Epidemiology and pathogenesis of pituitary adenomas.

Pituitary tumorigenesis is characterized by initiation, implying spontaneous or acquired mutations and promotion, implying that tumour expansion is sustained by intrinsic or extrinsic promoting factors. Pituitary tumours have a doubling time of 100 to 700 days. Seventy per cent of cases occur in 30 to 50-year-old patients, but tumours with highest growth rate (prolactin and ACTH-secreting adenomas) are also encountered in patients < 20 years. Pituitary adenomas occur at a greater frequency in females but there is no vivo evidence for a direct role of sex hormones. Oestrogen can amplify tumour growth factors in pituitary cell lines. One-third of GH-secreting adenomas have elevated cAMP resting levels which appear to be caused by a somatic mutation in the Gs protein associated with the GHRH receptor. This maintains the adenylate cyclase system in a turned-on state. Patients with mutant Gs protein have higher GH levels, reduced GHRH response, elevated TRH response, good suppressibility by SRIH and smaller tumour size compared to other acromegalic patients. Hypothalamic releasing hormones may be able to sustain pituitary tumour development since some acromegalic patients with GHRH-secreting tumours may harbour a pituitary GH-secreting adenoma. A lack of inhibitory factors may also have a promoting role in tumour progression.