  This work studies the impact of systematic uncertainties associated to
interaction cross sections on depth dose curves determined by Monte Carlo
simulations. The corresponding sensitivity factors are quantified by changing
cross sections in a given amount and determining the variation in the dose. The
influence of total cross sections for all particles, photons and only for
Compton scattering is addressed. The PENELOPE code was used in all simulations.
It was found that photon cross section sensitivity factors depend on depth. In
addition, they are positive and negative for depths below and above an
equilibrium depth, respectively. At this depth, sensitivity factors are null.
The equilibrium depths found in this work agree very well with the mean free
path of the corresponding incident photon energy. Using the sensitivity factors
reported here, it is possible to estimate the impact of photon cross section
uncertainties on the uncertainty of Monte Carlo-determined depth dose curves.
