Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_13_3245 - A Continuum Approach to Account for Uncertainties in Defining the Clinical Target Volume

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

A Continuum Approach to Account for Uncertainties in Defining the Clinical Target Volume
N. Shusharina1, D. Craft1, Y. L. E. Chen2, H. A. Shih1, and T. Bortfeld1; 1Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, 2Department of Radiation Oncology, Massachusetts General Hospital, Boston, MA

Purpose/Objective(s): The purpose of this study is to put forward a new approach that deals with delineation uncertainties of the clinical target volume (CTV) definition. Instead of a standard contour-based CTV we introduce a continuum distribution of probability of finding tumorous cells at a certain distance from the gross tumor volume (GTV) and call it the clinical target distribution (CTD).

Materials/Methods:: We test the new approach in clinical cases of glioma, and bone sarcoma patients. The CT scans of previously treated patients along with the contours of GTV and CTV were presented to radiation oncologists, one specializing in brain tumors and another in sarcoma. They were asked to draw the lines that would correspond to their levels of delineation uncertainty, or, equivalently, to the probabilities of tumor spread. Both physicians expanded the previously delineated CTV by approximately 20% by drawing the line of 0% probability to find the tumor at further distances. The levels of 80%, 60%, and 40% were drawn taking into account anatomic structures surrounded the GTV. IMRT treatment plans were created and optimized to deliver prescription dose to the space between the drawn levels with the coverage objective weighted with the corresponding probability. These plans were compared with the plans optimized to deliver the same dose to traditionally defined CTV.

Results: The CTD-based plan for glioma was optimized using three different levels of the mean brain dose. The physician appreciated a naturally occurring option of adjusting the coverage to different levels of tumor probability by choosing an appropriate tradeoff between the coverage and organ sparing. The CTV- and CTD-based plans with similar coverage were different by the mean brain dose being lower for the CTD by 23%. For the sarcoma case, both plans achieved the goal of sparing the OARs, although the dose distribution within extended target was different. The CTD-based approach was superior to the conventional CTV approach in covering regions marked with higher probabilities of finding tumor cells by shaping the lower dose regions around OARs so that the dose fall-off followed the decrease of tumor probability.

Conclusion: CTD allows to find the most suitable tradeoff between target coverage and sparing of surrounding healthy organs at the treatment planning stage, without having to modify or redraw a CTV. Owing to the variable probabilities afforded by the CTD, a more flexible and more clinically meaningful sparing of critical structure becomes possible.

Author Disclosure: N. Shusharina: None. D. Craft: None. Y.E. Chen: Employee; Beth Israel Deaconess Medical Center. H.A. Shih: Employee; Dartmouth Hitchcock. Honoraria; UpToDate. Chief, CNS & Eye Radiat Oncol Services; Massachusetts General Hospital. Associate Medical Director; MGH Proton Therapy Center. Associate Director; Harvard Radiation Oncology Program. T. Bortfeld: Research Grant; RaySearch AB, Stockholm Sweden.

Nadya Shusharina, PhD

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