The potential for increased sparing of normal tissue in parallel opposed techniques with a multileaf collimator

A multileaf collimator (MLC) can be used in parallel opposed techniques as a direct replacement for standard-shaped beam blocks. However, improved shielding is possible if the MLC field is designed to fit a target rather than to mimic a straight-edged block. This study has compared the treatment areas produced by the MLC and by conventionally blocked fields with the target area for 43 parallel opposed treatments. It was found in every case that the MLC treated less than 10% excess tissue, and, in over 70% of patients, the excess was less than 5%. The conventional fields, however, treated more than 10% excess tissue in 70% of patients. The effect of MLC orientation and the benefits of using an MLC are discussed.