There are trillions of cells making up the various organs and tissues of the human body. The vast majority of them arise during embryonic development from a single cell, the zygote. Embryonic development is a complex process in which cells proliferate and divide following a very controlled genetic process that we have acquired through millions of years of evolution. A remarkable fact is that this cell proliferation does not end in adulthood. There are still cells capable of dividing, such as the hematopoietic cells that form white blood cells to protect us from infectious agents or red blood cells which carry oxygen to the tissues and carbon dioxide from the tissues to the lungs. But no matter how good machinery is, soon or later it can fail. And unfortunately cells are no exception.
When they lose control they are unable to stop dividing, leading to the development of abnormal growths that we generically call ‘tumors’. Some tumors are benign, which means that they are not able to spread to other parts of the body, but can be dangerous if their growth puts pressure on vital organs. Other tumors, however, are malignant, or cancerous. If they are not diagnosed and treated in time, they can invade the surrounding tissue or spread to other organs and tissues. The tumor arising in the original site is known as the primary tumor, while the growth of the spreading cells is the secondary, or metastatic, tumor. Based on a series of diagnostic tests, such as physical examination, imaging tests and histological evaluations, which looks at cell characteristics, the doctor is able to easily identify the primary tumor. This gives the cancer its name and defines the most appropriate treatment in each case.
For example, when breast cancer has spread to the liver we say that the patient has breast cancer with liver metastases. It will therefore be treated as breast cancer and not liver cancer. Sometimes, however, the primary tumor is too small to produce symptoms or be seen in imaging tests. What’s more, the metastatic tumor cells have completely lost the histological features of the original tumor. In these cases, where the primary tumor remains hidden, it is not possible to name the tumor and it is also very difficult for the doctor to establish the most appropriate treatment. In these cases the patient is said to have a cancer of unknown primary origin (CUP). Fortunately, technological advances from many years of research and development have resulted in the emergence of new diagnostic tests.
We have gone from the X-rays discovered in the late nineteenth century to the CT scans of the twentieth century. And we have gone from simple but very useful histological characteristics to identifying specific markers such as mutations (irreversible DNA modifications), gene expression profiling (which genes are expressed and which are not) and, more recently, encouraging results with DNA methylation profiles, a particular type of epigenetic modification (potentially reversible DNA modifications). It is these advances, combined with new generation drugs, which have helped reduce cancer mortality in the past two decades.