Radiation and chemotherapy primarily affect which type of cells first?

• A. Mature neurons
• B. Cardiac myocytes
• C. Rapidly dividing cells (oral mucosa, GI tract, bone marrow)
• D. Skeletal muscle

Answer: (C)

Radiation and many chemotherapy drugs preferentially kill cells that are actively dividing. These treatments damage DNA and interfere with mitosis, so cells in the process of replication are most vulnerable. Tissues with a high rate of turnover—such as the lining of the mouth (oral mucosa), the gastrointestinal tract, and the bone marrow—contain many rapidly dividing cells and thus are affected first. This explains common early side effects like mucositis, diarrhea, and bone marrow suppression.

In contrast, mature neurons, cardiac myocytes, and skeletal muscle are largely non-dividing in adults. They don’t undergo much DNA replication, so they resist early cytotoxic effects. They may experience late toxicity only with higher or prolonged exposure.