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Development of Accurate EPID in Vivo Dosimetry for Photon Therapy

H Takei1*, T Isobe1 , Y Watanabe2 , T Hasegawa2 , E Sato3 , H Hara2 , H Muraishi2 , T Sakae1 , (1) University of Tsukuba, Tsukuba, Ibaraki, (2) Kitasato University, Sagamihara, Kanagawa, (3) Kyorin University, Mitaka, Tokyo


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To develop a new tool for in vivo dosimetry during a treatment using electronic portal imaging device (EPID) with high accuracy. A primary photon beam was reconstructed from an EPID image and the dose distribution in a phantom was calculated using Monte Carlo simulation (MC).

Methods: A 002LFC (CIRS) thorax phantom was irradiated with a 6 MV photon beam from Trilogy (Varian Medical Systems) linear accelerator in a 10×10 cm² area to simulate a lung cancer treatment. The center of the right lung was placed at the isocenter. An EPID image was acquired during the irradiation. The dose distribution was measured in an axial plane with a EBT3 (Ashland) film as a reference. DICOM CT data set of the phantom was imported to our in vivo dosimetry system built with a Geant4.9.6 patch04 MC code. The primary beam was reconstructed from the EPID image acquired in the actual irradiation using our newly developed method. The dose distribution in the phantom was calculated and compared with the reference.

Results: The calculated dose agreed with the measured dose within 2% in the soft tissue. 5±2% and 16±8% of disagreements were observed at the center and the edge of the lung, respectively, because of the dose absorption by the film. The maximum dose difference of 9±4% was observed in the penumbra region. The system calculated the dose distribution accurately within the irradiation field. It has a potential for a verification of the delivered dose to the patient. The agreement of the penumbra dose can be improved by further optimization of the reconstruction method of the primary beam.

Conclusion: A new tool for EPID in vivo dosimetry was developed with high accuracy for photon therapy.


Dosimetry, Treatment Verification, Transmission Measurements


TH- External beam- photons: portal dosimetry

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