The pre-metastatic niche is only half of the story of metastasis (it's the biological one)

Recently, Cancer Research UK posted an article on their blog in which they explain, in layman's terms, recent trends and ideas in research into metastatic spread. The focus of that article is on the concept of a 'pre-metastatic niche', the idea that the primary tumour emits signalling molecules that prime certain organs for the arrival of metastatic cells. We find this line of thought very interesting, as it could, at least in part, explain patterns of metastatic spread, but have strong opinions about how the ideas were presented and the lack of acknowledgment of the other factors that could be at play.  

First, the reader is given a condensed historical background, in which the surgeon Stephen Paget is given credit for having solved the riddle of metastatic patterns 150 years ago. His method of studying metastatic spread in breast cancer is briefly mentioned, however, as is often the case when the seed-soil hypothesis is mentioned, these old 'truths' do not seem to be carefully checked. For example, a much more recent study from by Dr. J Pickren (reported in The Principles of Metastasis by L. Weiss, p. 231, recently reviewed here), which reports a 4:1 ratio between splenic and hepatic metastases (compared to the 14:1 ratio that Paget observed). Another fact not accounted for by Paget in his analysis, is that the liver not only receives arterial blood, but also blood from the gut organs via the portal vein, thereby increasing the chance of it receiving circulating tumour cells (CTCs). If micro-metasases are present in the gut, then these secondary CTCs will most likely lodge in the liver increasing the risk of developing liver metastases. Lastly, Paget only studied a single location of primary tumours, making general conclusions difficult to draw - especially as the connectivity differs greatly between organs. These simple observations should make it clear that Paget's hypothesis is nothing more than an indication of what might be the case in certain circumstances, rather than a settled fact.

From reading the article one also gets the impression that CTCs are drawn to certain organs in the body (e.g. the caption of the 2nd figure reading "Tumour cells are selective about where they end up." or later in text "...which wandering tumour cells find irresistible."). This is not in agreement with what we know today (and have known for the last 30 years) about the dynamics of metastasis formation.

Figure 1: Human vascular system network topology schematic. It is evident by inspection of the network diagram that tumors originating in the gut and lung experience significantly different flow patterns and order in which they experience filtration at capillary beds than tumors originating in other parts of the ‘body’. The alternate pathways (purple) define the fraction of cells which evade arrest (filtration) at a given capillary bed. There are scant measurements of this in the literature, and none for clinical studies.

On the contrary CTCs have little influence over where they end up, instead the correct picture is that of a primary tumour releasing astronomical numbers of CTCs into the blood stream (roughly 100 million cells per day, of which most die in the blood stream), and that these cells are distributed according to physiology of the circulatory system. 

This means that each organ (except the lung and liver) receive a fraction of CTCs in direct relation to their relative blood supply, and only at this point, at which the cancer cells flow through the capillary bed of the organ, can organ specific mechanisms influence the fate of the cancer cell. This means that any explanation of why patterns of metastatic spread look as they do needs to first take into account the characteristics of the circulatory system, and only then the organ specific mechanisms such as the formation of a pre-metastatic niche.

These facts suggest (at least to us) that one should view the formation of the pre metastatic niche from a more passive point of view. The signals secreted by the primary tumour induce a systemic inflammatory response - which may or may not effect all organs. The evidence suggests that some distant sites respond in a way that makes them more hospitable to the CTCs that happen to pass though them and hence these cells are more likely to form overt metastases - but to present this as an active process is to stretch the data and to anthropomorphize to a dangerous extent.

When attempting to synthesize and communicate difficult scientific information to the public, it is always tempting to present a small slice of the story - and indeed, this is good practice as only so much can be communicated effectively at one time.  But when doing this, it is essential to point out where the limits of our understanding are, and not oversell current hypotheses as the 'truth'.  Science is, and always has been, a steady progression toward understanding, paved by models that are (we hope) less and less wrong.  The way we think today is not likely to be the same as the way we think in 10 years time.