A cyst is never cancerous - it is by definition a fluid filled lesion.
However, I would get a copy of the report of the MRI so you can read just what is on her pituitary.
Why are you going to see a neurologist? A neurologist does not treat pituitary tumors. cysts, or lesions of any type. Please go ahead and cancel that appointment as it will be full of nothing. Instead, make an appointment with a neuro-endocrinologist who can actually test your daughter for any hormonal abnormalities that have occurred as a result of this cyst and if need be, send her on to a proper surgeon who is experienced in pituitary lesions so it can be removed. Even if you are sent to a surgeon by the endo, ask a lot of questions about experience as you want a lot of it.
Pituitary lesions (I had two) are generally not cancerous but the effects from the hormones can be where the symptoms come from and sometimes but not always those effects vary in importance so she needs to see a proper doctor.
Growth hormone is often effected and at her age, that would be a huge effect on growth and development.
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