Because most chemotherapeutic agents have no effect
on the pituitary itself, they do not inhibit ACTH secretion,
which may actually increase with therapy.This phenomenon,
known as Nelson’s syndrome, has been well
documented in humans with pituitary tumors treated
with bilateral adrenalectomy. Nelson’s syndrome refers
to rapid enlargement of a pituitary mass that occurs after
loss of negative feedback from adrenal cortisol production,
which has an inhibitory effect on ACTH release.
While this phenomenon has been suggested to occur in
dogs, studies have shown no correlation between
treatment with mitotane and pituitary size or rate of
pituitary tumor growth. However, because the exact
mechanism behind the development of Nelson’s syndrome
is not completely understood, the potential for tumor expansion
as a result of adrenal corticolysis or decreased cortisol production seems plausible.