How would the presence of active rheumatoid arthritis on methotrexate impact your choice of fractionation for a patient with prostate cancer?

It depends on the dose of MTX.

For 7.5-10 mg weekly doses have not changed volume or dose but if on a higher dose of MTX then I would discuss with rhematologist for possible holding or decreasing dose of MTX (as in that range has known radiosensitizing effect)