Cancer is an umbrella term for diseases that involve uncontrolled cell growth.
In most cases, prostate tumours grow relatively slowly. It usually takes years for tumours to become large enough to be detectible, and it takes even longer for them to spread out of the prostate.
MedicineNet.com - What is Cancer?
The term “cancer” refers to uncontrolled growth of abnormal cells in the body. In the case of Prostate Cancer (PCA) the cells of the glandular tissue may multiply out of control and form a tumour.
Normally, cells undergo a predictable and orderly cycle of cell growth and renewal. As old cells die off, new cells form to replace them. This process is controlled by genes within each cell’s DNA.
When genes become damaged, cells may begin to grow and reproduce in an uncontrolled manner. We describe these cells as “malignant” or “cancerous”.
The term “tumour” is derived from the Latin word for “swelling”. A tumour forms when abnormal cells group together into a lump. Tumours may be classified as “benign” or “malignant”.
A benign tumour is localized, does not spread into other tissues, and grows slowly. Typically, benign tumours are not a cause for medical action unless they are interfering with the normal functioning of neighbouring tissues. For example, a benign tumour may become large enough to push against neighbouring organs.
The term "malignant" means cancerous. A malignant tumour is a lump of cancer cells. A malignant tumour may grow quickly or slowly depending on the grade of the cancer cells. It is possible for some cells from a malignant tumour to break away from the tumour and spread into other areas of the body - this is called metastasis.
Metastasis refers to a secondary tumour that forms as a result of cancer cells from the primary tumour traveling via the blood or lymphatic system to another part of the body. With prostate cancer, cells that break free from the prostate tumour often find their way into bone.