[Cat 1] Demonstrate knowledge of the function and interpret the values of specific dose measurement methods used for radiological procedures. Explain the implications of measured dose parameters, both in terms of overall risk and the risk to specific tissues and organs. Be aware of the relative radiation doses from different radiological procedures, and how they compare to natural background radiation doses.
[Cat 1] Examine the mechanism of how radiation interacts with tissue to cause biological damage, and the parameters used to quantify this damage.
[Cat 1] Demonstrate knowledge of the hereditary and genetic implications of radiation exposure.
[Cat 1] Demonstrate knowledge of the stochastic effects of radiation and the factors which influence it. Assess the approximate risk from a radiation exposure and explain how to convey this risk in a simple manner to patients and other staff.
[Cat 1] Demonstrate knowledge of the deterministic effects of radiation and the factors which influence it.
[Cat 1] Identify the procedures that may deliver large doses of radiation.
[Cat 1] Demonstrate knowledge of the effects of radiation on the developing embryo and foetus at various stages of gestation. To be aware of which procedures may deliver large doses to the embryo/foetus, and the actions to be taken in considering dose to a pregnant patient, prospectively or retrospectively.
[Cat 2] Explain the importance and application of the dose descriptors:
• Dose-area product (DAP)
• CT Dose Index (CTDI)
• Dose length product (DLP)
Dose Quantities
1. Exposure
Exposure (X) is the amount of ionisation created in air.
2. Air KERMA
Kinetic Energy Released per unit MAss is the amount of energy transferred from radiation to matter, measured in Joules (J) per kg.
This is better expressed as the SI unit Gray (Gy) where 1 Gy = 1 J.kg-1
3. Absorbed dose
Absorbed dose (D) is the amount of energy deposited in matter, measured in Gy. It is not however a good indicator of likely biological effect.
Recall alpha particles are heavily charged and slow, so deposit energy densely in their path through tissue, i.e. they have a high linear energy transfer (LET). Hence, 1 Gy of alpha radiation is more damaging than 1 Gy of photon radiation by a factor of 20.
When radiation interacts with tissue/matter (as opposed to air), there is a small amount of back-scattered radiation (about 35% for diagnostic x-rays), therefore absorbed dose > air kerma.
4. Equivalent dose
Equivalent dose (HT) is the amount of radiation dose to tissue which accounts for the relative biological effects (RBE) of different types of ionising radiation by averaging the absorbed/organ doses (D) and multiplying by a radiation weighting factor (WR).
Equivalent dose (HT) = D × WR
Measured in Sievert (Sv), where 1 Sv represents a 5.5% chance of eventually developing cancer, D = absorbed dose, WR = radiation weighting factor.
| Radiation Type | Radiation Weighting Factor (WR) |
| X-rays and γ rays | 1 |
| Electrons and positrons | 1 |
| Neutrons | 5 – 20 (varies with energy) |
| α particles, heavy nuclei | 20 |
Equal absorbed dose does not mean equal biological damage.
5. Effective dose
Effective dose (E) is the tissue-weighted (WT) sum of the equivalent doses in all specified tissues and represents an overall stochastic health risk to the whole body. The tissue/organ weighting factors depends on the radiosensitivity of the organ to stochastic effects of radiation.
Effective dose (E) = Σtissues(D × WR × WT)
Measured in Sievert (Sv), where 1 Sv represents a 5.5% chance of eventually developing cancer, D = absorbed dose, WR = radiation weighting factor, WT = radiation weighting factor.
| Organs | Tissue weighting factors | ||
|---|---|---|---|
| ICRP26 1977 |
ICRP60 1990 |
ICRP103 2007 |
|
| Gonads | 0.25 | 0.20 | 0.08 |
| Red Bone Marrow | 0.12 | 0.12 | 0.12 |
| Colon | – | 0.12 | 0.12 |
| Lung | 0.12 | 0.12 | 0.12 |
| Stomach | – | 0.12 | 0.12 |
| Breasts | 0.15 | 0.05 | 0.12 |
| Bladder | – | 0.05 | 0.04 |
| Liver | – | 0.05 | 0.04 |
| Oesophagus | – | 0.05 | 0.04 |
| Thyroid | 0.03 | 0.05 | 0.04 |
| Skin | – | 0.01 | 0.01 |
| Bone surface | 0.03 | 0.01 | 0.01 |
| Salivary glands | – | – | 0.01 |
| Brain | – | – | 0.01 |
| Remainder of body | 0.30 | 0.05 | 0.12 |
| Total | 1.00 | 1.00 | 1.00 |
Limitations:
- Effective dose is derived by models (e.g. Monte Carlo modelling) and simulations using anthropomorphic phantoms representing idealised anatomical forms in terms of size, shape and position of tissue. Therefore, it calculates radiation risks to the reference person based on population averages, not individual risks which depends on factors such as age and sex.
