Radiation is an integral part of modern medicine, especially in diagnostic imaging and cancer treatment. To ensure patient safety and proper risk assessment, radiation exposure is quantified using specific dose concepts. Among these, Absorbed Dose, Equivalent Dose, and Effective Dose are the most important.
This blog explains these three terms in a simple and practical way, especially for students and professionals in radiology and medical physics.
1. Absorbed Dose (Gray – Gy)
The absorbed dose refers to the amount of radiation energy deposited per unit mass of tissue.
Unit: Gray (Gy)
Definition:
1 Gray = 1 joule of radiation energy absorbed per kilogram of tissue
Absorbed dose only measures physical energy deposition. It does not consider the type of radiation or its biological impact.
📌 Example:
X-rays and gamma rays delivering the same energy to tissue will have the same absorbed dose in Gy.
2. Equivalent Dose (Sievert – Sv)
Different types of radiation do not cause the same biological damage, even if the absorbed dose is identical. This is where equivalent dose becomes important.
Unit: Sievert (Sv)
Formula:
Equivalent Dose = Absorbed Dose × Radiation Weighting Factor (Wr)
Radiation weighting factors account for the biological effectiveness of radiation:
| Radiation Type | Wr |
|---|---|
| X-rays / Gamma rays | 1 |
| Beta particles | 1 |
| Alpha particles | 20 |
📌 Meaning:
Alpha particles cause much greater biological damage than X-rays for the same absorbed dose.
3. Effective Dose (Sievert – Sv)
The effective dose estimates the overall risk to the whole body, considering that different organs have different sensitivities to radiation.
Unit: Sievert (Sv)
Formula:
Effective Dose = Σ (Equivalent Dose × Tissue Weighting Factor)
Some tissue weighting factors include:
| Organ | Tissue Weighting Factor (Wt) |
|---|---|
| Lungs | 0.12 |
| Bone marrow | 0.12 |
| Thyroid | 0.04 |
| Brain | 0.01 |
📌 Why it matters:
Effective dose allows comparison of risk from different medical procedures, such as CT scans, X-rays, and nuclear medicine studies.
Why Effective Dose Is Most Important
Used in radiation protection guidelines
Helps estimate long-term cancer risk
Enables comparison between different imaging modalities
Widely used in patient dose communication
Conclusion
To summarize:
Absorbed Dose (Gy): How much energy is deposited
Equivalent Dose (Sv): Accounts for radiation type
Effective Dose (Sv): Estimates overall biological risk
Understanding these concepts is essential for radiology students, technologists, medical physicists, and healthcare professionals to ensure safe and effective use of radiation.
🎥 For a visual explanation, watch the full video here:
👉 https://youtu.be/VZKC37mJa4U
