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Radiation toxicity — early and late effects, and organ tolerance
Early toxicity hits rapidly dividing tissues and heals, while late toxicity comes from vascular damage and fibrosis, depends on fraction size and is usually irreversible; every radiation plan is built around dose-volume limits for healthy organs.
Why it matters in the exam
The exam asks definitions such as TD5/5, risk factors for radiation pneumonitis, cardiac toxicity in breast radiation and dose limits for key organs. Clinical scenarios also appear: cough and dyspnoea a few months after chest radiation, an electric-shock sensation on neck flexion, or rectal bleeding a year after prostate radiation.
The key understanding is that late tissues are sensitive to fraction size (low α/β ratio), so a high dose per fraction mainly increases late toxicity. Concurrent chemotherapy, vascular or lung disease and immunotherapy increase toxicity, and some drugs cause "radiation recall" in a previously irradiated field.
Key points
- Early toxicity hits rapidly dividing tissues and resolves, and late toxicity comes from fibrosis and vascular damage and is usually permanent.
- TD5/5 is the dose causing a complication in 5% of patients within 5 years, and is used to define organ tolerance.
- Radiation pneumonitis appears 1 to 6 months after treatment, relates to V20 and concurrent chemotherapy, and is treated with prednisone.
- The Lhermitte sign is a transient electric sensation on neck flexion, while radiation myelopathy is rare, severe and permanent.
- Heart dose raises coronary risk with no safe threshold, and the salivary glands are preserved when the mean parotid dose is below 26 Gy.
Summary table
| Issue | What to know | Exam detail |
|---|---|---|
| Early versus late | Early: skin, mucosa, bowel and marrow, during treatment and the following weeks | Late: months to years, fibrosis and vascular damage, depends on fraction size |
| TD5/5 and TD50/5 | The dose causing a complication in 5% (or 50%) of patients within 5 years | Used to define organ tolerance |
| Radiation pneumonitis | 1 to 6 months after radiation; dry cough, dyspnoea and fever; opacity matching the field | Risk: high V20, mean lung dose, concurrent chemotherapy, prior lung disease; treatment: prednisone |
| Spinal cord | Accepted maximum dose of about 45 to 50 Gy with conventional fractionation | Lhermitte sign — electric sensation on neck flexion, transient and reversible; myelopathy — rare and severe |
| Heart | Coronary disease, pericarditis and valve disease, years after left breast or lymphoma radiation | No safe threshold; risk rises with mean heart dose |
| Head and neck and pelvis | Dry mouth, jaw osteoradionecrosis, hypothyroidism; proctitis and cystitis | A mean parotid dose below 26 Gy preserves saliva production |
| Second tumours and radiation recall | Sarcoma or carcinoma in the field, years after treatment | Radiation recall: inflammation in an old field after doxorubicin, taxanes or gemcitabine |
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
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Sources
- Hall and Giaccia, Radiobiology for the Radiologist, 8th ed. (2018) — Clinical response of normal tissues
- QUANTEC: Quantitative Analyses of Normal Tissue Effects in the Clinic, 2010 — Organ-specific dose-volume reports
- Gunderson and Tepper's Clinical Radiation Oncology, 5th ed. (2021) — Late effects of cancer treatment
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.