Tumors

Abnormal sections of tissue in the body are called tumors or neoplasms.  Some tumors are benign (not normally dangerous) and some are malignant (e.g. composed at least partly of cancer cells).  Occasional benign tumors are diagnosed as pre-cancerous if the doctor has good reason to believe cancer will develop.

Lymphomas and leukemias are cancers in the body’s fluids.  The malignant cells are mixed in with the healthy ones.  Sarcomas and carcinomas are cancers in solid organs and referred to as solid tumors.

Tumors in solid cancers change as they get larger.  The very small ones, including microscopic tumors that cannot normally be detected, are clumps of cells with no dedicated supporting blood vessels.  This is called the avascular phase of growth.   Oxygen and nutrients for the cells travel by diffusion.

The capacity for diffusion transport  is low,  Tumors can grow to about 1 to 2 cubic millimeters before the cells start running low on nutrients.  The body must produce new blood vessels to feed the cancer cells.  The growth of new blood vessels is called angiogenesis

The body makes new blood vessels all the time – whenever a person puts on weight and his or her belly expands or whenever a papercut heals, angiogenesis is happening.  Scientists have explored the mechanics of this process and it is very complex.  The body produces chemical signals to start and stop blood vessel growth.  Chemicals that stop blood vessel growth are called angiogenesis inhibitors.

Larger tumors are referred to as being in the vascular phase.  Once blood vessel networks form, the tumor can grow large,  Vascular phase tumors are more invasive and metastasis happens after the tumor has reached this stage.  Local invasion is the term for growth of the tumor and crowding out of nearby tissue.  It can produce pain and other medical consequence.  Metastasis involves spread of the cancer to other parts of the body, but this is through the birth and growth of new tumors, not an extension of the original tumor.

There are over a hundred types of cancer and among the differences are tumor growth rates – some very fast and some slow to the point of almost being benign.  Within a tumor can be both malignant and non-malignant cells.  And there is even more heterogeneity.  Malignant cells in the tumor can have different biological characteristics (morphological and phenotypes).  This heterogeneity and the fact that cancer cell mutate faster than healthy cells are key to the resilience of cancer and why so many cases of cancer develop resistance to chemotherapy treatment.

Traditional chemotherapy drugs are cytotoxic – they kill cells.  In the 1990s a new approach to cancer treatment was formulated.  The idea was to stop the cancer by stopping tumor growth, and stopping tumor growth by stopping the growth of new blood vessels.  By giving the patient angiogenesis inhibitors – either identical to the chemicals the body naturally produces for that purpose or other chemicals that inhibit generation of blood vessels – the oncologist can stop the cancer in its tracks,  This new paradigm was called anti-angiogenesis therapy or angiogenesis inhibition therapy.  It caused a great deal of excitement in the medical community. That excitement has faded as angiogenesis inhibition results in clinical practice were less than revolutionary.  However, several angiogenesis inhibitors have been approved by the FDA as cancer drugs and they are in use today.

Physiologists who look deep into tumors tell us they are heterogeneous.  Further, within the tumor the immediate environment around each cell shapes its characteristics.  Within a tumor are variations in oxygen levels in the blood and fluid.  If drugs are administered, there will be variations in drug levels, too.

This environment actually encourages the development of tumor resistance, as treatments that kill a majority of cells leave a few alive that multiply.  Scientist think that the resistance is due to both genetic mutations and expressions of genes.

Upon diagnosis, a tumor may be assigned a grade, which is not the same as the stage of the cancer.  Oncologists have special grading systems for some types of cancer; it there is no specific system for a cancer, the generic system of G1 (least abnormal) to G4 (most abnormal) can be employed.  Non-malignant cells tend to be specialized for the tissue they are in.  Cancer cells are less specialized.  A tumor mass that looks more like normal tissue under a microscope is called well-differentiated and can be classified a G1, while undifferentiated tissue of mature cancer is classified as G4.