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Evaluation of a Screening Method for Adaptive Radiotherapy of Head and Neck Cancer Patients

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S Gros

S Gros*, J Roeske , M Surucu , Loyola Univ Medical Center, Naperville, IL


SU-H1-GePD-J(A)-3 (Sunday, July 30, 2017) 3:00 PM - 3:30 PM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: To validate an Adaptive Radiotherapy tool recently developed for Head and Neck (HN) cancer patients that monitors external body contour changes from kV-CBCTs.

Methods: Nine HN patients who were rescanned during treatment for adaptive re-planning at 40 Gy ( median dose) were retrospectively analyzed. The initial treatment plan was recalculated on the rescan CT to evaluate the dosimetric and geometric changes of GTVs and OARs. The first kV-CBCTs external contours were compared to the kV-CBCTs matching the rescan CTs with a recently developed method that quantifies changes in external anatomy by the ‘difference in radial distance’ from isocenter axis. The radial difference (RD) is displayed in cylindrical coordinates as 2D intensity maps. Regions of interests (ROIs) corresponding to the parotids were defined on each patient’s RD maps. The correlations between the ROIs radial differences and changes in parotids doses were investigated.

Results: The median RD from the 18 parotids ROIs was 3.35 mm (range: -0.3 to 11.5 mm). A subset of 7 parotids had a median ROI RD greater than 5 mm; the corresponding parotid mean dose increased by an average of 6.4 Gy. The remaining 11 parotids, with a median ROI RD <5 mm, showed a mean parotid dose increase of 2.6 Gy (p=0.09). Twelve parotids had an initial mean dose <26 Gy. With a median RD of 3.7 mm, six of these 12 OARs exceeded the 26 Gy mean dose limit with an average parotid mean dose increase of 9.8 Gy. Initial data suggest a linear increase between RDs and parotids mean doses.

Conclusion: These preliminary results demonstrate the relevance of monitoring external patient contours from CBCTs to help guide Adaptive Radiotherapy decisions for HN cancer patients. A radial difference > 4 mm suggests a potential excess of the dose constraint for a single parotid mean dose.

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