Patient dosimetry techniques

Question 1

What is the equation for entrance surface dose?

Answer 1

ESD = Tube output (dose/mAs) x mAs x ISL correction x BSF.

Question 2

What is DAP? How does it vary with distance?

Answer 2

• Dose area product.
• Dose (or integral of dose as it is non-uniform) x Area of beam.
• It gives a measure of the total energy entering the patient assuming all energy is absorbed.
• It is invariant with distance assuming negligible contribution from backscatter, air interaction components and extra-focal radiation.

Question 3

How is CTDI_air measured? What is the equation for CTDI_air?

Answer 3

• With a 100 mm pencil ionisation chamber.
• CTDI_air = 1/s . int(D(x).dx) where s is the nominal slice width and the integral is that of the dose across the chamber.

Question 4

How is CTDI_w measured? What is the equation for CTDI_w?

Answer 4

• With a 100 mm pencil ionisation chamber and standard head and body phantoms with insert points for the chamber.
• CTDI_w = 1/3 . CTDI_100centre + 2/3 . CTDI_100periph.

Question 5

What is CTDI_vol?

Answer 5

CTDI_vol is the CTDI_w corrected for pitch or couch increment and the mAs used in the scan. CTDI_w will be measured for one rotation and for a different mAs, hence the correction required.

Question 6

What is DLP?

Answer 6

Dose length product is the CTDI_vol multiplied by irradiated length.

Question 7

What is the difference between incident air kerma and entrance surface air kerma? How are they related?

Answer 7

• Incident air kerma: Air kerma along central beam at position of patient without patient (i.e. no backscatter).
• Entrance surface air kerma: Air kerma along the central beam at position of patient including backscatter from patient.
• Entrance air kerma = Incident air kerma x Backscatter factor.

Question 8

What three general ways can patient dose estimates be measured or collected?

Answer 8

• Phantom measurements.
• On-patient measurements.
• Equipment dose indicators.

Question 9

What values would typically be determined using a phantom for radiography and fluoroscopy? What kind of factors should be used when determining patient dosimetry estimates using phantoms?

Answer 9

• Radiography: Entrance air kerma.
• Fluoroscopy: Entrance air kerma rate.
• Clinically relevant factors.

Question 10

What are the pros/cons of phantom measurements for patient dosimetry estimates?

Answer 10

Pros:
- Clinical staff not required.
- Results for standard sizes which can be easily compared (e.g. for different sets of equipment).
Cons:
- Does not represent actual patient data unless irradiation conditions (parameters, dimensions, setup etc.) are exactly the same.

Question 11

How is accuracy provided with equipment DAP estimates of patient dose? What are the pros and cons of equipment patient dose measurements like this?

Answer 11

• DAP calibration.

Pros:
- Easy if equipment is installed.
- ‘Real’ patient data.

Cons:
- Calibrations required.
- Results influenced by spread in patient sizes.

Question 12

What are the pros and cons of electronic dose data collection?

Answer 12

Pros:
- Easy to collect large amounts of data.
- Continuous data collection possible.
- Plenty of software available for data analysis.

Cons:
- Accuracy of data relies on dose indicators and other factors such as examination names etc.
- Not all equipment compatible.

Question 13

What are some typical uncertainties associated with measuring dose to a phantom or a single patient?

Answer 13

• Measurement setup (e.g. uncertainty in measurement position).
• Calibration, stability and energy dependence of dosimeter (e.g. temperature and pressure corrections).
• Precision of reading.
• Uncertainty in backscatter factors.
• TLD correction factors.

Question 14

What are some typical uncertainties associated with determining average dose to a cohort of patients?

Answer 14

• Measurement setup (e.g. uncertainty in measurement position).
• Calibration, stability and energy dependence of dosimeter (e.g. temperature and pressure corrections).
• Precision of reading.
• Uncertainty in backscatter factors.
• TLD correction factors.
• Dose variations between patients (e.g. due to size, examination complexity or patient numbers).

Question 15

What are ‘type A’ uncertainties? How can they be reduced?

Answer 15

• Random errors. Uncertainties relating to the standard deviation on the mean of multiple measurements. They are random and relate to the Gaussian distribution of measurement values.
• They can be reduced by increasing the number of measurements.

Question 16

What are ‘type B’ uncertainties? How can they be reduced?

Answer 16

• Systematic errors which could shift a measurement value (e.g. incorrect focal spot mark on tube housing for focus-skin distance measurement).
• Reducing type B errors is more difficult and relies on proper experimental technique and controlling the measurement process.

Question 17

What is the typical percentage uncertainty for clinical measurements of dose for a patient collective?

Answer 17

Approximately 25%.

Question 18

Why are diagnostic reference levels employed? What kind of patients and examinations do they relate to?

Answer 18

• Typical dose distributions are right-skewed (i.e. some patients get higher doses to the most typical). Due to this variation, use of dose limits is inappropriate. A diagnostic reference level flags when a dose is higher than typical to decide if the dose is excessive given the particular circumstances. It does not always mean action is required.
• They relate to standard patients (e.g. size) and standard, frequently performed examinations.

Question 19

Which DRLs are given in terms of an effective dose?

Answer 19

Mammography.

Question 20

Which statistical manipulation is used to determine a NDRL? Why? Is it always appropriate?

Answer 20

• Third quartile.
• Due to the right-skewed nature of dose distributions.
• Not always. The method used may need updating in the future due to the fact there is likely to become a point when doses can no longer be reduced.

Question 21

How are LDRLs typically set? When should an investigation be undertaken in relation to exceeding an LDRL?

Answer 21

• Setting LDRLs depends on the amount of data available. They will typically be set by the mean of the distribution of the room mean doses in an organisation. Values are rounded upwards with no more than 2 significant figures.
• Investigations should take place when an LDRL > NDRL and if an individual room LDRL > 20% and 2x the standard error on the mean.

Question 22

Why are patient dose audits carried out?

Answer 22

• IR(ME)R requirement.
• Get an idea of radiological practice.
• Compare doses to those applied in other hospitals/countries.
• To verify/define or check compliance DRLs.
• Identify areas potentially requiring optimisation.

Question 23

What initial checks need to be made on patient dose audit data?

Answer 23

• Exam specifications (e.g. exam names may be different for the same exam at different departments, multiple CT phases etc.).
• Dose units.
• Data sample (e.g. exclusion of very small/large patients, complex cases etc.). Not as much of an issue with very large electronically collected datasets as a small number of outliers will have little effect on the final result.
• General data sense checks.

Question 24

What is a typical minimum sample size for a patient dose audit?

Answer 24

10 patients.

Question 25

What should be considered with regards to the feedback of patient dose audit data?

Answer 25

• Regular feedback to optimisation group.
• Comparisons with other departments and DRLs.
• Link to equipment QA measurements.
• Uncertainties in data (e.g. number of significant figures used).
• Clarity in dose terminology.