ion recombination robert brackenridge, the princess royal hospital, hull & east yorkshire nhs...
TRANSCRIPT
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
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
Ion chamber design
• Radiation liberates charge within a medium
• Collection of charge allows determination of dose
• Liberated charge collected by electrodes held at potential
Ion chamber design
Ion chamber signal corrections
• Calibration factor uses standard conditions
• Corrections needed for local environmental conditions and incomplete charge collection
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
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
How large a potential?
Saturation voltage
• Between 200-400 volts
• Charge collection maximised
• Ionisation by collision minimised
• Significant recombination may occur especially for linac delivered radiation
Recombination corrections
• Accept and correct for such losses
• Calculated analytically
• Measured using the ‘Two voltage technique’
Measuring ion recombination
• Measured using the ‘Two voltage technique’ for a NE 2751 Farmer chamber
• Result – 2.8 %!
• Previously measured 0.7 %
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
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 %
Why the large value?
• Faulty experimental setup?
• Independent check– Confirmed 2.8 %
• Measure again– Again 2.8 %
Possible explanation
• A displaced central electrode?
• A series of diagnostic x-rays obtained
Bent electrode?
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
Boag’s formula
• Ion recombination value most sensitive to electrode spacing
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!
Any questions?