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Dose Comparison of Elekta Infinity 4D Symmetry Scan Versus Cone Beam CT

I Koistinen1*, M Koistinen2, (1) University of Maine, Orono, ME, (2) CHEM Center for Rad Oncology, Millis, MA


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

Room: AAPM ePoster Library

Purpose: To compare the dose delivered to a patient from a standard 3D CBCT and the 4D CBCT that more accurately tracks tumor excursion.

Methods: Elekta’s Infinity Linear Accelerator has the capability to acquire 4D images for more precise tracking of tumors using the Symmetry CT on it’s XVI system. The IMRT lung phantom from Standard Imaging was put on a Standard Imaging Respiratory Gating Platform set with varying speeds from 15-30 breath per minute and tumor excustion from 5 - 20 mm excursion. Gated scans were obtained on the Philips Big Bore CT to verify settings and allow treatment on the linac. The settings used for the CBCT were the settings used clinically for chest CBCT. The doses were measured using PTW ionization chambers at the center of the phantom between the lungs and Best Medical MOSFET dosimeters at the surface. The MOSFET was placed under 1 cm of bolus to provide dose build-up.

Results: dose from the center of the phantom was 1.52cGy (range 1.47-1.55 cGy)from the CBCT and 2.97cGy (range 2.86-3.07cGy) from the symmetry scan. The increase in dose at the surface of the phantom was even greater with 1.45cGy (range 1.35-1.69cGy) from CBCT and 11.70cGy (range 10.73-12.92cGy) from the Symmetry scan. These numbers were consistent across all breathing speeds and tumor movement.

Conclusion: work indicates that the dose from the Symmetry scan is not excessive and it has been shown to be useful for initial verification of a mobile tumor. The increased dose does indicate that it should not be used for daily verification and measurement of dose to pacemakers should be done when a Symmetry scan is performed.


Absolute Dosimetry, Cone-beam CT


IM/TH- Cone Beam CT: 4DCBCT

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