How do you treat locally advanced medullary thyroid cancer with postoperative RT?

What dose and fractionation is appropriate for medullary thyroid carcinoma with extrathyroid extension, bilateral cervical lymph node levels 2-5 with extranodal extension, paratracheal node involvement with extranodal extension, and thymus involvement s/p excision with positive margins throughout? Gross unresected disease remains in the mediastinum levels 4, 7, and bilateral hilar lymph nodes. Also what is the appropriate literature for treatment guidance?