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  4. Physical Characteristics of Annihilation and Related Background Photons Produced in X-Ray Irradiation with Medical Linear Accelerator

Physical Characteristics of Annihilation and Related Background Photons Produced in X-Ray Irradiation with Medical Linear Accelerator

K Takahashi*1, T Hanada2, H Miyauchi1,4, Y Tanaka1, M Hashimoto3, T Hasegawa3, (1)Graduate School of Medical Sciences, Kitasato University, Kanagawa, Japan, (2)Department of Radiology, Keio University School of Medicine, Tokyo, Japan, (3) Faculty of Allied Health Sciences, Kitasato University, Kanagawa, Japan, (4)Cancer Institute Hospital of JFCR, Tokyo, Japan

Presentations

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

Room: AAPM ePoster Library

Purpose:
To evaluate the physical characteristics of annihilation photons and related background photons produced during X-ray irradiation with a medical linear accelerator on the basis of Monte Carlo simulation and measurement for the purpose of considering the effective usage of the annihilation photons.

Methods:
A rectangular phantom with a total thickness of 20 cm consisting of an RT-3000-New phantom (R-TECH) and a water-equivalent material was placed on a couch at the isocenter position of a medical linear accelerator, TrueBeam (Varian). Scintillation detectors were used to measure energy spectra of photons produced in X-ray irradiation (10 MV) with various field sizes (1 cm × 1 cm–10 cm × 10 cm) and dose rates (100–600 MU/min). A Monte Carlo simulation code based on Geant4 ver.10 was used to simulate the production and detection of the annihilation and bremsstrahlung photons in the target, primary collimator, jaws, phantom, couch, floor, and detector.

Results:
The annihilation photon peak was clearly identified in the measured energy spectra. The detected count rates depended on the field size and dose rate. The signal to background ratio around the 0.511 MeV peak was better in smaller filed size. The source distribution was disclosed by Monte Carlo simulation. The relative contribution from the jaws was comparable to that from the phantom. The background from the couch and floor was less than 20%. The measured energy spectra included the background contribution, which was not simulated in the present study.

Conclusion:
We disclosed the physical characteristics of annihilation and related background photons and successfully identified the annihilation photon peaks in energy spectra measured with scintillation detectors for a medical linear accelerator.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by JSPS(Japan Society for the Promotion of Science) KAKENHI Grant Number JP 17K16484

Keywords

Pair Production, Monte Carlo, Scintillators

Taxonomy

TH- External Beam- Photons: portal dosimetry, in-vivo dosimetry and dose reconstruction

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