LITTLETON, Mass.--(BUSINESS WIRE)--Mevion Medical Systems, a global leader in compact proton therapy, today announced that the world’s first clinical proton therapy treatment delivered inside an existing conventional radiation therapy vault has been completed at Stanford Medicine Cancer Center. The treatment was delivered on the MEVION S250-FIT Proton Therapy SystemTM. The milestone establishes a new category of proton therapy delivery: a compact single-room system that fits within existing radi