Experimental determination ofpCoperturbation factors for plane-parallel chambers

Evaluation of a modified formalism for precision electron beam dosimetry compared to TG-51 and TRS-398

PURPOSE

This study aims to evaluate a modified formalism for electron beam reference dosimetry and to compare the results with the cases of previous protocols TG-51 and TRS-398.

METHODS

Measurements were performed using three types of cylindrical chambers (TW30013, FC65-G, and A1SL) and plane-parallel chambers (PPC) (Roos, Advanced Markus, and NACP-02) with electron beams of energies 6, 12, and 18 MeV. Absorbed dose to water at the reference depths was determined following the modified formalism using different chambers and methods, including the direct application of 60Co calibration or cross-calibration for PPCs in a high-energy electron beam. Data were also analyzed following TG-51 and TRS-398 formalisms for comparison.

RESULTS

Following the modified formalism, all the results obtained using cylindrical chambers, 60Co calibrated well-guarded PPCs and cross-calibrated PPCs were in good agreement within the estimated uncertainty (0.7 - 0.9 %) at all the energies. However, one PPC exhibited strange behavior, producing 3 % higher doses when applying 60Co calibration. This was confirmed due to chamber-to-chamber variation in Pwall. The modified formalism showed good agreement with TG-51 and TRS-398, but yielded consistently higher doses. The difference was reduced by applying the updated EPOM (effective point of measurement) shift to the previous protocols.

CONCLUSIONS

This study supports the reliability of the modified formalism for electron beam reference dosimetry with chambers calibrated under 60Co, thereby reducing uncertainty. The modified formalism yielded consistent results within smaller uncertainty (0.7 - 0.9 %). However, it is recommended to check each PPC's chamber-to-chamber variation.

AAPM WGTG51 Report 385: Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy electron beams

An Addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water is presented for electron beams with energies between 4 MeV and 22 MeV (1.70 cm ≤ R 50 ≤ 8.70 cm $1.70\nobreakspace {\rm cm} \le R_{\text{50}} \le 8.70\nobreakspace {\rm cm}$ ). This updated formalism allows simplified calibration procedures, including the use of calibrated cylindrical ionization chambers in all electron beams without the use of a gradient correction. Newk Q $k_{Q}$ data are provided for electron beams based on Monte Carlo simulations. Implementation guidance is provided. Components of the uncertainty budget in determining absorbed dose to water at the reference depth are discussed. Specifications for a reference-class chamber in electron beams include chamber stability, settling, ion recombination behavior, and polarity dependence. Progress in electron beam reference dosimetry is reviewed. Although this report introduces some major changes (e.g., gradient corrections are implicitly included in the electron beam quality conversion factors), they serve to simplify the calibration procedure. Results for absorbed dose per linac monitor unit are expected to be up to approximately 2 % higher using this Addendum compared to using the original TG-51 protocol.

Monte Carlo-calculated beam quality and perturbation correction factors validated against experiments for Farmer and Markus type ionization chambers in therapeutic carbon-ion beams

Objective. In current dosimetry protocols, the estimated uncertainty of the measured absorbed dose to waterDwin carbon-ion beams is approximately 3%. This large uncertainty is mainly contributed by the standard uncertainty of the beam quality correction factorkQ. In this study, thekQvalues in four cylindrical chambers and two plane-parallel chambers were calculated using Monte Carlo (MC) simulations in the plateau region. The chamber-specific perturbation correction factorPof each chamber was also determined through MC simulations.Approach.kQfor each chamber was calculated using MC code Geant4. The simulatedkQratios in subjected chambers and reference chambers were validated through comparisons against our measured values. In the measurements in Heavy-Ion Medical Accelerator in Chiba,kQratios were obtained fromDwvalues of60Co, 290- and 400 MeV u-1carbon-ion beams that were measured with the subjected ionization chamber and the reference chamber. In the simulations,fQ(the product of the water-to-air stopping power ratio andP) was acquired fromDwand the absorbed dose to air calculated in the sensitive volume of each chamber.kQvalues were then calculated from the simulatedfQand the literature-extractedWairand compared with previous publications.Main results. The calculatedkQratios in the subjected chambers to the reference chamber agreed well with the measuredkQratios. ThekQuncertainty was reduced from the current recommendation of approximately 3% to 1.7%. ThePvalues were close to unity in the cylindrical chambers and nearly 1% above unity in the plane-parallel chambers.Significance. ThekQvalues of carbon-ion beams were accurately calculated in MC simulations and thekQratios were validated through ionization chamber measurements. The results indicate a need for updating the current recommendations, which assume a constantPof unity in carbon-ion beams, to recommendations that consider chamber-induced differences.

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

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.

[Does the measured dose change when applying the new DIN 6800-2 (2008) versus the edition from 1997?]

New edition of DIN 6800-2 (1997) has been published in March 2008. The concept of absorbed dose to water has been retained unchanged. In many points modern data and approaches were adopted to international dosimetry protocols. For the first time values for the pertubation correction factors of plane parallel chambers are given in a dosimetry protocol. This enables the customer based on a Co-60 calibration factor to measure absorbed dose to water without any cross-calibration. In this paper new edition will be presented and compared with the old one. But main focus is set on the question, is there any deviation in the determination of dose when applying both protocols to same measured values. For photon beams and for in Germany common used types of ionization chambers the deviations are not larger than about 0.3% and for other types not larger than 0.5%. However, in electron beams partly larger deviations up to 0.5% and for some types of ionization chambers even more than 1% may occur.