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Accuracy of Step-Function Based CBCT-Density Calibration and Dose Calculation in a Raystation Treatment Planning System

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A Gopal

A Gopal*, H Xu , C Kalavagunta , S Chen , University of Maryland School of Medicine, Columbia, MD


SU-E-201-6 (Sunday, July 30, 2017) 1:00 PM - 1:55 PM Room: 201

Purpose: To study the accuracy of cone-beam CT (CBCT) density calibration and dose calculation in a Raystation treatment planning system.

Methods: The accuracy of CBCT dose calculation is affected by the accuracy of the CBCT-density calibration, which is quite sensitive to the additional scatter in CBCT imaging. Raystation features CBCT dose calculation based on a user guided step-function based CBCT density calibration. The step-function thresholds are anchored about reference material types like air, soft-tissue and bone that can be easily adjusted for each CBCT image according to the patient anatomy. In this study, water equivalent scatter blocks and a Catphan phantom containing reference density inserts were used to create head, chest, and pelvis phantoms, and a series of simulated plans were generated on treatment planning CT scans of these phantoms. CBCT imaging (Varian OBI) was used to position and irradiate the phantoms with the simulated plans. The CBCT images were used to characterize CBCT-density curves that were used to generate step-function thresholds for clinical use. CBCT doses were computed and compared with the doses from the original plans as well as with dose measurements at a reference point performed with an ion chamber.

Results: The CBCT-density curves varied significantly between the head, chest and pelvis geometries. However, step-function thresholds obtained from these curves correlated well with patient geometries for various anatomical sites. The plan doses computed on CT and CBCT images showed excellent agreement in mean and differential doses for simulated target and organ volumes. The CBCT dose calculations were within 2% of the measured dose for all plans.

Conclusion: The use of carefully selected site specific step-function thresholds for CBCT-density calibration minimizes uncertainties from variable scatter conditions and user subjectivity. It enables accurate CBCT dose calculations that are vital to dose tracking and adaptive planning.

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