Methods, probe assemblies and systems are provided for treating tissue a margins surrounding interstitial spaces created via the removal of tumors. The interstitial space may be in any tissue, e.g., breast tissue, and the interstitial space may be created by removing abnormal tissue. A hydrophilic electrode is compressed and introduced (e.g., percutaneously) into the interstitial cavity. An electrically conductive liquid (e.g., saline) is applied to the electrode, such that the electrode absorbs the electrically conductive liquid. The electrode is expanded into contact with the tissue margin, and electrical energy (e.g., radio frequency (RF) energy) is conveyed to the electrode, thereby ablating the tissue margin.