Dosimetric comparison of centered and off-centered posterior neck electron fields

Approximately rectangular low or medium energy electron fields at extended SSD are often used to boost over the spinal cord in the treatment of head and neck cancer after cord tolerance is reached. A separate abutting photon field is used to continue treatment anterior to the spine. Typically, the electron and photon fields have different central axes and the electron cutout is symmetrically centered in the cone. However, a good match between the photon and electron fields is achieved more readily if the central axis of the electron field is located at the center of the area treated by the photon field. This displaces the electron cutout toward the edge of the cone. We measured and compared the percent depth dose (PDD), output factors and profiles for matched pairs of centered and off-centered rectangular and square cutouts for the 6, 9 and 12 MeV beams of three Varian linacs. The 10 x 10 and 15 x 15 cones were used at SSDs of 100 cm and 110 cm. Differences between centered and off-centered cutouts of the same dimensions were less than 1% for PDD and profiles and less than 5% for output factors. Therefore, the same central axis can be used for abutting photon and electron fields, without requiring extra dosimetric data to account for the off-center location of the electron cutout.