Calculated beam quality correction factors for ionization chambers in MV photon beams

Experimental and Monte Carlo-based determination of magnetic field correction factors k B , Q $k_{B,Q}$ in high-energy photon fields for two ionization chambers

BACKGROUND

The integration of magnetic resonance tomography into clinical linear accelerators provides high-contrast, real-time imaging during treatment and facilitates online-adaptive workflows in radiation therapy treatments. The associated magnetic field also bends the trajectories of charged particles via the Lorentz force, which may alter the dose distribution in a patient or a phantom and affects the dose response of dosimetry detectors.

PURPOSE

To perform an experimental and Monte Carlo-based determination of correction factors k B , Q $k_{B,Q}$ , which correct the response of ion chambers in the presence of external magnetic fields in high-energy photon fields.

METHODS

The response variation of two different types of ion chambers (Sun Nuclear SNC125c and SNC600c) in strong external magnetic fields was investigated experimentally and by Monte Carlo simulations. The experimental data were acquired at the German National Metrology Institute, PTB, using a clinical linear accelerator with a nominal photon energy of 6 MV and an external electromagnet capable of generating magnetic flux densities of up to 1.5 T in opposite directions. The Monte Carlo simulation geometries corresponded to the experimental setup and additionally to the reference conditions of IAEA TRS-398. For the latter, the Monte Carlo simulations were performed with two different photon spectra: the 6 MV spectrum of the linear accelerator used for the experimental data acquisition and a 7 MV spectrum of a commercial MRI-linear accelerator. In each simulation geometry, three different orientations of the external magnetic field, the beam direction and the chamber orientation were investigated.

RESULTS

Good agreement was achieved between Monte Carlo simulations and measurements with the SNC125c and SNC600c ionization chambers, with a mean deviation of 0.3% and 0.6%, respectively. The magnitude of the correction factor k B , Q $k_{B,Q}$ strongly depends on the chamber volume and on the orientation of the chamber axis relative to the external magnetic field and the beam directions. It is greater for the SNC600c chamber with a volume of 0.6 cm³ than for the SNC125c chamber with a volume of 0.1 cm³ . When the magnetic field direction and the chamber axis coincide, and they are perpendicular to the beam direction, the ion chambers exhibit a calculated overresponse of less than 0.7(6)% (SNC600c) and 0.3(4)% (SNC125c) at 1.5 T and less than 0.3(0)% (SNC600c) and 0.1(3)% (SNC125c) for 0.35 T for nominal beam energies of 6 MV and 7 MV. This chamber orientation should be preferred, as k B , Q $k_{B,Q}$ may increase significantly in other chamber orientations. Due to the special geometry of the guard ring, no dead-volume effects have been observed in any orientation studied. The results show an intra-type variation of 0.17% and 0.07% standard uncertainty (k=1) for the SNC125c and SNC600c, respectively.

CONCLUSION

Magnetic field correction factors k B , Q $k_{B,Q}$ for two different ion chambers and for typical clinical photon beam qualities were presented and compared with the few data existing in the literature. The correction factors may be applied in clinical reference dosimetry for existing MRI-linear accelerators.

Evaluating the impact of ionization chamber-specific beam quality correction factor in dosimetry of filtered and unfiltered photon beams

Aim:

The response of ionization chamber changes when used at beam quality Q which is different from beam quality Q_o (usually ⁶⁰Co) that was used at the time of its calibration. Hence, one needs to apply beam quality correction factor (k_{Q, Qo}) during dosimetric measurements. However, k_{Q, Qo} data are unavailable for novel ion chambers in the literature. Moreover, most of such data do not differentiate between filtered (flat) and unfiltered (unflat) beams. In addition, literature-based data do not differentiate among different pieces of the ion chambers of the same make and model. Hence, the purpose of our study was to determine the ion chamber-specific experimental values of k_{Q, Qo} and to evaluate their impact in dosimetry.

Materials and Methods:

In this work, the value of k_{Q, Qo} were measured for six ionization chambers of three different types in 6, 10, and 15 MV filtered (with flattening filter [WFF]) as well as 6 and 10 MV unfiltered (flattening filter free [FFF]) photon beams. The measured values of k_{Q, Qo} were compared with Monte Carlo-calculated values available in the literature. The uncertainties in measurement of k_{Q, Qo} values were also evaluated.

Results:

For 6 MV FFF beam, the measured value of k_{Q, Qo} was found to be consistently lower than 6 MV WFF beam for all Sun Nuclear Corporation ion chambers, while it was higher as per the theoretical data. The inter-chamber variation in k_{Q, Qo} values was observed for the same model of the ion chambers. The maximum difference between absolute dose values on using the theoretical and experimental k_{Q, Qo} values was up to 3.23%.

Conclusion:

The measured absolute dose values by the ion chamber of a given make and model were found different due to the use of its theoretical and experimental k_{Q, Qo} values. Furthermore, the variation in response of different pieces of ion chambers of the same make and model cannot be accounted for theoretically, and hence, the use of theoretical k_{Q, Qo} data may introduce an inherent error in the estimation of absorbed dose to water. This necessitates the use of measured value of k_{Q, Qo} for each ionization chamber.

Monte Carlo calculation of quality correction factors based on air kerma and absorbed dose to water in medium energy x-ray beams

Clinical dosimetry is typically performed using ion chambers calibrated in terms of absorbed dose to water. As primary measurement standards for this quantity for low and medium energy x-rays are available only since a few years, most dosimetry protocols for this photon energy range are still based on air kerma calibration. For that reason, data for beam quality correction factors [Formula: see text], necessary for the application of dose to water based protocols, are scarce in literature. Currently the international IAEA TRS-398 Code of Practice is under revision and new [Formula: see text] factors for a large number of ion chambers will be introduced in the update of this protocol. Several international groups provided the IAEA with experimental and Monte Carlo based data for this revision. Within the European Community the EURAMET 16NRM03 RTNORM project was initiated for that purpose. In the present study, Monte Carlo based results for the beam quality correction factors in medium energy x-ray beams for six ion chambers applying different Monte Carlo codes are presented. Additionally, the perturbation factor p _Q , necessary for the calculation of dose to water from an air kerma calibration coefficient, was determined. The beam quality correction factor [Formula: see text] for the chambers varied in the investigated energy range by about 4%-5%, and for five out of six chambers the data could be fitted by a simple logarithmic function, if the half-value-layer was used as the beam quality specifier. Corresponding data using different Monte Carlo codes for the same ion chamber agreed within 0.5%. For the perturbation factor p _Q , the data did not obey a comparable simple relationship with the beam quality specifier. The variation of p _Q for all ion chambers was in the range of 3%-4%. Compared to recently published data, our p _Q data is around 1% larger, although the same Monte Carlo code has been used. Compared to the latest experimental data, there are even deviations in the range of 2%.