Where must the beam output calibration for a clinical photon beam be performed?

The beam output calibration for a clinical photon beam must be performed in a water phantom for a 10 cm x 10 cm field because this setup closely simulates the conditions under which radiation therapy is delivered to patients.

Water phantoms are used because water has similar dosimetric properties to human tissue, which allows for accurate measurement and calibration of the treatment beams. The 10 cm x 10 cm field size is a standard reference field area commonly used in dosimetry, ensuring consistency and comparability of measurements across different treatment planning systems and clinical scenarios.

By calibrating in a water phantom, dosimetrists can account for factors such as scatter, dose distribution, and attenuation, which are critical for accurately determining the necessary dose to achieve the desired therapeutic outcome while minimizing exposure to surrounding healthy tissue. Using this method increases the reliability and safety of radiation therapy treatments.