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Evaluation of a Method for Dose Estimations to Moving Pancreatic Tumors Treated with Gated Carbon Ion Beams

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G Meschini

G Meschini1*, M Seregni1 , A Vai2 , J Phillips3 , G Baroni1,2 , M Ciocca2 , S Molinelli2 , H Paganetti4 , A Pella2 , F Valvo2 , G Sharp4 , M Riboldi1 , (1) Politecnico di Milano, Milano, Milano, (2) Centro Nazionale di Adroterapia Oncologica, Pavia, Pavia, (3) University of California, San Francisco, San Francisco, CA, (4) Massachusetts General Hospital, Boston, MA


SU-I-GPD-T-137 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To evaluate the validity and limitations of a method for the estimation of dosimetric variations induced by internal target motion in gated carbon ion treatments.

Methods: Three patients with pancreatic tumors treated with scanned carbon ion beams gated around end-exhale were considered. Deformable image registration was applied between the end-exhale (CT_EX) and end-inhale (CT_IN) volumes of the planning 4DCT to calculate tumor motion and propagate contours. The physical dose distribution (D_EX) planned on CT_EX was transformed from CT coordinates in the Water Equivalent Depth (WED) space, translated according to tumor motion and finally transformed back to CT coordinates. To quantify the dosimetric variations caused by breathing motion, D_EX was compared with the dose recalculated using the treatment planning system on CT_IN (D_REC). Similarly, the WED-based estimation of the dose on CT_IN (D_WED) was compared with D_REC to assess the method accuracy. The area between DVH curves as a fraction of the planned maximum dose to the tumor (AB_DVH) was considered as the comparison metric.

Results: For the gross target volumes, the maximum AB_DVH among the three patients was 8.8% when comparing D_REC to D_EX. Estimations performed with the WED-based method were more consistent with D_REC, resulting in a maximum AB_DVH of 3.1% when comparing D_WED to D_REC. For organs at risk (OARs, typically cord, stomach, duodenum and bowel), the maximum AB_DVH resulted in 4.7% (D_REC vs. D_EX), and 2.3% (D_WED vs. D_REC).

Conclusion: The DVH analysis showed that, in presence of dose variations due to breathing motion in the abdomen, the WED-based dose estimations are comparable to TPS dose calculations in target volumes. Since the WED-based method considers only target translations, significant differences occur for the OARs when the impact of deformations on the beam path could not be neglected.

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