A medical assembly and method are provided to effectively treat abnormal tissue, such as, a tumor. The target tissue is thermally ablated using a suitable source, such as RF or laser energy. A cooling shield is placed in contact with non-target tissue adjacent the target tissue, and actively cooled to conduct thermal energy away from the non-target tissue. In one method, the cooling shield can be placed between two organs, in which case, one of the two organs can comprise the target tissue, and the other of the two organs can comprise the non-target tissue. In this case, the cooling shield may comprise an actively cooled inflatable balloon, which can be disposed between the two organs when deflated, and then inflated. The inflatable balloon can be actively cooled by pumping a cooling medium through it. In another method, the cooling shield can be embedded within the non-target tissue. In this case, the cooling shield can comprise one or more needles. If a plurality of needles is used, they can be embedded into the non-target tissue in a series, e.g., a rectilinear or curvilinear arrangement. The needle(s) can be actively cooled by pumping a cooling medium through them.