Tissue depth to lung for electron beam boost therapy of the breast

Radiation therapy of the breast frequently employs electron beam boost therapy of the tumor bed. The electron energy is typically chosen based on the location of the tumor and tissue depth to lung within the electron field. This paper proposes a simple technique to estimate the tissue depth to lung using a port film taken orthogonal to the electron beam axis and patient axis for arbitrary electron beam gantry angles and patient table angles. The port film is taken with the patient in standard position (table angle of 0 degrees) and the gantry at right angles to the electron field axis, clearly showing the depth to lung. The mathematical solution for arbitrary electron field gantry angle and patient table angle is presented.