Program Information

Positioning Errors Resulting From Using Bony Anatomy Alignment for Treating SBRT Lung Tumor

C Frame

C Frame*, G Ding , Vanderbilt University, Nashville, TN

Presentations

SU-E-J-94 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To quantify patient setups errors based on bony anatomy registration rather than 3D tumor alignment for SBRT lung treatments.

Method: A retrospective study was performed for patients treated with lung SBRT and imaged with kV cone beam computed tomography (kV-CBCT) image-guidance. Daily CBCT images were registered to treatment planning CTs based on bony anatomy alignment and then inter-fraction tumor movement was evaluated by comparing shift in the tumor center in the medial-lateral, anterior-posterior, and superior-inferior directions. The PTV V100% was evaluated for each patient based on the average daily tumor displacement to assess the impact of the positioning error on the target coverage when the registrations were based on bony anatomy. Of the 35 patients studied, 15 were free-breathing treatments, 10 used abdominal compression with a stereotactic body frame, and the remaining 10 were performed with BodyFIX vacuum bags.

Results: For free-breathing treatments, the range of tumor displacement error is between 1-6 mm in the medial-lateral, 1-13 mm in the anterior-posterior, and 1-7 mm in the superior-inferior directions. These positioning errors lead to 6-22% underdose coverage for PTV - V100% . Patients treated with abdominal compression immobilization showed positional errors of 0-4mm medial-laterally, 0-3mm anterior-posteriorly, and 0-2 mm inferior-superiorly with PTV - V100% underdose ranging between 6-17%. For patients immobilized with the vacuum bags, the positional errors were found to be 0-1 mm medial-laterally, 0-1mm anterior-posteriorly, and 0-2 mm inferior-superiorly with PTV - V100% under dose ranging between 5-6% only.

Conclusion: It is necessary to align the tumor target by using 3D image guidance to ensure adequate tumor coverage before performing SBRT lung treatments. The BodyFIX vacuum bag immobilization method has the least positioning errors among the three methods studied when bony anatomy is used for registration.



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