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The Imaging Dose to Patients Resulting From the ExacTrac X-Ray System

G Ding

G Ding*, G Luo, Vanderbilt University, Nashville, TN

SU-E-J-163 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

There is a lack of reported data on imaging dose from Novalis ExacTrac use for image guided radiotherapy. This investigation is to assess the imaging doses to patients resulting from a variety of clinical default imaging acquisition protocols in the ExacTrac x-ray imaging system.

Novalis TX ExacTrac x-ray system (BrainLab, Feldkirchen, Germany) is investigated. This system consists of two floor-mounted kilovoltage X-ray tubes projecting obliquely onto two flat-panel detectors mounted on the ceiling. The dose measurements were performed using an ion chamber inserted in a phantom. The air kerma calibration factor of the ion chamber for x-ray beams specified by the half-value layer was obtained from an Accredited Dosimetry Calibration Laboratory. The imaging dose from clinical default protocols including Cranium, Thorax and Abdomen were studied. The phantom, in the size of 30Χ12Χ30 cm3, was made from slabs of water-equivalent materials for low energy x-rays, PW-LR (CIRS, Norfolk, VA).

The field size of x-rays from each tube is 20x20 cm2 at the isocenter. Small differences of 5-10 % in measured radiation dose at the isocenter between two x-ray tubes were observed. The doses measured in the phantom at a depth of 1 cm from the treatment table were 0.05 cGy, 0.16 cGy, and 0.19 cGy resulting from a pair of obliquely beams of Cranium-heavy, Thorax-Heavy and Abdomen-Heavy clinical protocols respectively. It was found that there was up to 77% dose reduction by selecting Abdomen-Light instead of Abdomen-Heavy acquisition protocols.

Although the imaging doses to patients resulting from a single pair of beams are low, the sum of multiple acquisitions (10-20 pairs) is still significant, which can be common for one treatment fraction if non-coplanar beams are used. The imaging doses strongly depend on the image acquisition protocols. During treatment, the low dose protocols should be chosen, given that image quality is not compromised.

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