An apparatus for guiding a biopsy instrument into tissue of a patent comprises a cannula, a guide device, a support structure, and a locking assembly. At least a portion of the biopsy instrument is insertable into the cannula. The guide device comprises at least one guide hole. The at least one guide hole is configured to receive the cannula. The support structure is configured to position the guide device relative to a breast of a patent. The locking assembly comprises a lock arm that is configured to engage the cannula to restrict translational movement of the cannula within the guide hole of the guide device.