A modified formalism for electron beam reference dosimetry to improve the accuracy of linac output calibration

Abstract

Purpose To present and demonstrate the accuracy of a modified formalism for electron beam reference dosimetry using updated Monte Carlo calculated beam quality conversion factors. Methods The proposed, simplified formalism allows the use of cylindrical ionization chambers in all electron beams (even those with low beam energies) and does not require a measured gradient correction factor. Data from a previous publication are used for beam quality conversion factors. The formalism is tested and compared to the present formalism in the AAPM TG‐51 protocol with measurements made in Elekta Precise electron beams with energies between 4 MeV and 22 MeV and with fields shaped with a 10 × 10 cm2 clinical applicator as well as a 20 × 20 cm2 clinical applicator for the 18 MeV and 22 MeV beams. A set of six ionization chambers are used for measurements (two cylindical reference‐class chambers, two scanning‐type chambers and two parallel‐plate chambers). Dose per monitor unit is derived using the data and formalism provided in the TG‐51 protocol and with the proposed formalism and data and compared to that obtained using ionization chambers calibrated directly against primary standards for absorbed dose in electron beams. Results The standard deviation of results using different chambers when TG‐51 is followed strictly is on the order of 0.4% when parallel‐plate chambers are cross‐calibrated against cylindrical chambers. However, if parallel‐plate chambers are directly calibrated in a cobalt‐60 beam, the difference between results for these chambers is up to 2.2%. Using the proposed formalism and either directly calibrated or cross‐calibrated parallel‐plate chambers gives a standard deviation using different chambers of 0.4%. The difference between results that use TG‐51 and the primary standard measurements are on the order of 0.6% with a maximum difference in the 4 MeV beam of 2.8%. Comparing the results obtained with the proposed formalism and the primary standard measurements are on the order of 0.4% with a maximum difference of 1.0% in the 4 MeV beam. Conclusions The proposed formalism and the use of updated data for beam quality conversion factors improves the consistency of results obtained with different chamber types and improves the accuracy of reference dosimetry measurements. Moreover, it is simpler than the present formalism and will be straightforward to implement clinically.

Modified electron beam output calibration based on IAEA Technical Report Series 398

“…The standard deviation of the average dose per monitor unit from different types of chambers on the Elekta Synergy Platform and Elekta Versa HD is 0.5% and 0.4%, respectively. This result is in line with standard deviation which is provided by the previous work 12 . Therefore, it can be seen that the dose per monitor unit using a modified calibration for electron beam in this study gave good results.…”

“…Data for $${k_{Q,{Q_{\rm{o}}}\;}}$$ for cylindrical and parallel‐plate chambers are obtained from Table 7.III in document TRS‐398. Data for the chamber are most similar to the Exradin A1SL, IBA CC13, and Exradin A11, which are Exradin A1, Wellhofer IC‐10, and Exradin P11, respectively 12 …”

“…Data for the chamber are most similar to the Exradin A1SL, IBA CC13, and Exradin A11, which are Exradin A1, Wellhofer IC‐10, and Exradin P11, respectively. 12 …”

“…In addition, the variability of relative ion chamber perturbation correction of the ionization chamber on the cylindrical chamber yields a value of less than 0.4%, which is not worse than the parallel‐plate chamber. 11 Furthermore, there is a measurement work by Muir 12 of modified calibration for electron beam calibration based on AAPM TG‐51. He found that the use of cylindrical chambers at low energy gave acceptable results.…”

Dosimetric accuracy of a cross-calibration coefficient for plane-parallel ionization chamber obtained in low-energy electron beams using various cylindrical dosimeters

“…As presented in Figure 1, when measurements are performed at Zref depth, the higher the electron beam energy, the higher discrepancies in PDD curves. If one follows the new concept of calibration 25 , the discrepancies could be maintained at the level of 0.5%.…”