Determination of the ion recombination correction factor for intraoperative electron beams

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• 作者：Majid Ghorbanpour Besheli^a ; ^b ; ^{majid.ghorbanpour@med.uni-duesseldorf.de" class="auth_mail" title="E-mail the corresponding author} ; Ioannis Simiantonakis^a ; ^b ; Klemens Zink^c ; Wilfried Budach^a
• 关键词：Intraoperative radiation therapy ; ion recombination correction factor ; Two-Voltage-Analysis ; free-electron fraction
• 刊名：Zeitschrift für Medizinische Physik
• 出版年：2016
• 出版时间：March 2016
• 年：2016
• 卷：26
• 期：1
• 页码：35-44
• 全文大小：815 K

文摘

The ion recombination correction factor (k_s) is determined for the Advanced Markus chamber exposed to electron beams produced by a dedicated intraoperative radiation therapy (IORT) accelerator at medium dose-per-pulse values. The authors evaluate five different methods. Three of them are known as Boag's modified expressions, which are based on the two-voltage-analysis method and include the free-electron component. In the fourth method the IAEA TRS-398 protocol is applied, which uses the same two-voltage-analysis method but ignores the free-electron component, and finally the fifth approach is known as the Jaffé plot. k_s values were obtained in the range of 4 mGy/pulse to 42 mGy/pulse and were compared with k_s values determined by means of radiochromic films, which are independent of the dose rate. It was found that k_s values that resulted from the three Boag's modified expressions and the TRS-398 protocol deviated by on average 1.5% and 1.4%, respectively, from the reference k_s values based on film dosimetry. These results are within the estimated relative uncertainty of ±3%. On the other hand, the absolute deviation of each method depends on the dose-per-pulse value at which the method is investigated. In conclusion, in the medium dose-per-pulse range all Boag's modified expressions could be used for k_s determination. Above a dose-per-pulse value of 35 mGy/pulse, the TRS-398 approach should be avoided. At 27 mGy/pulse and a maximum operation voltage of 300 V the k_s value resulting from the Jaffé plot showed a 0.3% deviation from the reference value. More investigation on the Jaffé plot is necessary at higher dose-per-pulse values.