artistic rendition of horizon and tumor cells

Solid tumors

It all starts with a single mutation. The DNA strand is now compromised. A mutation/duplication of a single nucleotide or a nucleotide depletion disrupts the DNA sequence. This change in the genome is irreversible and can lead to the transformation of a normal cell into a tumor cell. Cancer can spread to nearby tissues or remote organs through the lymph system and bloodstream.

Solid tumors contain abnormal and heterotypic cells that communicate through tight and gap junctions. In contrast with liquid tumors, as the cells multiply, they form a “mass” called a solid tumor and usually do not contain pockets of fluid, pus, air, or other substances. Solid tumors can be either non-cancerous (benign), pre-malignant (cells that have the potential to become malignant), or malignant (cancerous).

Solid tumors represent approximatively 90% of adult human cancers. They can develop in many parts of the human body, including the breast, lung, prostate, colon, melanoma, bladder, and kidney (Figure 1).

Examples of localized solid tumors:

  • Carcinomas
  • Sarcomas
  • Lymphomas
  • Carcinosarcomas

The genomic background of pediatric tumors is different from that of adult tumors. The same tumor types tend to have completely different mutation profiles compared to their adult counterparts. Statistics have revealed that in children, solid tumors make up about 40% of all cancers. The most common and aggressive type of solid tumor found in children is a brain tumor.

graph showing cancers and the estimated percentage of new cases
 Click image to enlarge
Figure 1. Cancers and the estimated percentage of new cases. Adapted from American Cancer Society: Cancer Facts and Figures 2018.

The multi-stage mechanism of metastatic tumors starts with breakthrough of primary cells and entry into the lymphatic system or peripheral circulation followed by infiltration of underlying tissue where they develop into tumors. Nearly all solid tumors require the formation of a neo-vasculature that allows them to grow and spread through vascular metastasis. Metastatic tumors represent the highest threat to cancer patient mortality (Figure 2).

Solid tumor anatomy or tumor microenvironment (TME) does not consist of cancer cells alone; it includes other cells as well:

  • Cancer stem cells—possess stem cells properties and can reproduce over many generations
  • Typical tissue cells—originate from the primary tissue or organ
  • Mitotic division—actively growing cancer cells
  • Necrotic process—dead cancer cells creating a necrotic area of tissue
  • Connective or supportive tissue cells—provide structural support for the tumor (fat, cartilage, bone, and blood vessels)
  • Immune cells—active (killing cancer cells) or passive (turned off by the tumor cells)

It is crucial to comprehend the heterogeneity of solid tumor’s microenvironment to tackle drug discovery and study of resistance and mutations.

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