At 1546hrs Dorchester Central dispatched units for the MVC with rollover in front of DARK HORSE ANTIQUES @ 2923 OCEAN GTWY in the Cambridge 1-4 Box Area. Chief 1 arrived on location with A 4 vehicle MVC, 1 on it's roof with entrapment and established command. Units (Rescue 1-1 & Rescue Engine 1-1) arrived on location and formed the rescue group to stabilize & cut 1 patient out of the vehicle on it's roof. Crews then worked with EMS to treat all additional patients, clean up debris, and assist with traffic control. DCEMS with A MCI (Mass Causality Incident) declared obtained multiple patient refusals and transported 1 patient Priority 2 to PRMC. Command was terminated (leaving the scene to MSP) at 1649hrs and all units were placed in service without incident. Rt50 West was shut down for approx. 90mins for the operation.