The radiobiology of Papillon-type treatments.

The dominant influence in all types of contact therapy is physical rather than radiobiological in nature and is associated with the rapid fall-off of dose with increasing depth in tissue. Even at a depth of only 20 mm the dose has typically fallen to around 15% of the surface dose, meaning that the deeper (and, potentially, dose-limiting) structures are physically spared. Papillon treatments therefore owe their clinical success largely to this simple characteristic and the radiobiological issues, although of interest, might be dismissed as being of secondary importance. However, consideration of the associated radiobiology is useful as it provides a deeper insight into why Papillon-type treatments are effective and also helps to identify the circumstances in which more careful planning of treatment might be required. Some of the most relevant issues are discussed in this paper. The essential points are introduced in a qualitative manner and then followed by some quantitative assessments.