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Correlating Daily CBCT 3D Gamma Density with Daily Volumetric and Dosimetric Changes in Head and Neck Patients

M Sharma*, A Witztum, J Pan, J Chan, T Solberg, E Hirata, UCSF Comprehensive Cancer Center, San Francisco, CA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: In this study we evaluated the daily CBCT based changes in patient anatomy. The organ-specific 3D gamma density analysis in MobiusCB (Varian Medical System) was correlated with the predicted deformably mapped volume and dose changes in the RayStation Adaptive planning module.

Methods: Three HN patients with 99 daily CBCT images and at least one replan were retrospectively studied. The patients were prescribed a total dose of 69.96, 66.0 and 59.4 Gy in 33 fractions respectively to the PTV1, PTV2 and PTV3. The location of the tumor site was consistent among the patient cohort. The daily CBCT images were sent to MobiusCB and RayStation (RS) TPS. The CBCT images were deformably registered and the Dose-Tracking module in RS was used to get the relevant dose metrics based on the planning directives. For each patient, the daily changes in volume and dose metrics of planning target volumes (D95 and D99 for PTV1, PTV2 and PTV3) and organs-at-risk (OARs) (Dmean for parotids, oral-cavity and mandible, D0.03cc for spinal cord and mandible) were evaluated. A constant Gamma Criteria (0.2g/cc/3mm) was used in MobiusCB to find the daily changes in organ-specific 3D gamma density.

Results: The CBCT based daily organ-specific 3D gamma density varied significantly for a few OARs throughout the course of treatment. A strong to moderate correlation was found between the organ-specific 3D gamma density in regard to volume changes, and changes in dose metrics for parotids (0.8, -0.81), oral-cavity (0.71, -0.42); and PTV3 (0.65, -0.51). The volume changes for all the organs were strongly correlated to the dosimetric changes (-0.55 to 0.87).

Conclusion: Using MobiusCB organ-specific 3D gamma density and deformable image registration between daily CBCT and planning CT could flag clinically significant anatomical changes. Further studies will determine organ-specific gamma passing thresholds and help identify the appropriate time for re-planning.

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