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Improved Normal Tissue Sparing by Limiting Jaw Opening During Volumetric Modulated Arc Therapy for Cancer of the Scalp

J Zhang

J Zhang1,2*, M Leu3 , J Cho-Lim1,2 , W Inouye1,2 , S Lee1,3,4 , (1) Department of Radiation Oncology, VA Long Beach Healthcare System, Long Beach, CA, (2) Department of Radiation Oncology, University of California, Irvine, Orange, CA (3) Department of Radiation Oncology, VA Greater Los Angeles Healthcare System, Los Angeles, CA, (4) David Geffen School of Medicine, University of California, Los Angeles, CA


SU-K-FS2-12 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: Four Seasons 2

Purpose: To improve dosimetric outcome of volumetric modulated arc therapy (VMAT) for diffuse scalp cancer by implementing a segmented organ-at-risk (OAR)-avoiding approach with optimal jaw opening and collimator angle.

Methods: Radiation dose coverage for a patient with superficially extensive squamous cell carcinoma of the frontal scalp was analyzed using CT-simulated images with 1-cm bolus placed over the lesion. After contouring of clinical target volume, optic nerves, chiasm, lens, brainstem, and brain, the dosimetry plans were compared using three separate techniques: 7-field IMRT, 2-arc VMAT, and jaw-limited (J-L) 2-arc VMAT. Each arc of the J-L VMAT technique was separated into 3 to 5 segmented sub-arcs with the jaw settings and collimator angles adjusted to conform tangentially over the scalp while sparing the majority of the brain medially. A dose of 55Gy in 22 fractions was prescribed to at least 95% of the planned target volume.

Results: All three plans were analyzed at the same V55Gy. Three dose volumes for the brain were compared: V10Gy were 42.8%, 48.4% and 7.9% for IMRT, VMAT and J-L VMAT, respectively; V20Gy were 29.8%, 29.7% and 5.7%; V30Gy were 12.1%, 6.4% and 3.4%. Dmax for the optic nerves were 5.9 Gy, 3.6 Gy and 1.3 Gy for IMRT, VMAT and J-L VMAT, respectively; Dmax for the lens were 5.4 Gy, 2.8 Gy and 2.2 Gy; and Dmax for the brainstem were 4.5 Gy, 3.3 Gy and 0.6 Gy. The J-L 2-arc VMAT plan was thus used for actual radiotherapy. At 6-week post-treatment, complete tumor response was achieved while skin cosmesis attained excellence in comparison with pre-treatment condition.

Conclusion: Although fixed-field IMRT or conventional VMAT planning could be automated to produce readily acceptable dose-volume parameters via inverse planning, the integral dose to the brain and other OARs can be further minimized by implementing the J-L VMAT technique.

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