We appreciate the comment regarding the definition of familial colorectal cancer type X (FCCTX). We agree that the original term by Lindor et al1 of FCCTX was used to define families meeting Amsterdam I criteria with microsatellite-stable (MSS) tumors who have a lower risk of colorectal cancer than families with Lynch syndrome. This study used microsatellite status as a surrogate for whether there was an MMR gene defect. There has been inconsistency in the definition since, with both Lindor2 in 2009 and Lynch et al3 in 2015 describing FCCTX as Amsterdam criteria “without an MMR defect,” not Amsterdam criteria with MSS tumors. Because there is no systematic method to define syndromes nor is there an authoritative body determining terminology, there has been, and will continue to be, some differences in opinion regarding definitions of evolving syndromes. There is no consensus on whether an expanded definition to include Amsterdam II families or Amsterdam families who have tumors without germline MMR mutations but with MSI (microsatellite instability) is appropriate. We agree with your preference for defining those who meet Amsterdam criteria without an MMR gene defect but with MSI as likely having Lynch syndrome.4 Until there is consensus on these issues, it will be important for those studying the topic to clearly define FCCTX with respect to whether Amsterdam I or II is used and whether the absence of a gene defect, the presence of MSS, or both is required.
We also appreciate the discussion regarding surgical options. It is important to note that these guidelines are developed based on systematic review of available evidence and not the consensus of expert opinion. Each method can give important guidance to the reader. However, to our knowledge, no study has been published examining either primary or tertiary prophylaxis. Accordingly, only secondary prophylaxis is included in this work, because there are insufficient data to provide guidance on primary or tertiary prophylaxis.