Chemotherapy vs. Radiation Therapy: Mechanisms, Differences, and How They're Combined

Both chemotherapy and radiation therapy kill cancer cells, but through fundamentally different mechanisms and spatial scales.

Mechanism of Action

• Chemotherapy: Systemic drugs targeting rapidly dividing cells (DNA synthesis, microtubule function, topoisomerase inhibition). Circulates throughout the body.
• Radiation Therapy: High-energy photons/electrons/particles causing DNA double-strand breaks locally. Delivered to a defined target volume.

Key Differences

Aspect	Chemotherapy	Radiation Therapy
Scope	Systemic (whole body)	Local/regional
Primary Use	Metastatic disease, adjuvant, neoadjuvant	Local control, definitive, palliative
Typical Side Effects	Nausea, alopecia, myelosuppression, neuropathy, fatigue	Skin reaction, fatigue, site-specific (e.g., esophagitis, cystitis)
Schedule	Cycles every 2–3 weeks	Daily fractions (Mon–Fri) for weeks

Combination Strategies

• Concurrent Chemoradiation: Radiation sensitized by chemo (e.g., cisplatin + RT for H&N, cervical, anal, lung).
• Sequential: Neoadjuvant chemo → chemoradiation → adjuvant chemo (e.g., pancreatic, esophageal).
• Sandwich: Chemo → surgery → chemo ± radiation.

Decision Factors

Cancer type, stage, molecular profile, patient fitness, organ function, and goals (curative vs palliative) determine the optimal modality mix.