Small Fields

Question 1

Why do small fields require special consideration?

Answer 1

Normal assumptions don’t hold, ie calibration chain, isodoses following TPS, TRP dose in prescription, MLC calibration at isocentre, different chamber readings, FWHM field size definition doesn’t apply, no primary standard

Question 2

What field size counts as small?

Answer 2

If the width is such that lateral electronic equilibrium no longer holds or if standard measuring devices are no longer suitable

Question 3

What applications use small fields?

Answer 3

IMRT, Cyberknife, Tomotherapy, SRS

Question 4

What happens when the flattening filter view is restricted?

Answer 4

The source of scatter reduces
Energy spectrum changes
Fluence drops sharply
Rate of reveal is collimator dependent

Question 5

What happens when the extended source is partially occluded?

Answer 5

The output drops and the gaussian shape of the beam at the flattening filter is lost

Question 6

What happens if lateral equilibrium is lost?

Answer 6

TPS may not model appropriately

It is energy dependent

Question 7

What type of detector should be used for small fields?

Answer 7

Diode to prevent volume averaging effects
Film but scanner limits resolving power to ~1mm
Mini liquid ionisation chamber
Mini ionisation chambers (0.01cc, no metal electrode)
Need to correct for anisotropy and scan hysteresis

Question 8

How does the output change for small fields?

Answer 8

Drops 10% pr jaw for a 2cm wide field as it occludes the source
The TPS needs to model source size to predict the central axis dose and that the FWHM definition no longer applies for the field size

Question 9

How is field size defined for small fields?

Answer 9

Use collimator position as individual jaw 50% < FWHM

Question 10

How are relative dose measurements performed?

Answer 10

Take ratio of lateral electronic equilibrium loss

The material, position and size of detector introduces errors which need to be corrected for

Question 11

Why are there different dose measurements in large and small fields?

Answer 11

MC simulations show density dependent differences in stopping power ratios between detector and water. Can be up to 26% difference for 0.5cm field when comparing materials with the same stopping power but different density
Also, volume averaging effects

Question 12

What issues do small detectors have?

Answer 12

Cause density perturbation
Produce very small signals
Cable/Stem signals can be unacceptable

Question 13

What is the method for measuring PDD in small fields?

Answer 13

Check source alignment
Measure on beam axis, possibly with serpentine scanning
Corroborate with multiple methods
Correct energy dependence for increase in low energy scattered photons

Question 14

What factor can be ignored when using an ionisation chamber to measure Spc?

Answer 14

Th perturbation factor

Question 15

What needs to be introduced when measuring Spc with a diode?

Answer 15

Factor for cavity theory:

KdetA,Aref = (d.(Sdelta/denstiy)med,det + (1-d)(uen/density)med,det)A,Aref

Question 16

How does the mass energy absorption factor change?

Answer 16

Decreases in harder beam and depends on QI. Changes quickly at low energy

Question 17

How is Sc measured in small fields?

Answer 17

Ratio of primary collision water KERMA in free space between clinical and reference fields.

Question 18

What issues are faced when measuring Sc in small fields?

Answer 18

Need LEE
Complete phantom must be enclosed by smallest field
Rapid fall of dose as source occluded and beam hits phantom edge but this doesn’t mean the measurement is correct - must extrapolate to zero
Need to ensure in centre of beam - use EPID to verify
Cable is directly exposed

Question 19

What are the recommendations for measuring Sc in small fields

Answer 19

Use small detectors: ionisation chamber (air or liquid), diodes
Use high density cap if energy <18MV
Scan for max signal, and ensure the sensitive volume is at the centre of the detector
Confirm data extrapolates to zero corroborate with multiple detectors, overlap farmer chamber with unit density phantom

Question 20

How is Sp measured in small fields?

Answer 20

Scp/Sc, this compensates for errors in Sc
O’Connor’s density scaling theorem: if the field size and SSD are scaled inversely with density the scatter to primary ratio is constant, so Sp(p1xr1) = Sp(p2xr2) if p1r1=p2r2

Question 21

What issues can the TPS have for small fields?

Answer 21

Leaf edge modelling errors
Assumptions of FWHM for small fields
Too coarse a selection of calculation matrix
Inaccurate modelling of source size
Assumed shielding for small fields