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 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 DateValue Oct June March Dec Feb Feb Jan Nov

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?