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Isodose Volume Comparison of Two Common Volume Derivation Methods

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J Leasure

J Leasure*, V Ulizio , D Pearson , University of Toledo, Toledo, OH


SU-I-GPD-T-574 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The purpose of this study is to compare two volume derivation methods used within the Pinnacle planning system and determine if one is more accurate than the other. These volume derivations become very important when they are used to derive isodose volumes for the calculation of conformity and gradient indices, especially for small stereotactic lesions.

Methods: A spherical test object of known measured dimensions was scanned in the CT Simulator and then sent to the planning system. A ROI (region of interest) was then contoured for the object. Dose was then computed. Isodose volumes were converted to ROIs using the dose covering the entire object and by using the tabular DVH (dose volume histogram). The volumes from each method were then compared to the known object volume. The volume derivation methods were also compared to each other for prescription isodose lines over a wide range of target sizes for 82 stereotactic treatment plans. The percent difference was computed and visual inspection was done. The uncertainty in the two methods was then compared.

Results: The known volume of the test object was 2.158 cc as measured with a micrometer. The DVH method found a value of 2.05 cc and the ROI method from this isodose found a value of 1.96 cc. The percent difference between the two methods was 4.46%. The percent difference for the 82 treatment plans ranged from 2.73-27.81%, with the greatest errors being for very small lesions.

Conclusion: The error seen in the ROI contours created from isodose lines is due to the contour being created on a worse resolution dose grid than the original isodose line is represented with. Due to this deviation the DVH method was determined to be more accurate and is to be used for any isodose volume studies.

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