Cancer Overview: Cell Cycle, Tumor Types, and Histology

Clinical Correlation: Treatment Sensitivity Understanding the cell cycle is vital for radiation therapy. Cells exhibit varying levels of sensitivity to radiation depending on their phase. Cells are most sensitive to radiation during the G2 and M phases, whereas they are most resistant during the S phase.

Tumor Classification: Benign vs. Malignant

When cell regulation fails, tumors (masses of abnormal cells) can form. Clinicians classify these tumors based on their growth patterns, their ability to spread, and how closely they resemble the original tissue.

Benign Tumors

Benign tumors are generally considered less dangerous because they are well-differentiated, meaning the cells look and function very much like the normal cells from which they originated. They are typically slow-growing and often encapsulated, which means they tend to push surrounding tissues aside rather than infiltrating them. Crucially, benign tumors do not invade nearby tissues and do not metastasize (spread to distant sites).

Malignant Tumors

Malignant tumors are characterized by their ability to invade and destroy nearby tissues rather than simply compressing them. A hallmark of malignancy is metastasis, the process by which cancer cells spread to distant parts of the body.

Malignant cells often undergo dedifferentiation, a process where they lose their specialized structure and normal appearance. In extreme cases, cells become anaplastic (undifferentiated). An anaplastic cell has changed so significantly that it bears no resemblance to the original parent cell and has lost its normal biological functions.

Exam Tip: Grading vs. Staging

• Grading is determined by examining cells under a microscope to see the degree of differentiation (from well-differentiated to anaplastic).
• Staging (such as the TNM system) describes the size of the tumor (T), the involvement of lymph nodes (N), and the presence of metastasis (M).

Histological Classification: Where Does the Cancer Start?

Tumors are further categorized by their histology, which refers to the specific type of tissue from which the cancer arises.

Sarcomas

Sarcomas arise from mesenchymal cells, which are the cells that form connective tissues. Examples of tissues that give rise to sarcomas include:

• Bone
• Cartilage
• Muscle
• Fat

It is important to note that while blood and lymphatic cancers also originate from mesenchymal tissue, they are given specialized names: leukemias (blood) and lymphomas (lymphatic system).

Carcinomas

Carcinomas are much more common and arise from epithelial cells. These are the cells that cover body surfaces and line the internal organs and body cavities. Carcinomas are subdivided into two main types:

1. Squamous Cell Carcinoma: These originate from squamous epithelium (e.g., squamous cell carcinoma of the lung).
2. Adenocarcinoma: These originate from glandular epithelial cells, which are specifically responsible for producing secretions (e.g., adenocarcinoma of the stomach).

Table 1.3: Histologies Associated With Common Anatomic Cancer Sites