Dosimetric Performance of Newly Developed Farmer-Type Ionization Chamber in Radiotherapy Practice

Characteristics Performance of Newly Developed Parallel Plate Chamber in Electron Beams

Background

To investigate the dosimetric performance of newly developed parallel plate chamber in electron beams.

Materials and Methods

Rosalina Instruments India Private Limited (Mumbai, Maharashtra, India) has designed and fabricated PRATT2 parallel plate chamber. The various dosimetric characteristics, including pre- and post-irradiation leakage, stability, polarity effect, chamber response with bias voltage, dose linearity, dose rate effect, and chamber absorbed dose calibration, were performed for the developed chamber. The electron beam energies of 4, 6, 8, and 15MeV were used in this study.

Results

The pre- and post-irradiation leakage of the developed chamber was within the acceptable limit. The chamber shows good stability over the electron beams used in this study. The maximum error in polarity effect was 0.7% for the developed chamber. The chamber shows the good linear response with dose, and its response is independent of the dose rate for all electron beams. The beam quality correction factor (kQ, Q0) was determined for the all electron beam energies, which was used for determination absorbed dose in electron beams.

Discussion

The developed parallel plate chamber (PRATT2) is suitable for dosimetry of electron beams in radiotherapy. The chamber is cost effective and shows precise and reproducible response. The study carried out confirms that the newly designed and fabricated ion chamber can be used in the measurement of absorbed dose for therapeutic electron beams.

Quality assurance of six cylindrical and two parallel plate chambers by radioactive check device: Influence of chamber age to its performance

PURPOSE

To evaluate the performances of eight ionization chambers, (two types) manufactured in a time span of 14 years, with radioactive source devices and photon beams.

METHODS

The measurements were performed with six Farmer and two parallel plate ionization chambers with appropriate radioactive check devices and accompanying inserts. Some tests were performed with photon beams at linear accelerator. Physical visibility check, stability, linearity of response, directional dependence, determination of leakage current, ion recombination and polarity effect and influence of background radiation tests were conducted and analyzed.

RESULTS

The performance of all Farmer ionization chambers was in agreement to IEC standard as well as for parallel plate type chambers. Long-term stability of older planparallel chamber was somewhat worse than expected.

CONCLUSION

Radioactive check devices have proven to be easy to use in clinical environment. According to results of this work Farmer type ionization chambers showed good quality regardless of the time of production, which make them long-term suitable for clinical dosimetry in radiotherapy. Also, for parallel plate type of ionization chambers the results showed fine agreement with the IEC standard, except long-term stability for older chamber, which needs further investigation.

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.