Verification techniques and QA

Question 1

Define random and systematic errors, and how can they be reduced? (6 marks)

Answer 1

Random errors:

• if it is a random error then it does not propagate through the system (otherwise it would be systematic).
• it varies each time and is not repeatable e.g. the random variation in patient positioning.
• can be reduced in terms of positioning errors by imaging the patient before each treatment and ensuring they are in the correct position.

Systematic errors:

• they propagate through the system and appear as a constant offset in the accurate value or position.
• for example an incorrect delineation from the clinicians or laser mis-alignment.
• image patient on consecutive days and re-adjust position based on the constant offset from the accurate position.

Question 2

Give four different verification methods and place them in order of decreasing resolution, as well as order of decreasing contrast? (4 marks)

Answer 2

Best to worst res:

kV film -> kV EPID -> MV EPID -> CT DRR

Best to worst contrast:

kV film -> kV EPID -> CT DRR -> MV EPID

DRR = digitally reconstructed radiographs…

Question 3

Give a recent development in verification techniques? (2 marks)

Answer 3

Advances in tumour motion tracking for SABR treatments (imaging of a dose cloud).

Question 4

What is meant by the terms QA, QC and QMS? (3 marks)

Answer 4

QMS = principles and processes which guide delivering a service.

QA = establishes and ensures that standards are maintained, such as a document outlining tolerances and specifies the frequency of tests.

QC = actually performing tests and analysis to ensure that the required standards are met.

Question 5

Why do we do QC in RT? (2 marks)

Answer 5

To ensure that the right amount of dose is going to the right place, e.g. 95-107 % prescribed dose to 5-10 mm tolerance.

Question 6

What QA is performed on CT and the TPS system? (5 marks)

Answer 6

CT QA: filtration measurements, checking geometry is correct (couch and laser positioning), image quality (CNR, res etc),

TPS QA: ensuring that data transfer has occurred without error including beam set-up etc, independent dose checks, recalculating a standard plan

Question 7

What is the FMEA approach to QA? What issues are there with it? (4 marks)

Answer 7

Failure Model Effect Analysis: most errors occur to issues with the process as a whole. Try to map out processes and identify, and quantify, where errors are most likely to occur and address these problems.

Issues are the time required and complexity of the whole system makes this difficult. Arbitrarily quantifying the potential for errors may not be that repeatable.