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Intrafractional Change of the Relationship Between Internal Fiducials and External Breathing Signal in Pancreatic Cancer Stereotactic Body Radiation Therapy

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N Pettersson

N Pettersson*, J Murphy , D Simpson , L Cervino , University of California, San Diego, La Jolla, CA


SU-F-J-137 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

The use of respiratory gating for management of breathing motion during stereotactic body radiation therapy (SBRT) relies on a consistent relationship between the breathing signal and the actual position of the internal target. This relationship was investigated in patients treated for pancreatic cancer.

Four patients with pancreatic cancer undergoing SBRT that had implanted fiducials in the tumor were included in this study. Treatment plans were generated based on the exhale phases (30-70%) from the pre-treatment 4DCT. The margin between the internal target volume (ITV) and the planning target volume was three mm. After patient setup using cone-beam CT, simultaneous fluoroscopic imaging and breathing motion monitoring were used during at least three breathing cycles to verify the fiducial position and to optimize the gating window. After treatment, fluoroscopic images were acquired for verification purposes and exported for retrospective analyses. Fiducial positions were determined using a template-matching algorithm. For each dataset, we established a linear relationship between the fiducial position and the anterior-posterior (AP) breathing signal. The relationships before and after treatment were compared and the dose distribution impact evaluated.

Seven pre- and post-treatment fluoroscopic pairs were available for fiducial position analyses in the superior-inferior (SI) and left-right (LR) directions, and five in the AP direction. Time between image acquisitions was typically six to eight minutes. An average absolute change of 1.2±0.7 mm (range: 0.1-1.7) of the SI fiducial position relative to the external signal was found. Corresponding numbers for the LR and AP fiducial positions were 0.9±1.0 mm (range: 0.2-3.0) and 0.5±0.4 mm (range: 0.2-1.2), respectively. The dose distribution impact was small in both the ITV and organs-at-risk.

The relationship change between fiducial position and external breathing signal has been observed to be about 1 mm in four pancreas SBRT patients, leading to small dose distribution impact.

Funding Support, Disclosures, and Conflict of Interest: Pettersson and Cervino are funded by a Varian Medical Systems grant.

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