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Evaluating Head-And-Neck 4pi Non-Coplanar Plans From the Perspective of Proton Therapy

K Sheng

K Sheng1*, P Dong1, A Gautam2, C Cheng2, D Ruan1, D Low1, M Cao1, S Lee1, P Kupelian1, (1) UCLA School of Medicine, Los Angeles, CA, (2) Midwest Proton Therapy Institute, Bloomington, IN

MO-G-137-6 Monday 5:15PM - 6:00PM Room: 137

Purpose: Recent rapid development of non-coplanar planning such as 4pi radiotherapy on robotic radiotherapy platform has shown marked improvements in the plan quality comparing against current coplanar plans. The efficacy of 4pi radiotherapy to significantly reduce radiation dose to normal tissue intrigues an evaluation from the proton therapy perspective.

Methods: A post-surgical head-and-neck patient with a tumor involving posterior right maxillary sinus was studied. A single-level 60 Gy was prescribed to the PTV. Four treatment plans were developed including volumetric modulated arc therapy (VMAT, clinically used) using 2 partial arcs, two 4pi non-coplanar plans using 30 1 MV and 6 MV beams, respectively, and a scanning 3-field proton plan. The 4pi plans were generated using the convolution/superposition method and an in-house optimization program. Both VMAT and the proton plans were generated on Eclipse (Varian).

Results: All photon plans achieved 95% coverage of the PTV and less than 10% hot spots in the PTV. The proton plan showed greater dose heterogeneity in the PTV and greater high dose spillage to the surrounding normal tissue. The comparison of the maximum doses between VMAT, 6 MV 4pi and proton plans shows that for the left cochlea, it was reduced from 15.6 to 4.6 and 0 Gy; the chiasm, it was from 31 to 10.6 and 7.2 Gy; for the left lens, it was reduced from 6.2 to 1 and 1 Gy; for the left optical nerve, it was reduced from 31.5 to 16.4 and 10.9 Gy; for the brain stem, it was reduced from 28.6 to 14.5 and 14.2 Gy.

Conclusion: The 4pi plan capacity to spare normal organs was benchmarked against the proton plan in a head-and-neck plan. 4pi plan showed remarkable potentials in approaching the proton therapy level of critical organ sparing while maintaining superior PTV coverage.

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