Cancer is a complex group of diseases characterized by the uncontrolled growth and spread of abnormal cells. These cells can form masses known as tumors, which can invade surrounding tissues and spread to distant parts of the body through metastasis. Understanding these complexities helps explain why eradicating cancer remains a formidable scientific endeavor.
Cancer’s Diverse Nature
Cancer is not a single disease; it encompasses over 200 distinct types, each with unique biological characteristics. This diversity originates from genetic mutations, changes in the DNA sequence of cells. These mutations can affect specific genes like oncogenes and tumor suppressor genes. Oncogenes, when activated, promote uncontrolled cell growth, acting like an accelerator.
Conversely, tumor suppressor genes normally regulate cell division and trigger cell death, acting like a brake. When inactivated, cells can grow out of control. Cellular heterogeneity within a single tumor means different cancer cells can possess varying genetic profiles and behaviors. This allows some cancer cells to evolve and adapt, potentially becoming resistant to treatments. Genetic differences can be amplified through clonal expansion, leading to populations of cells with distinct characteristics. This diversity and adaptability make a single “one-size-fits-all” cure impossible.
Treatment Complexities
Developing effective cancer treatments is challenging due to the difficulty of selectively targeting cancer cells while leaving healthy cells unharmed. Conventional treatments like chemotherapy and radiation therapy often damage rapidly dividing healthy cells alongside cancer cells, leading to severe side effects such as fatigue, hair loss, and damage to bone marrow or the gut lining. While newer targeted therapies aim to specifically attack molecular pathways active in cancer cells, they also present challenges.
Cancer cells can develop drug resistance over time, making initial treatments ineffective. This resistance can be intrinsic, meaning cells are naturally resistant before treatment, or acquired, developing during therapy. Mechanisms of resistance include changes in drug metabolism, alterations in the drug’s target, activation of alternative survival pathways, and enhanced DNA repair. Delivering drugs to all parts of a tumor or to specific locations can be difficult due to factors like the dense tumor microenvironment and poor vascularization, which can limit drug penetration.
Evading the Immune System
The body’s immune system is naturally equipped to identify and destroy abnormal cells, including early cancer cells. However, cancer cells have evolved mechanisms to evade this immune surveillance. One way cancer cells hide is by reducing the presentation of specific molecules, like MHC-I proteins, on their surface, which normally display cancerous antigens to immune cells. Without these signals, T cells may not recognize the cancer cells as a threat.
Cancer cells can also suppress immune responses by creating an immunosuppressive microenvironment around the tumor. This environment is rich in certain immune cells, such as myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs), which release molecules that inhibit anti-tumor immune cells. Some cancer cells can even co-opt immune cells to their advantage, promoting tumor growth and spread.
The Challenge of Complete Eradication
Eliminating every single cancer cell is difficult, contributing to the challenge of a complete cure. A major reason for this is metastasis, the process by which cancer cells break away from the original tumor and spread to distant parts of the body through the bloodstream or lymphatic system. Even if the primary tumor is successfully treated, microscopic, undetectable cancer cells can remain scattered throughout the body. These disseminated cells can lie dormant for years before reactivating and growing into new tumors, leading to cancer recurrence.
Surgical removal, while often a primary treatment, faces limitations in achieving complete eradication. Surgeons aim to remove the entire tumor along with a margin of healthy tissue to ensure no cancer cells are left behind. However, some tumors may be located near vital organs or major blood vessels, making complete removal risky or impossible without causing significant damage. Microscopic cancer cells that have already spread, or those too small to be detected by imaging, mean surgery alone may not be sufficient.