Large cystic lesion is demonstrated suprasellar region distorting the floor of the third ventricle, interpeduncular cistern and the midbrain. The cerebral aqueduct is distorted and the lateral ventricles are large for age. No periventricular oedema to suggest transependymal CSF spread. There is indentation on the posterior aspect of the optic chiasm and splaying of the optic tracts. No associated enhancing or nodular components. The contents of the cystic lesion follows CSF signal on all sequences including DWI/ADC. No perceptible wall of the lesion.
The pituitary and infundibulum are normal. No pituitary lesion. The remainder of the imaged brain (please note this study has been targeted to the pituitary, and as such the whole brain has not been imaged) appears unremarkable.
Conclusion: Suprasellar cystic lesion is almost certainly an arachnoid cyst, with localised mass effect as described.