  The Radiotherapy treatment planning optimization process based on a
quasi-Newton algorithm with an object function containing dose-volume
constraints is not guaranteed to converge when the dose value in the
dose-volume constraint is a critical value of the dose distribution. This is
caused by finite differentiability of the dose-volume histogram at such values.
A closer look near such values reveals that convergence is most likely not at
stake, but it might be slowed down.
