Dosimetric Comparison of Coplanar, Non-coplanar, and Mixed-Arc VMAT for Head and Face Skin Cancers: A Multi-scenario Analysis.

Abstract

Background/aim: This study compared dosimetric differences in target coverage and organs-at-risk (OARs) sparing among coplanar (co-VMAT), non-coplanar (nonco-VMAT), and mixed-arc (mxd-VMAT) volumetric modulated arc therapy (VMAT) for stereotactic radiation treatment of head and face skin cancers (HFSC).

Patients and methods: Five patients with HFSC, presenting with tumors located in critical areas near OARs were selected to represent distinct clinical scenarios. At least three competing VMAT plans per case (up to five for extensive tumors) were generated. The planning target volume (PTV) was obtained by applying a 1 mm isotropic expansion to the clinical target volume (CTV), except for portions extending beyond the body contour. Dosimetric parameters, including PTV indices [Dmax, D2%, D98%, V95%, conformity index (CI), and homogeneity index (HI)], dose to surrounding healthy tissues, beam-on time (BOT), and monitor units (MU) were evaluated and compared under identical optimization conditions.

Results: Nonco-VMAT improved CI, HI, and OAR sparing for the first (left temporal-zygomatic) and third (nasal pyramid) patients. For the second patient (right frontal and zygomatic targets), mxd-VMAT was optimal for the frontal target, while nonco-VMAT was superior for the zygomatic target. Co-VMAT provided the highest plan quality for the fourth (occipital) patient, though mxd-VMAT slightly reduced OAR doses. For the fifth patient (scalp and vertex), co-VMAT achieved the best balance between target coverage and OAR sparing.

Conclusion: This study highlights the potential benefits of non-coplanar arcs in HFSC treatment. VMAT arc arrangement should be tailored to tumor location, as the inclusion of non-coplanar arcs can enhance plan quality for both target coverage and OAR protection in specific cases. However, non-coplanar techniques may prolong treatment duration due to couch rotations and increased MU, potentially reducing patient tolerability.