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Quality Assurance of Imaging Dose of Computed Tomography Dose Index for Cone Beam Computed Tomography System Using a 0.6 Cc Farmer-Type Ionization Chamber

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M Itano

M Itano1*, H Tachibana2 , S Shirasaki1 , j ota3 , Y Uchida2 , T Yamazaki4 , (1) Funabashi Municipal Medical Center, Funabashi, Chiba, (2) National Cancer Center, Kashiwa, ,(1) Funabashi Municipal Medical Center, Funabashi, Chiba, (4) Chiba Univercity Hospital, Chiba City, Chiba,Japan, (2) National Cancer Center, Kashiwa, Chiba, (4) Inagi Municipal Hospital, Inagi, Tokyo


SU-I-GPD-T-474 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: AAPM Task Group 142 recommends quality assurance of imaging dose at least annually. In general, the weighted computed tomography dose index (CTDIw) is measured with a pencil (100 mm) ionization chamber. However, most of the hospitals providing radiation therapy don’t have the 100 mm ionization chamber. We developed the methodology to  measure the CTDIw with a 0.6 cc farmer-type chamber used commonly in the department of radiation oncology.

Methods: Clinac iX (Varian Medical systems, Palo Alto, USA) with on-board imager device was used. First, the dose profiles were measured using with a two-dimensional detector to obtain the profile coefficients. The CTDIw of the kV-CBCT were measured using a 0.6 cc farmer-type ionization chamber with absorbed dose to water calibration factor in 16 and 32 cm diameter standard cylindrical Perspex phantoms respectively. Subsequently, the CTDIw were measured using a 100 mm ionization chamber. The measurements were used with the five working modes of the OBI system (standard-head, low dose-head, pelvis-spot light, low dose-thorax and pelvis). The measurements of CTDIw with the two ionization chambers were compared.

Results: The differences of CTDIw from the two ionization chambers for the five working modes were -4.6%, 4.3%, 2.9%, 2.5% and -2.1%, respectively. The dosimetry using a 0.6 cc farmer-type ionization chamber was within 5% of measurements with a 100 mm ionization chamber.

Conclusion: Our methodology for CBCT imaging dose would provide efficient procedure with tolerable accuracy.

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