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Experimentally Quantification of Interplay Effect for Lung SBRT with VMAT

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E Sandrini

E Sandrini1,2*, C Silva1 , A da Silva2 , (1) Clinicas Oncologicas Integradas, Rio De Janeiro, Rio de Janeiro, (2) Coope- Instituto Alberto Luiz Coimbra de Pos-Graduacao e Pesquisa em Eng, Rio De Janeiro, Rio de JAneiro


SU-I-GPD-T-533 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Volumetric modulated arc therapy (VMAT) allows fast delivery of stereotactic radiotherapy for lung tumors with flattening filter free (FFF) beams. We investigated discrepancies between the calculated and delivery dose distributions, as well as the dosimetric impact of leaf interplay with breathing- induced tumor motion.

Methods: It was investigated 30 VMAT plans created with the Eclipse treatment planning system (version 13) in three fractions of 18Gy. All plans were generated and delivered based on three energies: 6X MV photon beam, 6FFF MV and 10FFF MV x-ray beam from a TrueBeam accelerator equipped with high-definition MLC. For each energy the plans were created using (1) 1 coplanar full arc (full), (2) 2 coplanar full arcs (2full), (3) a 2 coplanar partial arcs (half), (4) a 2 coplanar partial arcs plus 30° (half30) and (5) a 2 coplanar partial arcs plus 60° (half60); and calculated with Acuros XB (AXB) and Anisotropic Analytical Algorithm (AAA). Static and dynamic measurements were performed with a farmer chamber in a Quasar motion phantom with amplitudes of 5, 10, 15, 20 mm and with a motion period of 8s. Dynamic measurements were compared with the static measurements and with the calculated dose distribution.

Results: For plan calculated with AXB and AAA algorithm the interplay effect is negligible when the motion amplitudes was equal or less than 15mm (<2%) and equal to 5mm (<1%), respectively. For motion amplitudes of 20mm it was found a difference up to 10% with both algorithms. It was not found a significant difference between 6X, 6FFF and 10FFF. However, partial arcs showed higher values comparable to full arcs.

Conclusion: Our study suggests that the risk of underdosing/overdosing the tumor due to interplay effects is not a significant problem with VMAT delivery. The interplay effect does not result in unexpected dose variation.

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