Nelson's syndrome is most likely seen after -

Adrenalectomy performed by an expe surgeon effectively cures ACTH-dependent Cushing's syndrome, but in patients with pituitary-dependent Cushing's syndrome this can result in Nelson's syndrome. In Nelson's syndrome, the loss of negative feedback from endogenous coisol results in growth of the pituitary tumour, often leading to an invasive pituitary macroadenoma (which causes local mass effects) and very high ACTH levels (which cause pigmentation). The risk of Nelson's syndrome is repoed as being reduced by pituitary irradiation in some series, but not all . Ref - Davidson's medicine 23e p669