Secondary standard calibrations in terms of absorbed dose to water in Co-60 beam

At LNK Ghent the secondary standard calibrations in term of absorbed dose to water in ⁶⁰Co beam are done according to the TRS 398 protocol (Ref. 1). Three NE2571 ionization chambers are used at LNK Gent as secondary standard. These chambers are regularly calibrated against the water calorimeter (primary standard) of VSL Delft, The Netherlands. The combined expanded uncertainty on the N_D,W calibration factor for the chamber to be calibrated is 1.3 % for k = 2 (confidence level 95.4 %). For the routine calibrations no correction is done for the polarity of the voltage (k_pol) and for the recombination (k_s). The polarization voltage requested by the user is used during the calibration. The recombination effect is considered negligible (les than 0.1 %) for the ⁶⁰Co beam quality.

Using the notations from the TRS 398 protocol, the absorbed dose to water (D_w,Q) at the reference depth z_ref for a user beam quality Q and in the absence of the chamber is given by:

D_w,q = M_corr,Q N_D,W,Q0 k_Q,Q0

where Q₀ is the reference beam quality (⁶⁰Co in the present file), M_corr,Q is the corrected reading of the electrometer in the user beam quality Q and N_D,W,Q0 is the calibration factor of the ionization chamber. k_Q,Q0 corrects for the effects of the difference between the reference beam quality Q₀ and actual user quality Q . k_Q,Q0 = 1 when the user beam quality Q and is the same as the reference beam quality Q . Then Eq.1 becomes:

D_w,q0 = M_corr,Q0 N_D,W,Q0

Because all the calibrations described here are done in a ⁶⁰Co beam the beam quality index Q₀ is neglected and Eq. 2 becomes:

D_w = M_corr N_D,W

where M_corr is the reading of the electrometer under the reference conditions used in the standard laboratory and N_D,W is the calibration coefficient in terms of absorbed dose to water in the reference beam quality Q_0. The reading of the electrometer has to be corrected for the influence quantities: atmospheric pressure, temperature, humidity, electrometer calibration, polarity effect and ion recombination. Therefore the reading of the electrometer M can be written as:

M_corr = M ∏k_i

where M is the uncorrected reading from the electrometer expressed in nC or rdg (readings) and ∏k_i is a product of correction factors for the influence quantities.

The absorbed dose to water at the reference depth z_ref can be written as:

D_wr = N_D,W M ∏k_i , where

∏k_i = k_TP k_h K_elec k_pol k_s and

• k_TP is the correction factor for temperature and atmospheric pressure
• k_h is the correction factor for the atmospheric humidity
• k_elec is the calibration factor of the electrometer in nC/nC or in rdg/nC (readings/nC)
• k_pol is the correction factor for the polarity effect
• k_s is the correction factor for the ion recombination

To verify the stability of the ionization chambers during the calibration procedure, the reference absorbed dose to water is measured with at least 2 secondary standard ionization chambers, one before and one after the measurement with the chamber to be calibrated. Therefore, the reference absorbed dose to water is an average of the two chambers.

The reference conditions used at LNK Ghent for the calibration of ionization chambers in terms of absorbed dose to water in ⁶⁰Co beam are in agreement with the current protocols used by the radiotherapy departments (TRS-398, NCS-8):

Bibliography

[1]. IAEA, "Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water", Technical Report Series 398 (Vienna IAEA) 2000.

[2]. NCS-18 -Netherlands Commission on Radiation Dosimetry – "Code of practice for absorbed dose determination in high energy photon and electron beams" – January 2008