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Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust.

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Page 1: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Ion recombination

Robert Brackenridge,

The Princess Royal Hospital,

Hull & East Yorkshire NHS Trust.

Page 2: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Synopsis

• Measurement of dose in radiotherapy

• How ion chambers enable this

• Theory of ion recombination

• Practical experience

• Summary, conclusions and questions

Page 3: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Measurement of dose

• Accurate and precise measurement critical in radiotherapy

• Ensures local calibration of treatment machines

• Enabling correct dose for treatments, avoiding accidents, and facilitating the availability of new treatments

Page 4: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Ion chamber design

• Radiation liberates charge within a medium

• Collection of charge allows determination of dose

• Liberated charge collected by electrodes held at potential

Page 5: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Ion chamber design

Page 6: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Ion chamber signal corrections

• Calibration factor uses standard conditions

• Corrections needed for local environmental conditions and incomplete charge collection

Page 7: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Dose determination

• Incident radiation causes ionisation of neutral gas molecules in chamber

• Dose to sensitive volume proportional to charge liberated within volume

• Collect charge with electric field

Page 8: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

How large a potential?

• Magnitude of potential an important choice• If potential too low,

– recombination of charge before collection, and dose is underestimated

• If potential too large, – ionisation by collision is induced, and dose is

overestimated

Page 9: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

How large a potential?

Page 10: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Saturation voltage

• Between 200-400 volts

• Charge collection maximised

• Ionisation by collision minimised

• Significant recombination may occur especially for linac delivered radiation

Page 11: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Recombination corrections

• Accept and correct for such losses

• Calculated analytically

• Measured using the ‘Two voltage technique’

Page 12: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Measuring ion recombination

• Measured using the ‘Two voltage technique’ for a NE 2751 Farmer chamber

• Result – 2.8 %!

• Previously measured 0.7 %

Page 13: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Previous results

Historical ion recombination values for NE 2571 chamber s/n 3234

Date Value

Oct 06 1.028

June 06 1.006

March 06 1.007

Dec 05 1.008

Feb 05 1.007

Feb 04 1.006

Jan 03 1.004

Nov 02 1.008

Page 14: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Practical experience

• Measured using the ‘Two voltage technique’ for a NE 2751 Farmer chamber

• Result – 2.8 %!

• Previously measured 0.7 %

• Previous value consistent over four years

• Calculated value (Boag) 0.6 %

Page 15: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Why the large value?

• Faulty experimental setup?

• Independent check– Confirmed 2.8 %

• Measure again– Again 2.8 %

Page 16: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Possible explanation

• A displaced central electrode?

• A series of diagnostic x-rays obtained

Page 17: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Bent electrode?

Page 18: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Boag’s formula

f ion = u / ln (1+u)

u = cmd^2 / V

c = constant

m = dose per pulse

d = electrode spacing

V = polarising voltage

Page 19: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Boag’s formula

• Ion recombination value most sensitive to electrode spacing

Page 20: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Summary and conclusions

• The importance of being able to measure dose• How ion chambers enable this• Corrections for environmental conditions and

incomplete ion collection• Two methods to determine collection efficiency• A possible explanation for large measured values• Don’t bend electrodes!

Page 21: Ion recombination Robert Brackenridge, The Princess Royal Hospital, Hull & East Yorkshire NHS Trust

Any questions?