A distinction is generally made between tumours of the blood (leukaemia, lymphomas etc.) and solid tumours which present as a solid mass in organs or connective tissue and which can be detected by imaging methods (ultrasonography, X-ray, scintigraphy) once they have reached a certain size. Tumours initially cause hardly any symptoms. For this reason they are usually not recognized until their growth constricts e.g. the function of vitally important organs.
Even in their early stages tumours, also known as primary tumours disseminate so-called circulating tumour cells (CTC) into the blood circulation. The CTC spread around the body via the bloodstream and later form metastases, which themselves can also disseminate CTC. Metastases generally arise in certain preferred target areas of the primary tumour. It is said, for example, that a lung tumour has metastasized into the liver. Sometimes metastases are found in the direct vicinity of the primary tumour or in the same organ. Primary tumours can usually be removed surgically without problems. Metastases which occasionally arise suddenly years later are a greater problem. Because of their great number at times they cannot be removed surgically and can also not treated with the medication that was used to treat the primary tumour.
What is the reason for this?
To answer this question it is helpful to look at the primary tumour, the CTC, the metastases and their interconnections in more detail.
A primary tumour tends to grow and produce new colonies. In order to do this it sends out the CTC as cellular messengers.
The first hurdle CTC has to overcome is the immune system which recognizes the CTC as extraneous material on the basis of certain genome based molecular structures on their surface and soon destroys 99.9% of them. These molecular structures function like barcodes recognized by certain antennas of the immune system.
However, in some of the CTC the primary tumour has altered the barcodes, with the result that the immune system no longer recognizes these as tumour cells and allows them to pass.
They now slip through the net and some of them retreat into the connective tissue or bone marrow, from where they sometimes return to the bloodstream years later at a particular signal. Also the CTC form in favourable places immediately new colonies, the metastases.
This means that primary tumours and metastases differ genetically to a certain extend and as a result of this, a therapy concept which may work for the primary tumour can seldom be used for the metastases, as these behave like new tumours with their own individual profile. The mediators between the two are the circulating tumour cells, the CTC. They carry the genetic information of the primary tumour, but above all the genetic information of the future metastases. Particular genetic information also forms the target here for medication, which will be shown in more detail below.
In rare cases no primary tumour can be located at the time of diagnosis. Only metastases are found. This is denoted as cancer of unknown primary tumour. The abbreviation CUP (cancer of unknown primary) has become generally accepted for this. By means of molecular biology techniques the primary tumour is found in approximately 40% of cases because certain tumours can be recognized by genetic markers. On the other hand it is probably better to deal just with the putative metastases, analyse these like tumours and treat them.