A Boston police officer was struck by a vehicle Tuesday night in Dorchester, police said. David Estrada, a Boston police spokesman, said the officer was struck near Massachusetts Avenue and Allstate Road at about 10:20 p.m. The officer suffered a knee injury and was taken to Tufts Medical Center in Boston, Estrada said. He had no word on whether the driver was cited or charged. It was also unclear what the officer, who was on duty, was doing when he was hit, police said.