What is the best treatment volume and dose for a marginal zone orbital lymphoma?

It would depend on the location- retrobulbar, conjunctival or lacrimal gland. Imaging, including MRI, helps in evaluating the site of origin and extent of involvement. Some advocate treating the entire orbit for all orbital lymphomas to be comprehensive, as the total dose is low, and this away we avoid geographical miss. But one can do conformal RT targeting the involved site only as long as imaging confirms this. For retrobulbar- just based on location, you end up treating most of the orbit. For primary conjunctival malt lymphoma one can treat with electron beam targeting the anterior structure only.