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Figure 13: A conceptual illustration indicating 3 regimes of outoffield dose. The highdose region (where effects are likely to be deterministic) is typically calculated well by modern treatment planning systems. The dose region several centimeters beyond the penumbra (where stochastic effects are dominant) is best calculated by Monte Carlo methods. Far from the primary field (where lowrisk stochastic effects are relevant), measurement of the leakage dose would best inform risk assessment  since it is relatively difficult to construct a Monte Carlo model that very accurately determines leakage dose at large distances 
