These are part of a medical radiation treatment system. Here is a link to a patent diagram of a similar assembly. http://patentimages.storage.googleapis.com/US7997553B2/US07997553-20110816-D00000.png
This sits at the end of the beamline, and allows the aiming/positioning of the beam in the right location.
Here’s a pic in situ. 
Edit: Added more info
