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Effects of ITV/PTV Electron-Density Overrides for Lung SBRT VMAT When Using Monaco XVMC and AcurosXB

G Owen

G Owen*, D To , A Harrison , P Charpentier , Y Yu , T Li , Thomas Jefferson University, Philadelphia, PA


SU-I-GPD-T-243 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To compare the effect of forcing the ITV/PTV density on MUs needed to deliver lung VMAT SBRT using Eclipse/AcurosXB and Monaco/XVMC.

Methods: : Plans for 10 lung volumetric-modulated arc therapy (VMAT) stereotactic body radiation therapy (SBRT) patients were created and optimized using both Monaco/XVMC v5.11 and Eclipse/AcurosXB v11.0. Gross tumor volume (GTV) was contoured on one breathing phase of 4DCT; internal target volume (ITV) was generated by combining GTVs from all 10 phases; and planning target volume (PTV) was expanded from ITV by 3-5mm. For each patient, three structure sets were created using the 4DCT average intensity projection (AveIP), forcing the ITV electron-density to 1.0 (fITV), and forcing the PTV electron-density to 1.0 (fPTV). Plans were reoptimized in both planning systems using the same optimization objectives, and normalized so the prescription dose covers 95% of the PTV. MUs required to deliver each plan were compared.

Results: The average number of MU for the AveIP plans was 3673±1272 for XVMC and 3123±600 for AcurosXB. Compared to AveIP plans the fITV plans had an average reduction in MU of 5.5±9.7% for XVMC and 4.3±2.3% for AcurosXB. For fPTV, the reduction of MU was more substantial, reaching 15.8±11.6% using XVMC and 10.7±5.9% using AcurosXB. The XVMC plans had higher standard deviations, indicating larger uncertainty in the change of MU. The reduction in MU did not correlate to the ratio of the ITV/GTV or PTV/GTV volumes based on current data.

Conclusion: Forcing either the ITV or PTV electron-density has a significant effect on the optimization and MUs of 4D lung SBRT plans, especially for PTV density overrides. The effect is more pronounced in Monaco/XVMC vs Eclipse /AcurosXB. The reduction is not found to be linked to ITV-to-GTV or PTV-to-GTV ratio.

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