A 7-year old patient p. its with headache, paralysis of upward gaze, loss of ilOtt perception and accommodation, nystagmus and failure of convergence. CT scan showed homogenous hyperdense lesion above the sella and in the posterior pa of the third ventricle. MRI showed that the lesions were homogenous and isointense on T1 weighted imaging, and isointense onT2 weighted imaging with intense contrast enhancement. The most likely diagnosis is: