Matching photon and electron fields with dynamic intensity modulation

A technique was developed to reduce the size and magnitude of the hot and c old spots in the abutting regions of photon and electron fields. The photon and electron fields were set up such that the photon field extended simila r to 2 cm into the electron held in the abutting region. The region of the photon beam that overlapped the electron field was modulated using a multil eaf collimator, effectively broadening the photon penumbra to make it compl imentary to the electron penumbra. The computer calculations were verified using him measurements for abutting a 6 MV photon beam with a 9 MeV electro n beam. A uniform dose was achieved at a prespecified depth of 2 cm, and do se uniformity was improved at the specified depth and beyond compared with unmodulated photon beams. A slight increase in dose inhomogeneity was seen at shallower depths. The overall areas of the hot and cold spots were signi ficantly reduced. The technique also reduced the sensitivity of dose homoge neity to setup errors such that the magnitudes of the hot and cold spots we re about half of those produced with unmodulated photon beam when an overla p or gap of 4 mm was introduced. The technique was applied to the treatment of a head and neck cancer and a lymphoma involving the right pleura with m arkedly reduced dose inhomogeneity in the abutting regions. (C) 1999 Americ an Association of Physicists in Medicine. [S0094-2405(99)03211-3].