Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_5_3158 - Do Interfraction Changes in Lung Target Volume Lead to Increased Pre-treatment Registration Variability?

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Do Interfraction Changes in Lung Target Volume Lead to Increased Pre-treatment Registration Variability?
J. Hyder1, G. Baran2, J. W. Burmeister3, P. A. Paximadis4, and M. M. Dominello3; 1Wayne State University School of Medicine, Detroit, MI, 2Karmanos Cancer Institute, Detroit, MI, 3Department of Oncology, Wayne State University School of Medicine, Detroit, MI, 4Lakeland Radiation Oncology, St. Joseph, MI

Purpose/Objective(s): Stereotactic body radiation therapy (SBRT) utilized in the management of early stage non-small lung cancer requires highly accurate patient setup. This is typically achieved with a registration between the original CT simulation image and pre-treatment onboard imaging. In a previous study it was demonstrated that interobserver variability may increase from first to last fraction for some patients. Here we attempt to investigate possible causes for this variation by evaluating anatomic and radiographic changes in target and surrounding normal tissue.

Materials/Methods: Prior to treatment delivery, a megavoltage CT (MVCT) is acquired for each patient and compared to the simulation CT for registration purposes. For each patient, a target volume was contoured from the MVCT acquired at the time of first and last fraction using a standardized threshold window level setting. This allowed evaluation of both target volume and changes in Hounsfield units (HU) during treatment. Changes in surrounding lung parenchyma were evaluated by defining a “PTV-GTV” structure to include a 5 mm expansion surrounding the target, with chest wall and GTV cropped out. These structures were evaluated within their corresponding MVCT using 3D Slicer (https://www.slicer.org) to obtain mean and standard deviation HU values within the confining surfaces. A total of 10 patients with primary non-small cell lung carcinoma who underwent SBRT were analyzed.

Results: A decrease in GTV, from first to last fraction, was noted for 6 of 10 (60%) patients with a mean absolute decrease of 0.53 cm3 (0.27-1.96 cm3) and a relative decrease of 26% (23-40%). An increase was noted for 1 patient of 0.23 cm3 (22%). HU values for both the PTV-GTV and target structures remained relatively constant for each patient when comparing first and last fraction. The mean change in GTV HU was -3.6 ± 23.3 HU while the mean change in PTV-GTV HU was 9.8 ± 15.8 HU.

Conclusion: In this study, we observed a change in GTV volume of 22-40% for 7 of the 10 patients examined during the course of SBRT. We have previously shown an increase in registration variability over the course of an SBRT treatment course. Relative changes in target and/or surrounding lung parenchyma may contribute to this increase in pre-treatment registration uncertainty. Examination of a larger sample size is now underway to further validate our results.

Author Disclosure: J. Hyder: None. J.W. Burmeister: Research Grant; NIH. Board Member; SDAMPP. Chair, Radiation Oncology Physics Exam Committee; American Board of Radiology. P.A. Paximadis: None.

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