Larger field sizes allow for more collimator surfaces to contribute to this scatter effect.

While patient interaction, beam energy, and air attenuation play roles in radiation therapy, they do not primarily account for the increase in beam output with larger field sizes in the context of 60Co units. Patient interaction refers to how the radiation interacts with the tissue, which comes into play after the radiation exits the machine. Beam energy is a fixed characteristic of the unit and does not change with field size. Air attenuation involves the scattering and absorption of the beam by air, but this does not have the same significant impact on beam output as collimator scatter does when considering larger field sizes.