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Automatic Patient Positioning and Gating Window Thresholds for Respiratory-Gated Pancreatic Cancer Stereotactic Body Radiation Therapy Using Fiducial Markers and Orthogonally Acquired Fluoroscopic Images

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N Pettersson

N Pettersson*, D Simpson , T Atwood , L Cervino , J Murphy , J Hattangadi-Gluth , UC San Diego, La Jolla, CA


SU-H4-GePD-J(A)-1 (Sunday, July 30, 2017) 4:30 PM - 5:00 PM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: In linac-based respiratory-gated stereotactic body radiotherapy (SBRT) of the pancreas, both the patient position and gating window thresholds are manually set and are therefore user-skill dependent. We present an automatic setup method and compare the clinically used setups to the automatic setups by our method.

Methods: We retrospectively analyzed 15 setup procedures in five pancreatic cancer patients with implanted fiducials. During clinically used setup, fluoroscopic images in anterior-posterior (AP) and lateral (LR) directions had been acquired simultaneously with the breathing amplitude.Template-based fiducial tracking in Matlab is used to find the fiducial positions on the acquired images. The actual fiducial positions are then compared to their ideal positions as defined by the fiducial contours in the treatment plan. We calculate the required table shifts and gating window thresholds with the objective to accurately position all fiducials at beam-on (gating accuracy, GA) while maximizing the duty cycle (DC).

Results: The differences between the clinically used patient setups and the automatic setups were -0.4±0.8 mm, -1.0±1.1 mm, and 1.8±1.3 mm in the LR, AP, and superior-inferior (SI) direction, respectively, with an associated 3D shift of 2.6±1.1 mm. Typical patient fraction-to-fraction variations for the clinical setups were 1-2 mm in the LR and AP directions, and 2-3 mm in the SI direction.The overall GA for the clinical setups was 90.4±10.7% compared to 99.7±0.9% for the automatic setups. In four fractions, the DC had to be lowered, in six fractions with GA>95%, the GA could be maintained with increased DC, and in five fractions the GA and the DC could be simultaneously improved.

Conclusion: Our automatic setup method has the potential to increase accuracy and decrease user dependence of setup for patients with implanted fiducials treated with respiratory-gated SBRT. If deemed necessary, it could be used at any time during treatment delivery.

Funding Support, Disclosures, and Conflict of Interest: Drs Hattangadi-Gluth and Cervino hold research funding from Varian Medical Systems

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