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Assessment of PTV Margin Reduction for Nasopharyngeal Carcinoma Using Deformable Image Registration and Dose Accumulation

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A McNiven

J McCusker1,2 , B Zhang1,2 , O Wong1 , J Lee1 , J Bissonnette1,2 , A Hope1,2 , J Waldron1,2 , S Bratman1,2 , M Giuliani1,2 , A McNiven1,2*, (1) Princess Margaret Cancer Centre, University Health Network,Toronto, ON, (2) Department of Radiation Oncology, University of Toronto


TU-C1-GePD-J(B)-4 (Tuesday, August 1, 2017) 9:30 AM - 10:00 AM Room: Joint Imaging-Therapy ePoster Lounge - B

Purpose: To assess reduced planning target volume (PTV) margins for nasopharyngeal carcinoma (NPC) using clinical image guided radiation therapy (IGRT), deformable image registration (DIR) and dose accumulation.

Methods: Retrospective re-planning was completed for ten NPC patients treated previously and positioned daily with cone-beam CT (CBCT). Four plans were generated per patient with different margin scenarios: including (a) current 5mm isotropic margins (clinical practice); (b) 3mm isotropic margins; (c) 3mm in all directions except for 0mm posterior; and, (d) 1mm isotropic margins. Planning goals were to achieve at least the same CTV coverage and organ at risk (OAR) sparing as the clinical plan. The impact of daily set-up variation and anatomical changes on target coverage and dose to the OARs was assessed using the difference between the estimated delivered dose (using DIR and dose accumulation) and the planned dose.

Results: Plans generated using senarios (b-d) yielded lower OAR doses, with maximum Dmax reductions of 9.4 Gy and 20.7 Gy for brainstem and chiasm, respectively. The average reduction in Dmax for the brainstem, chiasm, and optic nerves, were b) 4.3, 6.2, 6.1 Gy, c) 4.1, 7.0, 5.0 Gy and d) 3.8, 11.2, 8.5 Gy. After dose accumulation, the mean difference in CTV coverage (V70Gy) was 1.3 ± 1.1% for the clinical margins, with similar results for new schema of b) 1.1 ± 0.9%, c) 0.9 ± 0.9% and d) 0.4 ± 0.8%. In addition to maintenance in coverage, on average, the estimated delivered doses for OAR were less than planned for the estimated delivered dose.

Conclusion: Dose accumulation results indicate that a reduced PTV margin could be safely introduced to the clinic using current clinical IGRT procedures. Further evaluation of these margin scenarios on a larger patient cohort will be conducted to determine which schema is optimal for clinical implementation.

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