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A Comparison of VMAT and IMRT Treatment Plans for Four Clinical Sites at a Mixed-Machine Cancer Center

E Schnell

E Schnell*, S Ahmad , S Thompson , O Algan , C Bogardus , T Herman , T De La Fuente Herman , Oklahoma Univ. Health Science Ctr., Oklahoma City, OK


SU-I-GPD-T-377 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the optimal treatment technique for specific treatment sites: Head and Neck, Brain, Lung, and Pelvis, that can help strategically schedule and designate patient loads in a busy mixed-machine radiation treatment facility.

Methods: IMRT and VMAT plans (twenty each) equally distributed between the sites of interest were compared on the basis of tumor coverage and normal tissue dose using the QUANTEC guidelines. IMRT plans utilized seven or nine fields and VMAT plans used two or three arc fields. Prescription doses were appropriate for the treatment sites. The treatment plans were created by the same planner using 6 MV photon beam and Varian’s Eclipse v11 with the AAA calculation model.

Results: Relative to IMRT, the VMAT plans resulted in average PTV maximum doses 1.23% lower, 0.91% lower, 0.09% higher, and 0.22% lower for brain, head & neck, lung, and pelvis, respectively. For VMAT, the percent differences for maximum normal structure doses are lower, while mean doses are higher, compared to IMRT.

Conclusion: VMAT machine availability is important for patients that need faster treatment times due to medication and discomfort, or for patients with treatments such as hippocampus sparing brain that need specialized optimization. Organs with a wrapping interface with the target volume, or fully encompassed are better spared by using the VMAT optimization, while organs with a linear interface may be better spared with beam angle selection in IMRT. Plans with target volumes larger than the achievable size of the VMAT MLC lose the increased VMAT treatment efficiency when more than one isocenter is needed. Some treatment sites and individuals may clinically be treated in a superior way with VMAT. In most cases, planning, treatment efficiency, machine scheduling, and individual patient charactaristics should primarily be considered in the selection of machine and treatment techniques.

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