Practical pathology for thermal tissue applications

The development of minimally invasive medical devices, which employ hyperthermic and/or cryothermic modalities to treat a variety of organ system diseases, is a rapidly expanding field. Consultation with a knowledgeable pathologist during the development of these devices is crucial, as characterization of the treated tissues can support the device’s safety and future efficacy. The properties of thermally treated tissues often share a set of overlapping histopathologic characteristics, regardless of organ system. Several methods for optimally evaluating these thermal changes have been developed that depend on the tissue’s post-treatment time interval. For devices associated with hyperthermic collagen denaturation, bright field or polarized light microscopy of hematoxylin and eosin stained sections can be utilized to assess thermal spread within the tissue. For applications resulting in collagen denaturation with associated desiccation, trichrome staining may provide additional information. For cryothermic devices, these collagen-based methods are generally less informative. Tetrazolium-based tissue viability staining (triphenyltetrazolium chloride, TTC; nitroblue tetrazolium, NBT; NADH/NADPH staining) can be used to assess for the presence of associated tissue necrosis within either hyperthermically or cryothermically treated tissues. The advantages and limitations of several of these methods will be discussed.