The Effects of the Shape and Size of the Clinical Target Volume on the Planning Target Volume Margin
Figure 3
The method for calculating the margins for random errors adds up the probability that point O is at any position of the PTV to obtain the probability that point O is inside the PTV.
When point O is at position B, C, or D, part (position B and D) or even all (position C) of the CTV is outside the PTV.