R Yamauchi^1,2*, M Igari³, Y Kasai⁴, M Hariu⁵, Y Suda^6,2, T Kawachi⁷, T Katayose⁷, N Mizuno¹, R Miyasaka⁷, H Saitoh², (1) St. Lukes International Hospital ,Tokyo, JP, (2) Tokyo Metropolitan University, Tokyo, JP, (3) Saitama Medical University International Medical Center, Saitama, JP, (4) National Hospital Organization Tokyo National Hospital, Tokyo, JP, (5) Saitama Medical University Saitama Medical Center, Saitama, JP, (6) Tokyo Metropolitan Cancer and Infectious Diseases Center Komagome Hospital, Tokyo, JP, (7) Chiba Cancer Center, Chiba, JP,

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  R Yamauchi

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Ionization chambers are widely used for dosimetry in megavoltage photon beams. The radiation-induced cable effects are categorized as leakage current and Compton current. A few reports related to cable leakage have been published, however its mechanism has not yet been analyzed thoroughly. We proposed the quantitative measurement to separate the cable leakage from the electrometer readings in the previous report and it was 0.36 pC Gy?¹ cm?¹. The aim of this report is to estimate the cable leakage and investigate the mechanism of cable leakage by (1) Monte Carlo simulation and (2) theoretical analysis.

Methods: (1) The EGSnrc code system was used for the simulation. The cable was modeled in detail according to the internal structure. Photons were impinged on the water surface and number of electrons and positrons inflowing to the core wire or outflowing from the core wire were counted. The behavior of these particles in the core wire was analyzed and cable leakage was estimated. (2) The cable leakage was apparently observed as a positive charge by the measurement. The main cause of cable leakage can be concluded as electrons outflowing from the core wire. Therefore, electrons outflowing from the core wire by Compton scattering were estimated by the theoretical analysis.

Results: The cable leakage estimated by Monte Carlo simulation and theoretical analysis were 0.45 to 0.51 pC Gy?¹ cm?¹ and 0.99 pC Gy?¹ cm?¹, respectively. The good agreement between measurement and Monte Carlo simulation show that the cable leakage is due to recoiled electrons by Compton scattering at the core wire.

Conclusion: The mechanism of leakage was investigated by Monte Carlo simulation and theoretical analysis. It was confirmed by the two methods that the main mechanism of cable leakage is electron outflow by Compton scattering at the core wire.

Keywords

Compton Effect, Ionization Chamber, Monte Carlo

Taxonomy

TH- External Beam- Photons: General (most aspects)

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