Why is 1 mm Pb foil placed in the beam path during photon beam calibration?

The placement of 1 mm Pb (lead) foil in the beam path during photon beam calibration is primarily to introduce known electron contamination. When a photon beam interacts with any material, it can produce secondary electrons due to the photoelectric effect and Compton scattering. The lead foil serves to generate a controlled amount of electron contamination which can then be accounted for in the calibration process.

By adding a specific thickness of lead, dosimetrists can quantify the amount of secondary electrons being produced, which is essential for accurate dose delivery calculations. Understanding and managing electron contamination helps ensure that the photon beam is properly calibrated and that the measured doses at particular points will reflect what will be delivered in clinical settings.

The other options do not align with the primary use of the lead foil during calibration. For example, while improving beam uniformity and reducing patient exposure are important considerations in radiation therapy, they are not the primary purposes of placing lead foil in the beam path during calibration. Stabilizing photon energy is also not a function of the lead foil; rather, photon energy is determined by the x-ray generator settings and the materials the beam interacts with.