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Feasibility Study of Stereotactic Radiosurgery Using a Non-Commercial VMAT Treatment Planning System with a Non-Coplanar Delivery Technique


Y Song

Y Song1*, B Mueller2, C Obcemea3, C Burman4, B Mychalczak5, (1) Memorial Sloan-Kettering Cancer Center, New York, NY, (2) Memorial Sloan-Kettering Cancer Center, New York, NY, (3) Memorial Sloan-Kettering Cancer Center, New York, NY, (4) Memorial Sloan-Kettering Cancer Center, New York, NY, (5) Memorial Sloan-Kettering Cancer Center, New York, NY

SU-E-T-608 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
Conventional beamlet-based SRS procedures have several fundamental deficiencies. To address these issues, we recently conducted a study to explore the feasibility of implementing SRS using our newly-developed VMAT treatment planning system. Our goal was to investigate the possibility of replacing IMRT with VMAT using a non-coplanar delivery technique for intracranial SRS.

Methods:
A commercial SRS head phantom was CT-scanned. The hypothetical PTV and OARs were delineated on the CT scan. Other optimization-related dose tuning structures were also delineated. The VMAT plan was optimized with one coplanar full arc and two non-coplanar partial arcs. Our VMAT adopts an aperture-based technique to optimize both MLC apertures and MU weights simultaneously. The MLC interdigitation modulation was employed to increase the MLC relative speed and the dose gradient. For the comparison purpose, the conventional IMRT plan with 7 beams was also computed with the same optimization constraints.

Results:
VMAT-SRS plan exhibits superior target conformity compared to the IMRT plan. The conformity index PITV, defined as the volume covered by the prescription isodose surface (PI) divided by the planning target volume (TV): PITV = PI/TV, were 0.9713 and 0.9416, respectively. VMAT plan was also found to present a huge advantage over IMRT plan in delivery efficiency. The Monitor Units (MU) for the VMAT and IMRT plans were 2706 and 4296, respectively, for a prescription dose of 1800 cGy/fraction. The MU-to-cGy coefficient was 1.537 for the VMAT plan and 2.388 for the IMRT plan. The VMAT plan demonstrated superiority in sparing the OARs. The dose to the brainstem was reduced by 31.63% and the dose to the scalp was reduced by 19.79.

Conclusion:
Non-coplanar VMAT should replace the conventional IMRT to become the treatment of the choice for intracranial SRS procedures.

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