TNM classification for lung metastases

Although extensively discussed in the field of oncology, lung metastasis remains an important clinical dilemma full of uncertainty with many unresolved questions. The extensive literature shows that the level of evidence to perform LM is weak, and surgeons continue to operate on the basis of their own experience without formal guidelines. Furthermore, the recent results of the Pulmonary Metastasectomy colorectal (PulMiCC) trial showed no statistical differences on survival between operated and no-operated patients with lung metastases. It is obvious that we are caught in a dead-end, as it seems impossible to know whether or not surgery is indicated in pulmonary metastases or when other treatments should be preferred. This AIM of this paper is to highlight the rationale, the possible advantages and future direction of the TNM classification for lung metastases. For these reasons, we re-propose a TNM classification for lung metastases which was published initially in 2016. The originally idea of the TNM classification of LM was born to explain how the number of metastases and nodal involvement interfere between each other to influence survival. We anticipate that in the amended classification the T will describe the number of metastases. We will also include the acronymous lm (lung metastases) before the TNM (lmTNM) to differentiate it from the classical TNM classification for cancers. Time to metastasis (TM), defined as the time of appearance of lung metastasis, which is an important prognostic factor could also be added in the staging system to separate synchronous from metachronous lung metastases. Conclusion: We believe that the TNM classification for LM will simplify the indication for surgery and allow physicians, oncologist and surgeons to speak the "same language" in comparing their data and assessing the results of treatment of lung metastases.