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Dose-Based Treatment Table Shift by Prescription Dose Volume and Deformable Image Registrations with Daily Cone Beam Computed Tomography (CBCT) of Head and Neck Patients

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S Lee

S Lee1*, B Zhang1 , H Xu1 , I Lee2 , K Prado1 , W D'Souza1 , B Yi1 , (1) University of Maryland School of Medicine, Columbia, MD, (2) River Hill High School, Clarksville, MD


SU-H3-GePD-T-6 (Sunday, July 30, 2017) 4:00 PM - 4:30 PM Room: Therapy ePoster Lounge

Purpose: Geometry-Based Shift (GBS) is currently used in the clinic to correct patient positioning on the treatment table based on imaging guidance such as CBCT. Dose-Based Shift (DBS) is an alternative method that automatically searches the optimal shift based on the highest conformity index (CI). The aim of this study is to evaluate DBS and compare it with GBS in terms of PTV coverage.

Methods: Six advanced stage H&N patients, each with 4-7 CBCT image sets were obtained in this study. PTVs and OARs were contoured onto these CBCTs using conventional CT images for treatment planning with deformable image registration technique imbedded in the RayStation treatment planning system (TPS, Manufactured by RaySearch Laboratories AB, Sweden, V. 5.0.2). DBS performed isocenter shift in lateral, longitudinal, and vertical directions with 1-3 mm increments for each coordinate. Target (PTV) dose coverage estimation was performed for the 342 shifted positions and the final isocenter shift was determined based on the best CI values among these positions. The CI calculated 95% of prescribed dose (D95) which was converted into volume over target volume of the original (GBS) setup. PTV mean dose and D95 after the shifts using DBS and GBS respectively on the CBCT were compared.

Results: Higher CI was not always correlated to higher PTV coverage outcome. Out of total 33 CBCT image pairs for comparison, 14 CBCTs showed enhancement (average: 0.93±0.9%) in mean PTV dose with DBS and were closer to the prescription dose. The highest enhancement was 3.34% with DBS. 5 CBCTs showed higher D95 (average: 1.42±1.32%) for DBS. The highest D95 enhancement was 3.36%.

Conclusion: For some cases, the DBS may enhance target dose coverage than the GBS. To maximize the benefit of DBS, further study with different sites and rotational shift would be warranted.

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