Session Item

Patients treated with radiotherapy for H&N cancer unfortunately suffer from high rates of post treatment complications, due to the significant number of radiosensitive OARs within close proximity to large tumour volumes. Non-coplanar 4π-VMAT with increased degrees of freedom has the potential to create more conformal plans that would likely benefit this patient group. The aim of this study was to develop and independently test a quasi-optimal non-coplanar 4π-VMAT configuration.  

The study was divided into two main phases. In phase one, a cohort of ten previously treated H&N patients were replanned using a 12-arc plan; 11 non-coplanar arcs and a coplanar arc. The 12-arc plans were reduced to a single arc plan by using an iterative algorithm based on MU. The arc contributing the fewest MU was deleted from the plan, the plan re-optimised and the process repeated. Consequently, it was possible to determine how the number of arcs in a plan affects certain dose metrics and which couch rotation angles seem preferable for this patient group.  In phase 2, a preferred non-coplanar 4π-VMAT configuration developed from phase 1 was applied to an independent cohort of ten H&N patients, and the effect on OAR doses assessed. The resulting plans were delivered and timed for comparison with the standard 2-arc coplanar approach.  

The gradient index, along with spinal canal and parotid doses, were minimised when as few as three non-coplanar arcs were used. In addition, non-coplanar arcs with large couch rotation angles survived for longer during the iterative process and therefore were deemed superior. In phase two, statistically significant dose reductions were obtained when using a 4-arc solution consisting of a coplanar arc and three non-coplanar arcs with 90° and ±75° couch rotations. These included mean reductions in the doses to the spinal canal D_0.1cc (3.83±1.33 Gy, p < 0.001), brainstem D_0.1cc (5.15±3.71 Gy, p = 0.019), contra-lateral parotid D_mean (3.83±2.06 Gy, p = 0.005) and ipsi-lateral parotid D_mean (4.18±1.99 Gy, p = 0.002). The mean delivery time was 450±32 s, compared to 170±9 s for the original, 2-arc coplanar plans. The 4π-VMAT delivery time can likely be reduced via intelligent sequencing of arcs to minimise gantry/couch rotations and applying single button press delivery techniques. Assessing the dosimetric accuracy of these plans will form part of the ongoing work. 

By using a simple, iterative algorithm, a quasi-optimal non-coplanar class-solution was developed from a small cohort of patients. Non-coplanar VMAT for H&N cancer shows promise in reducing OAR doses and therefore side effects from treatment.