An apparatus for use in carrying out a prophylactic or treatment procedure on tissue comprises a head piece 1 having a receiver 2 for receiving an instrument such as an endoscope 10 and chamber 3 into which tissue is drawn by vacuum. There are two electrodes 11 within the chamber 2 and two electrodes 25 located on opposite sides of the entrance 5 into the chamber. The voltage applied to the inner and outer electrodes 11, 25 can be varied independently an allows tissue underlying the device to be treated. This is particularly important for colorectal cancers.