Purpose: HyperArc VMAT is used to treat brain lesions and gaining globe interest. To expand this modality, we demonstrated feasibility of the modified HyperArc module to treat locally recurrent/advanced or previously-irradiated head&neck cancers with stereotactic radiotherapy (SRT).
Methods: RANDO head phantom was imaged, planned, and irradiated (35Gy in 5 fractions,6MV-FFF beam) with eight HyperArc VMAT plans (4 right and 4 left neck tumors) at different anatomical locations (C1-C4) using the original HyperArc fixed-geometry. Mean tumor volume was 21.7cc(13.0–32.3cc). Average distance to isocenter from the central marker of the Qfix Encompass device down to the neck tumors was 25.8cm (21.8–28.0cm) for left and 24.3cm (21.0–27.1cm) for right-sided neck tumors and about 9cm from the lateral markers providing patient’s protection zone. The HyperArc fixed-geometry was modified by adding new partial arcs with 20°couch spacing and selectively assigning 5-unilateral arcs for these neck tumors down to C4-level avoiding entrance through the brain. Original plans were re-planned using modified HyperArc geometry. Plan quality and feasibility variables were compared.
Results: For similar target coverage, conformity, dose fall-off and heterogeneity, using modified HyperArc plans, mean/maximum doses to all critical organs (cord and skin) were reduced by 5% to 20%/10% to 30%. The 50% dose-spillage and brain dose was decreased by 2Gy/3Gy. Both plans showed similar total monitor units, modulation factor, beam-on time, and were delivered in less than 15min (including set up and CBCT imaging) with no collision issue. Six-degrees-of-freedom, couch corrections average was <1.0 mm and 1.5°.
Conclusion: Modified HyperArc VMAT geometry provided highly conformal dose distribution, rapid dose fall-off, excellent sparing of critical organs including brain. These highly accurate, fast, and efficient SRT treatments could be delivered for locally advanced/recurrent head&neck tumors to as low as C4-level. Our modified technique avoids collision issue with precise patient set and could become an attractive head&neck SRT.