Novel VMAT Planning Technique Improves Dosimetry for Head and Neck Cancer Patients Undergoing Definitive Chemoradiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> Intensity modulated radiotherapy (IMRT) allows for substantial sparing of organs-at-risk (OARs) without compromising delivery of curative-intent doses in patients with head and neck cancers. Volumetric modulated arc therapy (VMAT) allows for rapid treatment delivery and minimizes fractional treatment time. Given the intrinsic complexity of VMAT planning, standard two-arc techniques often fail to achieve planning objectives. We hypothesize that a simple, institutional approach improves OAR dosimetry compared to a standard two-arc technique. <h3>Materials/Methods</h3> Head and neck cancer patients who received definitive chemoradiotherapy were included in this single institution study. VMAT plans were generated per institutional protocol using 6 MV photon beams. Briefly, three single arcs were employed with assigned collimator angles of (C) 0°, (C) 90°, and (C) 90°. The maximum jaw extension of the (C) 0° field encompassed the entirety of the treatment volume. The inferior jaw on the first collimated arc was set with a maximum extension of 1 cm beyond the isocenter, while the superior jaw of the second collimated arc was set with a maximum extension of 1 cm beyond the isocenter. Comparison plans were generated for each patient using the manufacturer's recommended beam arrangement, comprising two collimated arcs - (C) 30° and a reciprocal (C) 330° - with jaw tracking enabled. Mean differences in target volume and OAR dosimetry between planning approaches were calculated. A two-tailed, paired Student's t-test was employed to determine statistically significant differences in dosimetry between standard and institutional plans. <h3>Results</h3> Ten patients were planned using institutional and standard two-arc protocols. Statistically significant mean improvements were observed in ipsilateral parotid mean dose (∆314.7 cGy, p = 0.007) and V30Gy (∆6.62%, p = 0.004), contralateral parotid mean dose (∆292.6 cGy, p = 0.006) and V30Gy (∆5.36%, p = 0.004), parotid volume receiving less than 20 Gy (∆7.36 cc, p = 0.010), pharyngeal constrictor mean dose (∆269.3 cGy, p = 0.003), ipsilateral brachial plexus maximum dose (∆96.0 cGy, p = 0.036), and global maximum dose (∆1.73%, p = 0.047) with the three-arc technique. No significant differences in PTV, spinal cord, mandible, cricopharyngeus, cochlear, or contralateral brachial plexus dosimetry were identified. Subjectively, dosimetrists reported shorter planning time and easier achievement of objectives with the three-arc approach. <h3>Conclusion</h3> Our institutional approach to head and neck VMAT conveys significant dosimetric improvements compared to a standard VMAT beam arrangement. This approach represents a simple and reproducible solution to planning for head and neck cancer patients which may reduce planning time. Additional research is warranted to further optimize this technique and quantify reductions in planning time.