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Potential Soft-Tissue IGTV Misalignment for Low-Frequency Breathing with Fast-Gantry MV CBCT

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C Kennedy

C Kennedy*, R Scheuermann , S Anamalayil , D Mihailidis , L Dong , J Metz , University of Pennsylvania, Philadelphia, PA


TU-C1-GePD-J(B)-3 (Tuesday, August 1, 2017) 9:30 AM - 10:00 AM Room: Joint Imaging-Therapy ePoster Lounge - B

Purpose: Potential soft-tissue IGTV misalignment for low-frequency breathing was evaluated for a prototype linac capable of fast-gantry (4 rpm) MVCBCT.

Methods: A 4DCT of the Quasar Respiratory Motion Phantom was acquired with 8 phase bins for 2 cm amplitude longitudinal motion of the 3 cm spherical tumor insert in simulated lung tissue. Phase bins were reconstructed with 3mm slice thickness, and the average CT of all bins was calculated for planning. An IGTV volume was created including the full range of tumor motion. The Quasar phantom was aligned for plan delivery on the prototype fast-gantry linac by performing a CBCT, then matching the phantom geometry and IGTV volume with fast tumor motion (24 bpm). Shifts away from this position were defined as misalignment of the IGTV. Fast MV CBCT’s (12 s, 200 degrees, 5 MU) were acquired with variable sinusoidal frequencies of 4-20 bpm (breaths per minute) to simulate variable patient breathing rates. The automated CBCT matching tool was used to calculate shifts relative to the planning CT using an intensity range of -700 to 100 HU and an ROI defined approximately 1-2 cm around the IGTV volume. Ten CBCT’s were acquired for scans < 10 bpm, and five CBCT’s were acquired for scans 10-20 bpm. The average and standard deviation of the longitudinal shift magnitudes are reported.

Results: Breathing rates from 10-20 bpm generally resulted in small misalignments from the correctly aligned IGTV position (<1.5mm). Breathing rates < 10 bpm sometimes result in larger misalignments > 2mm. For 4 bpm, the average misalignment was 2.1mm, with a maximum misalignment of 4.8 mm.

Conclusion: Fast-gantry MVCBCT has the potential to result in clinically-meaningful soft-tissue IGTV misalignment for low frequency breathing. Strategies such as breath-hold, 4DCBCT/gating, increased PTV margins, or alignment to bony anatomy should be considered for these patients.

Funding Support, Disclosures, and Conflict of Interest: This work was performed as part of a research agreement with Varian Medical Systems.

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