The two major types of cancer are carcinomas and sarcomas. To understand how they differ, imagine that the body is made of silly putty and we could reshape it—it would take the form of a doughnut. The doughnut hole would consist of the hole made if you enter the mouth, go down the throat and the esophagus, through the stomach, small intestine, large intestine and rectum, and out the anus, over the skin and back up to the mouth. If you’re-shaped the body (remember—it is made of silly putty), you would get a doughnut shape with the gastrointestinal tract as the hole in the middle of the doughnut.
The tissues that make up the outer layers of the doughnut, both on the skin’s surface and in the gastrointestinal tract as I described above (think of it as the icing of the doughnut) are referred to as epithelial tissue or epithelium—the Greek root “epi” refers to “on top of.” The tissues that are internal to the epithelium (the cake in the doughnut?) are referred to as connective tissue. These two layers of tissue (the icing and the cake) are separated in the body by a structure known as the basement membrane. When a pathologist looks at a biopsy from any organ and sees these two tissue types under the microscope, s/he sees or imagines (sometimes it is not visible) the basement membrane separating them. (Since this is a Jewish newspaper, I can feel free to analogize that to the raqia separating the heavens and the earth.)
Why is this important and relevant to cancer? First, let me say that all the tissues (and cells) in the body must fall into one of these two categories. Breast cells, prostate cells, white blood cells, fat cells, bone, neurons, whatever—they have to be either epithelial tissue or connective tissue—no other choices.
How do these two types of tissue differ? The epithelial cells in each organ differentiate in such a way as to perform the functions for which that particular organ is responsible. So, if we are in the breast, the cells which produce milk (the explicit function of the breast) are derived from epithelial tissue. Likewise, if we were in the pancreas, the cells of the pancreas that produce digestive enzymes to aid in digestion are also in the epithelium. The cells of the skin that provide protection to the body from the external environment—trauma, ultraviolet radiation, moisture, heat and cold, etc.—are likewise epithelial.
What about the connective tissue? The various types of connective tissue are responsible for providing support to the epithelial tissue—nutrition, water, oxygen, temperature control, immunological protection, structural support, removal of waste, etc. Thus, included in the category of connective tissue are bone, cartilage, blood, lymph and lymphocytes, fat and muscle.
Now that we have exhausted the doughnut analogy, let’s try another one—a building. Each building has rooms that are differentiated in such a way as to reflect the intentions for which the building is constructed. Thus, a residential building has bedrooms and living rooms; an office building has offices and reception areas; a school building has classrooms. These are the “epithelial cells” of the buildings. But each building also has “connective tissue” to support it—steel girders, electricity cables, water pipes, air conditioning and heat ducts, sewage.
So now, let us return to the topic we started with. Cancer can and does arise in both epithelial cells and in connective tissue. Because there are two major classes of tissue, there are, ipso facto, two major classes of malignancy. Cancers that occur in epithelial cells are called carcinomas. Cancers that occur in cells of connective tissue are called sarcomas. Of course, there are numerous subcategories of each of these, but fundamentally all cancers are either carcinomas or sarcomas.
The astute or knowledgeable reader will appreciate that since blood cells and lymphocytes are both of connective-tissue origin, I lump malignancies of those cell types (leukemias and lymphomas) under the broader rubric of sarcomas even though in day-to-day clinical practice the term “sarcoma” is generally reserved for malignancies of the structural anatomy (bone, muscle, fat, etc.); and hematologic malignancies (leukemia, lymphoma, myeloma) and brain tumors (which are also sarcomas) are generally categorized separately. We can return to this on another occasion for further discussion.
Just to liven up the discussion and make this dry pathologic stuff more interesting, let us consider some relevant statistics. Interestingly, about 80-85% of cancers are of epithelial origin, i.e., carcinomas. Thus, the main cancers we hear about are carcinomas—breast, colorectal, lung, prostate, pancreas, etc. Leukemias, lymphomas (interestingly a contraction for the older term lymphosarcoma), brain tumors, and bone/soft tissue sarcomas make up the remainder.
In contrast, in pediatric oncology, almost all of children’s cancers are leukemia/lymphoma, sarcomas, and brain tumors. Carcinomas are extremely uncommon in the pediatric population. Yet connective tissue malignancies seem to occur in similar proportions in children and adults.
So what protects children from carcinomas? Stay tuned, and let’s continue this discussion in our next episode of Thoughts on Cancer.
Alfred I. Neugut, M.D., Ph.D., is a medical oncologist and cancer epidemiologist at Columbia University Irving Medical Center/New York Presbyterian and Mailman School of Public Health in New York.
This article is for educational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment, and does not constitute medical or other professional advice. Always seek the advice of your qualified health provider with any questions you may have regarding a medical condition or treatment.
By Alfred I. Neugut