QA in RT

Question 1

Which laws means you have to do QA?

Answer 1

• IRR2017
• Approved code of practice (L121)
• Guidance notes: Equipment (PM77)

Recommendation: Manual of cancer standards 2004

Question 2

Dosimetric Accuracy level

What is it?

Who defines it?

Answer 2

• 5% uncertainty in delivery of dose to target volume
• Accuracy of dose delivery is 5-7% (95% CI)
• ICRU Report 24

Question 3

Geometric Accuracy level

What is it?

Who defines it?

Answer 3

• 5-10mm
• ICRU

Question 4

CT QA:

Lasers

Answer 4

• Lasers used to position tattoo markers
• need to be accurate

Question 5

CT QA:

Couch

Answer 5

Couch shifts used to get from reference tattoos to isocentre

shifts have to be accurate

Question 6

CT QA:

Geometric Planning

Answer 6

• CT is used to plan
• scan phantoms and make simple measurements to assess distortions

Question 7

CT QA:

Density

Answer 7

• Measurements of known electron densities
• Create CT to electron density curce

Question 8

What’s the best way to test the entire chain and validate the whole process?

Answer 8

Use a phantom and put it through every single step

e.g. IPSM phantom

Question 9

LINAC QA:

What do we measure performance against?

Answer 9

Performance standards

• IEC60976: mechanicals, flatness, symmetry

Standards established during Commissioning

• beam energy, output

Question 10

LINAC QA:

6 sources of mechanical uncertainty in LINAC

Answer 10

1. Jaw calibration (2mm)
2. light field (2mm)
3. lasers (1mm)
4. col rotation (0.5deg)
5. gantry rotation and sag (0.5deg)
6. table movements (2mm, 0.5deg)

Question 11

LINAC QA:

4 Dosimetric uncertainties in LINAC

Where do the tolerances come from?

Answer 11

1. Radiation Output - Absorbed dose to water/MU (3%)
2. Beam Energy - TPR20/10 (1%)
3. Beam Flatness and Symmetry - Baseline profile (%2)
4. Dose delivery stability - Ouput with rotation (0deg, 2%), ouput with linearity (2%)

IPEM 81

Question 12

Daily QC (x4)

Answer 12

1. Output constancy
2. Safety interlocks
3. laser alignment
4. Light field and & cross-wires

Question 13

• Monthly QC (x4)

Answer 13

• Phtons
• electrons
• mechanicals
• on-board imaging

Question 14

Photon monthly QC (x3)

Answer 14

1. Output
2. Energy
3. Flatness & Symmetry

Question 15

Electron monthly QC (x4)

Answer 15

• Output
• Energy
• Applicator output factors
• Flatness & Symmetry

Question 16

mechanicals monthly QC (x5)

Answer 16

1. Cross-wire
2. light field
3. lasers
4. jaws
5. couch