Assembly (10) for positioning an electrode assembly (16) for radiofrequency ablation, comprising a first linear motion guide (11) defining a first axis of motion (103) and comprising a first carriage (111) operable to move along the first axis of motion (103), an XY positioner (12) operable to move a second carriage (123) in a two-dimensional space defined by a second axis of motion (104) and a third axis (105) orthogonal to the second axis of motion. The XY positioner is positioned relative to the first linear motion guide (11) such that the second axis of motion and the third axis are orthogonal to the first axis of motion (103). A first coupling means (125, 14) couples the electrode assembly (16) to the second carriage (123). A second linear motion guide (13) defining a fourth axis of motion (106) is disposed such that the fourth axis of motion is parallel to the second axis of motion (104). The second linear motion guide (13) comprises a third carriage (131) operable to move independently of the second carriage (123). A second coupling means (133, 15) for coupling a tissue stabilizing instrument is mounted to the third carriage (131).