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A Novel Methodology for Deriving Set-Up Margins Using Dose Accumulation and Bidirectional Local Distance

A Frederick1,2*, S Quirk1,2, S Weppler1,2, M Roumeliotis1,2, (1) University of Calgary, Calgary, AB, CA, (2) Tom Baker Cancer Centre, Calgary, AB, CA

Presentations

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

Room: AAPM ePoster Library

Purpose:

This study presents the first application of the bidirectional local distance (BLD) to planned and accumulated isodose surfaces as a tool to derive set-up margins.


Methods:

Ten early-stage breast and three oropharyngeal cancer patients were retrospectively selected. The breast patients underwent accelerated partial breast irradiation, delivered using intensity modulated radiotherapy (27 Gy in 5 fractions) with daily cone-beam CT (CBCT) imaging. The oropharyngeal patients underwent volumetric modulated arc therapy (70 Gy in 33 fractions) with weekly CBCT imaging. For each patient, clinical treatment plans were re-optimized with 0 mm planning target volume (PTV) margins and normalized to achieve a minimum target coverage criterion (D98% = 95%). Deformable image registration of the planning CT to CBCTs enabled dose accumulation of these zero-margin plans. BLDs were computed between the planned and accumulated 95% isodose surfaces relative to each clinical target volume (CTV) contour point. An isotropic set-up margin was defined as the 95?? percentile of the BLDs in regions where the accumulated 95% isodose surface shifted towards the CTV. The treatment plans were re-optimized again with the derived set-up margin applied to the CTV and the resulting accumulated CTV coverage was evaluated using D98% = 95%.

Results:

The median (range) set-up margin derived using the BLD was 3 mm (2–8) for the breast patients and was 2 mm for all three oropharyngeal patients. When treatment plans were re-optimized with the derived set-up margin applied for each patient case, the median (range) accumulated CTV D98% was 96% (95–99). All 13 patients met the criteria for adequate CTV coverage.

Conclusion:

Application of the BLD to planned and accumulated isodose surfaces for zero-margin treatment plans can be used to derive appropriate set-up margins. These results can be combined with other uncertainties in the external beam radiotherapy workflow to define evidence-based PTV margins.

Download ePoster [PDF]

Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Natural Sciences and Engineering Research Council of Canada, and Alberta Innovates. There are no relevant financial disclosures or conflicts of interest to declare.

Keywords

Set-up Margins, Treatment Planning, Dose Accumulation

Taxonomy

TH- External Beam- Photons: Dose reconstruction over deforming anatomies

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