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Radiation physics and treatment techniques — IMRT, SBRT, protons and brachytherapy
A linear accelerator produces megavoltage photons that act on tissue mainly through Compton scattering; modern techniques — IMRT, SBRT, protons and brachytherapy — aim to concentrate a high dose in the tumour and lower the dose to healthy tissue.
Why it matters in the exam
Physics questions are known to rest on a single fact: which interaction is common in a linear accelerator, the energy range, the role of the multileaf collimator (MLC), the advantage of protons over photons, and how radiation behaves around a brachytherapy source. Those who remember the principles earn these questions easily.
Other questions are more clinical: what limits the dose in liver SBRT, when a spinal metastasis is better treated with conventional radiation rather than SBRT, and how stereotactic treatment is defined. It matters to distinguish SRS (a single dose to the brain) from SBRT (a few high fractions to the body), and permanent low-dose-rate brachytherapy from temporary high-dose-rate brachytherapy.
Key points
- In a linear accelerator in the 4 to 25 MV range, Compton scattering is the main interaction in tissue.
- MLC and IMRT shape the dose around the tumour and lower the dose to sensitive organs with inverse planning.
- Protons deposit most of their energy in the Bragg peak with almost no exit dose, so they reduce the dose to healthy tissue.
- SRS and SBRT give very high doses in a few fractions, with precise immobilisation and image guidance.
- In brachytherapy the dose falls by the inverse square of the distance from the source, so a high tumour dose comes with a low surrounding dose.
Summary table
| Technique or concept | Principle | Use and advantage |
|---|---|---|
| Linear accelerator and interactions | Photons of about 4 to 25 MV and electrons; in tissue in this range Compton scattering dominates | The photoelectric effect dominates at diagnostic energies; pair production at high energies |
| Skin sparing | The maximum dose lies below the skin (about 1.5 cm at 6 MV) | The skin gets a relatively low dose with photons; electrons suit superficial tumours |
| MLC and IMRT | Moving leaves that shape the field and vary intensity; inverse planning | Concave dose distribution around sensitive organs — salivary glands, rectum, bladder |
| Protons | Bragg peak: energy is deposited at a set depth with almost no exit dose | Less total dose to the body — children, skull base, re-irradiation |
| SRS and SBRT | Very high doses in few fractions (1 to 5), steep dose fall-off, immobilisation and image guidance | In liver SBRT the main limit is the dose to the remaining healthy liver volume |
| Brachytherapy | A radioactive source inside or next to the tumour; dose falls by the inverse square of distance | Permanent low dose rate (iodine-125 seeds in the prostate) or temporary high dose rate (iridium-192 in the cervix) |
| Spinal metastasis — conventional radiation versus SBRT | Conventional radiation is preferred in cord compression with neurological deficit, multilevel disease and short life expectancy | SBRT suits a single metastasis, re-irradiation and a radioresistant tumour |
A free exam question
מטופל עם,סרטן ערמונית עם מדדי PSA=18 , Gleason Score=8 (4+4), מחלה דו"צ. לפי הדמיה , ללא מחלה גרורתית. מה נכון לגבי טיפול קרינתי במטופל זה? (the question as examined, in Hebrew)
- א. ניתן לשקול מתן טיפול Androgen deprivation therapy ארוך טווח, ללא טיפול קרינתי, משום שצפויה הישרדות זהה לחולים שכן קיבלו קרינה
- ב. מומלץ על תוספת טיפול ב- taxotere , לטיפול הסטנדרטי, מאחר ש מפחית הופעת גרורות
- ג. תוספת טיפול קרינתי לבלוטות האגניות משפרת במקרים אלו הישרדות כוללת בערך ב- 8% ולכן יש המלצה לטיפול קרי נתי לאגן
- ד. ניתן לתת טיפול קרינתי עם ברכיתרפיה, בשילוב עם Androgen deprivation therapy ארוך טווח
In the full version for subscribers
- A simple algorithm
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- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Oncology, Diagnostic radiology.
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Sources
- Khan and Gibbons, Khan's The Physics of Radiation Therapy, 6th ed. (2020) — Interactions of ionizing radiation; Intensity-modulated radiation therapy; Proton beam therapy
- Gunderson and Tepper's Clinical Radiation Oncology, 5th ed. (2021) — Radiation physics; Stereotactic body radiotherapy; Brachytherapy
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.