ESR/alanine dosimetry: response for ultrahard photon radiation

The properties of the secondary standard measuring system, which is based on the detection of radiation-induced free radicals in alanine by means of electron spin resonance (ESR) spectroscopy, are well known for irradiations in the ⁶⁰Co reference field [1]. To be able to use the system also in the radiation fields generally used in therapy, the response for these radiation qualities must also be indicated. Since calibration is done in ⁶⁰Co, it is sufficient to determine the relative response r related to ⁶⁰Co.

For this purpose, a series of irradiations was carried out in PTB’s reference fields: approximately at the same time as the calibration irradiation, alanine dosimeter probes were irradiated in the electron linear accelerator - and this by means of ultrahard photon radiation. The nominal accelerating voltage thereby used was of 8 MV or 16 MV (in the following referred to as 8 MV and 16 MV in short). The transmission of the dose rate, which was previously determined with the aid of the water calorimeter, was carried out by means of an ionisation chamber (NE 2571, farmer-type chamber) and of the monitoring large-area transmission ionisation chamber of the accelerator. The calibration of the large-area transmission ionisation chamber was checked with the aid of the farmer-type chamber in-between the irradiations of the alanine probes. The reproducibility for the irradiation in the accelerator could thereby be quantified with a measurement uncertainty of 0.12%. The relative response was determined by relating the dose D^Co, which was obtained with the aid of the irradiated alanine probes after a ⁶⁰Co calibration, to the nominal dose D^nom, traced back to the calorimeter, i.e.

r = D^Co/D^nom.

(Here and in the following, D stands for the absorbed dose to water D_w). The measurement was repeated several times. For the relative response, values of r = 0.9959 ± 0.0034 and r = 0.9967 ± 0.0027 for 8 MV / 16 MV were yielded. The uncertainties which could be obtained for these measurements are smaller than the data which have been published to date (the indication states the standard measurement uncertainty). The essential contribution is thereby due to the uncertainty of the correction factor k_Q which is of 0.26% [2]. Between 8 and 16 MV, no significant changes of the relative response were observed. All in all, the response for ultrahard photon radiation is smaller than the response for ⁶⁰Co by approx. 0.4%. The most exhaustive measurements to date had been carried out by NPL (4 MV - 12 MV) stating a quality-independent response of r = 0.994 ± 0.007 for ultrahard photon radiation. PTB’s current measurement values are thus higher by 0.2% but all in all, the expected, extremely low dependence of the response on the radiation quality has been confirmed.

In addition, Monte Carlo simulation calculations were carried out. For this purpose, the experimental set-up was simplified and transferred onto a cylinder-symmetrical geometry so that the user programme dosrznrc of the programme package EGSnrc could be used. The representation of the structure of the alanine probes which is actually pitted was simplified and represented as a homogeneous mixture of alanine and binding material with the density of the tablets. The results of the simulation, however, show a surprisingly good agreement with the experimentally determined results; the calculated values for the relative response are r = 0.9963 ± 0.0028 and r = 0.9967 ± 0.0026 for 8 MV and resp. 16 MV. The uncertainties stated are purely statistical uncertainties which are obtained on the basis of the number of calculated histories.

These results allow the use of the secondary standard measuring system for the clinical use of ultrahard photon radiation.

Literature

1. Anton, M.:
   Uncertainties in alanine/ESR dosimetry at PTB,
   Phys. Med. Biol. 51 S. 5419-5440 (2006)
2. Krauss, A. and Kapsch, R.-P.:
   Calorimetric determination of k_Q factors for NE 2561 and NE2571 ionization chambers in 5 cm x 5 cm and 10 cm x 10 cm radiotherapy beams of 8 MV and 16 MV photons,
   Phys. Med. Biol. 52, S. 6243-6259 (2007)