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Feasibility Study of CBCT Dose Measurements with Tissue-Equivalent Thermoluminescense Sheet

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C Kurokawa

C Kurokawa1*, A Urushiyama2 , T Inoue3 , K Usui1 , S Sugimoto1 , (1) Juntendo University, Tokyo, ,(2) Rikkyo University, Toshima-ku, Tokyo, (3) Juntendo University Urayasu Hospital, Urayasu, Chiba


SU-I-GPD-T-311 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To evaluate CBCT imaging dose measured by a new type of tissue-equivalent Thermoluminescense sheet

Methods: CBCT dose distributions were measured using a TLD sheet, which has 18 x 24 cm2 dimension and 0.35 mm thickness. Before the measurements of CBCT imaging dose, the dynamic range was evaluated for our TLD sheet by an irradiation of 6 MV photon. These sheets were inserted in a tough water phantom (Taisei Medical) in the same manner as film, and the CBCT scan (X-ray Volumetric Imager (XVI) integrated with the Elekta Synergy linear accelerator) was performed under pelvis and prostate protocols. After the irradiation, the sheets were heated at 165 °C for 180 s., and generated light was detected with a prototype of a readout system (TOYO MEDIC, Tokyo, Japan). CBCT dose with heterogeneous phantom which included inserts of Pb, Al and tough bone was also evaluated.

Results: The TLD sheet had a wide dynamic range and its dose response was linear in the range of 0.02 to 20 Gy. Two-dimensional dose distributions per single and 3 times scans were obtained at several depths including surface and bottom surface which was between the phantom and a carbon fiber couch. Averaged surface dose for the single and 3 times scans were 12.1 and 37.3 mGy, respectively. The obtained dose at bottom surface was higher than that of the surface sheet due to a couch attenuation effect. Effect of phantom inhomogeneity on CBCT dose was remarkable especially for Pb inserts.

Conclusion: Although the measured dose for single scan was close to the lower detection limit, the signal was clear and it could be distinguished from the background. We thus consider that our sheet may be applicable for an in vivo dose verification of the nonuniform 2D dose distributions for CBCT.

Funding Support, Disclosures, and Conflict of Interest: The measurement device (TLD readout system) used in this study was provided by TOYO MEDIC, Tokyo, Japan.

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