Spreading through airspaces: should its existence play a role in thinking about lung metastases? COMMENT

Thoracic surgeons are familiar with the modes of diffusion of primary lung tumours; classically, nodal diffusion through the lymphatics and haematogenous metastases are recognized as the most frequent modes. They are known to determine outcomes and thus represent the basis of the tumour/node/metastasis staging system. Together with these diffusion modes, pleural invasion and lymphatic and vascular emboli have been recognized as other modes of spreading lung cancer; their prognostic significance has been established as well [1, 2]. Spreading through air spaces (STAS), although known for several years, has been investigated more frequently in recent years, and its prognostic significance is beginning to be established [3]. STAS is the evolution of the old concept of aerogenous metastasis, which has been known for several years, especially in the setting of the formerly named bronchoalveolar carcinoma. In 2015, the World Health Organization officialized the term by providing an apparently rigorous definition: tumour cells, including 1 or more micropapillary structures, solid nests or single cells, spreading beyond the edge of the tumour into air spaces in the surrounding lung parenchyma. If areas of uncertainty and relatively little evidence to support the definition have been evoked in the setting of lung cancer [4], even less is known about STAS as a mode of intraparenchymal diffusion of lung metastases.