A tissue ablation method for treating atrial fibrillation in a patient comprises locating an ostium of a pulmonary vein and positioning an interventional catheter adjacent the ostium. The interventional catheter has an energy source. Collateral tissue adjacent the ostium is located and tissue around the ostium is ablated with energy from the energy source so as to form a contiguous lesion circumscribing the ostium. The lesion blocks aberrant electrical pathways in the tissue so as to reduce or eliminate the atrial fibrillation. The ablating is modified so as to avoid ablating or otherwise damaging the collateral tissue.