I do not think median here qualifies a value (eg median height). Median in anatomy refers to midline structures.
https://www.imaios.com/en/e-anatomy/anatomical-structure/med...
Median plane refers to an antero-posterior sagittal plane that passes through the midline or the middle of the body and divides it into right and left equal halves. It is a.k.a. the midline sagittal plane.




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Note added at 6 hrs (2024-04-09 16:12:18 GMT)
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The fourth ventricle is a midline cavity located between the brainstem and the cerebellum (Matsushima, Rhoton, & Lenkey, 1982; Rhoton Jr, 2000)

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Note added at 6 hrs (2024-04-09 16:16:59 GMT)
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However brain structures can be displaced by e.g. a tumour, a bleed within the cranial cavity.
https://www.tandfonline.com/doi/pdf/10.3109/0001692560917082...
A tumour whose greater part is situated outside
the ventricular system generally causes a larger displacement of
the ventricles than one which is exclusively intraventricular, Eut there
are exception

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Note added at 6 hrs (2024-04-09 16:18:05 GMT)
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So here I think "median" indicates that the fourth ventricle is in its normal position, in the midline

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Note added at 1 day 6 mins (2024-04-10 09:53:31 GMT)
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sorry about the typo - my answer is - fourth ventricle in the midline, of normal volume