Surface dose for megavoltage photon beams outside the treatment field

Measurements made on photon beams from four different radiotherapy machines have demonstrated that skin dose several centimeters outside the boundary of a treatment field may be as much as 20% of the central axis maximum dose. This surface dose has been measured for an AECL Theratron 80, Siemens Mevatron VI, Varian Clinac 20, and CGR Sagittaire for distances up to 12 cm outside the field boundary and for depths up to the depth of maximum central axis dose. This dose has also been measured as a function of field size and of source-to-skin distance. For the lower energy photon beams, this radiation is significantly attenuated in the first 2-3 mm of tissue, while for higher energy beams, a buildup phenomenon with a dmax of 2-3 mm is observed. The magnitude of this radiation is approximately linearly dependent upon field dimension for all energies.