The field used megavoltage photons with bolus. Orthovoltage was not used because the field is too large and there is no indication of a junction. Electrons were not used as the reaction extends over an area with more than 30% obliquity.
The bolus resulted from the need to provide a high skin dose over the whole field. Given that this treatment is in the leg the likely clinical explanation is that there was a sarcoma here and post-operatively the skin has to get full dose.
This is a clinical example of the pig skin clonogen assay. An annulus of skin was obliterated to a very high dose to prevent epithelial migration, and then the central portion received its test dose. Following this the clonogens regrowing were counted and correlated with radiation effects.