What Is Cancer?

Cancer is a class of diseases characterized by the uncontrolled proliferation and spread of abnormal cells derived from the body's own tissues. More than 100 distinct disease entities fall under this umbrella, each defined by the tissue of origin, cell type, and molecular alterations that drive tumor growth. Despite their diversity, cancers share a common set of acquired biological capabilities, described by Hanahan and Weinberg in the widely cited Hallmarks of Cancer framework, which include sustaining proliferative signaling, evading growth suppressors, resisting programmed cell death, and activating invasion and metastasis.

Cancer draws its scientific foundation from cell biology, molecular genetics, pathology, and biomedical engineering. The oncology field has expanded significantly since the 1970s to encompass surgery, cytotoxic drugs, targeted molecular therapies, immunotherapy, and precision radiation delivery. Engineering disciplines contribute through medical imaging, radiation physics, drug delivery systems, and computational modeling of tumor dynamics.

Tumor Biology

A tumor is a mass of cells that has accumulated sufficient mutations to escape the normal constraints on growth and death. Solid tumors are typically described as benign (locally confined with no invasive capacity) or malignant (capable of invasion and metastasis through the lymphatic system or bloodstream). The transition from a precancerous lesion to an invasive carcinoma typically requires the progressive accumulation of driver mutations across several oncogenes and tumor suppressor genes, including TP53, KRAS, and BRCA1/2. Tumor heterogeneity, the coexistence of genetically distinct subpopulations within a single tumor, is a central reason that monotherapy frequently leads to drug resistance. As reviewed in cellular and molecular aspects of oncogenesis, tumor microenvironment interactions, including immune evasion and angiogenesis, are as important as the intrinsic cell properties.

Diagnostic Imaging and Detection

Early detection fundamentally improves outcomes in most cancer types. Medical diagnostic imaging modalities used in oncology include X-ray computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and single photon emission computed tomography (SPECT). PET and SPECT are functional modalities that reveal metabolic and receptor-level activity rather than anatomy alone, allowing clinicians to distinguish active tumor tissue from scar or necrosis. CT colonography, mammography, and low-dose lung CT screening each target specific high-risk populations. Biopsy with histopathological analysis remains the definitive diagnostic standard, with molecular profiling increasingly used to classify tumors by genomic subtype rather than tissue of origin alone.

Treatment Modalities

Cancer treatment is multimodal. Oncological surgery remains the primary curative intervention for most solid tumors diagnosed at an early stage. Chemotherapy uses cytotoxic agents that target rapidly dividing cells; classes include alkylating agents, antimetabolites, and topoisomerase inhibitors. Radiation therapy, including external beam and brachytherapy, deposits ionizing energy in tumor volumes using techniques such as intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery. Biomedical applications of radiation extend to radioimmunotherapy, which conjugates radionuclides to antibodies for targeted delivery. Targeted therapies act on specific driver mutations, such as tyrosine kinase inhibitors for BCR-ABL in chronic myeloid leukemia. According to the National Cancer Institute, combination regimens, chosen based on tumor type, stage, and molecular profile, now dominate treatment planning for most malignancies.

Applications

Cancer research and oncology have applications in a range of fields, including:

• Development of radiopharmaceuticals and imaging contrast agents
• Design of drug delivery nanoparticles and targeted therapeutics
• Computational modeling of tumor growth and treatment response
• Radiation treatment planning systems and accelerator technology
• Biosensor and liquid biopsy platforms for early detection
• Immunotherapy and CAR-T cell engineering