- Similarly, the tissue weight factors were developed for a population of both genders and wide range of ages
- The scan region (i.e. area irradiated) is inputted breakdown
Advantages:
- Best quantity for describing biological relevance of radiation exposure where different tissues/organs receive varying doses
- Related to the probability of health determine i.e. associated with the risk of radiation-induced cancer
- Can be used to compare the dose from different modalities
In the pregnant patient the effective dose to the fetus is most important. This is approximately equal to the absorbed dose or equivalent dose to the patient’s uterus.
6. Background radiation
We receive 1 – 2 mSv per year from naturally occuring and man-made sources of radiation.
Natural sources:
- Radionuclides in air (Radon)
- Internal radionuclides (Potassium-40)
- External Gamma (soil, rocks, building materials)
- Cosmic rays 0.3 mSv per year (about 4μSv.hr-1 when flying)
- Pilots receive 2 – 5 mSv per year vs. medical imaging staff <1mSv
Man-made sources:
- Medical radiation (X-rays, nuclear medicine)
- Approximately 0.4 mSv
- Consumer products (smoke detectors)
- Nuclear industry (Fallout 0.006 mSv, disposals 0.0009 mSv)
- Technologically enhanced sources
| Equivalent to | |||
| Radiology examinations | Effective dose range (mSv) | Exposure to natural background radiation (2 mSv per year) | 7 hour flights (0.05 mSv per 7 hours flight) |
| MRI and US | No radiation | N/A | N/A |
| X-ray tooth (dental film) | ~ 0.004 | < 1 day | < 1 time |
| X-ray jaw (OPG) | ~ 0.014 | < 3 days | < 1 time |
| X-ray chest (1 image) | ~ 0.02 | < 4 days | < 1 time |
| X-ray chest (2 images) | ~ 0.04 | < 8 days | < 1 time |
| X-ray extremities / X-ray skull / X-ray cervical spine (neck) | 0 to 0.1 | 0 to 18 days | < 2 times |
| X-ray thoracic spine (middle spine) X-ray lumbar spine (lower back) (1 image) X-ray abdomen X-ray pelvis Mammography (2 images) | 0.1 to 1 | 18 days to 6 months | 2 – 20 times |
| Barium swallow / Barium meal CT head CT cervical spine CT chest (without portal liver phase) | 1 to 5 | 6 months to 2.5 years | 20 – 100 times |
| Angiogram-coronary/pulmonary Angioplasty coronary Barium enema CT chest (with portal liver phase) CT renal (KUB) CT abdomen and/or pelvis (single image) CT thoracic spine or lumbar spine | 5 to 10 | 2.5 years to 5 years | 100 – 200 times |
| Angiogram-abdominal Aortography-abdominal CT chest/abdomen/pelvis CT abdomen / pelvis (multiple images) CT pulmonary angiogram / CT coronary angiogram | > 10 | > 5 years | > 200 times |
Diagnostic Reference Level
A diagnostic reference level (DRL) is an indicative dose that is not expected to be exceeded under normal imaging conditions for a given diagnostic task.
DRL’s are determined based on the results of wide-scale surveys of the median doses representing typical practice for a patient group (e.g. adults or children of different sizes) at a range of representative healthcare facilities for a specific type of examination or procedure. The information is then used to calculate the facility reference levels (FRLs) for those surveys. The DRLs are based on the 75th percentile of the resulting FRL distributions.
If a practice’s FRL exceeds the DRL for a particular protocol, this means that patients are receiving a higher dose than 75% of Australian imaging facilities for that procedure
Table: Quantities suitable for setting DRLs
| Quantity | Recommended symbols |
Recommended unit |
Other common symbols used in literature |
Closely similar quantity |
| Entrance surface air kerma |
Ka,e | mGy | ESAK | Entrance- surface dose (ESD)* |
| Incident air kerma |
Ka,i | mGy | IAK | |
|
Incident air |
Ka,r | Gy | CAK (Cumulative air kerma) |
|
| Air kerma-area product |
PKA | mGy.cm2 (radiography and dental), Gy.cm2 (fluoroscopy) |
KAP | Dose-area product (DAP)* |
| Volume computed tomography dose index |
CTDIvol | mGy | Volume CT air kerma index (Cvol)* |
|
| Dose-length product |
DLP | mGy.cm | Air kerma-length product (PKL)* |
|
| Mean glandular dose |
DG | mGy | MGD, AGD |
* “Air kerma” and “dose in air” are numerically equal in diagnostic radiology energy range.
** Also names “cumulative dose”, “reference air kerma” and “reference point air kerma” have been used in the literature. These quantities are not patient doses that can allow estimation of risk to individuals, but are dose indicators characterizing radiation exposure for the purposes of comparison of practice. There is no merit in setting DRLs in terms of other dose quantities, such as effective dose, that are derived from the well-defined monitoring quantities by coefficients that could vary depending on the particular dose model adopted.