Question

When an ion chamber is used for reference dosimetry measurements, what are the required corrections to convert electrometer reading into dose? Why we use each of them.

Answer 1

Electrometer calibration (Kelec ) Values An electrometer calibration correction factor is employed when the ionization chamber and the electrometer are calibrated separately. (If they are calibrated together, (Kelec) Q is not required.) The calibration certificate states that (Kelec) Q = 1.000 ± 0.25%. Uncertainties The estimated standard uncertainty is 0.14%, assuming a rectangular distribution with the variation limits set at ±0.25%.

Corrected reading M*Q

The components that we took into account were • charge reading, • monitor chamber, • pressure, • temperature, • relative humidity, • electrometer calibration, • polarity, and • saturation.

Charge reading

Values MQ = MDisplay − MLeakage ≅ MDisplay

The value of MLeakage can be determined by measuring the collected charge in the absence of radiation after and before the irradiation. That reading can then be used to verify that the reading from the electrometer is MDisplay ≥ 103 MLeakage, as recommended in TRS-398, and so on, accomplishing condition.

Uncertainties

(MQ) = u2 (MDisplay) + u2 (MLeakage)

Air kerma based protocol:

Air kerma based protocols use the air kerma in air calibration coefficient NK, Co-60 obtained for a local reference ionization chamber in a Co-60 beam at a standards laboratory. Routine ionization chambers are then cross-calibrated with the reference ionization chamber in a local Co-60 beam. Two steps are involved in an air kerma based protocol for the calibration of megavoltage photon beam

Absorbed dose to water based protocol:

All dosimetry protocols aim at determination of the quantity absorbed dose to water. It is therefore logical to provide ionization chambers directly with a calibration coefficient in terms of this quantity, rather than in terms of the air kerma in air, if at all possible. Recent developments have provided support for a change in the quantity used at present to calibrate ionization chambers and provide calibration coefficients in terms of absorbed dose to water ND,W for use in radiotherapy beams. Many PSDLs now provide ND,W