Impact of field number and beam angle on functional image-guided lung cancer radiotherapy planning

To investigate the effect of beam angles and field number on functionally-guided intensity modulated radiotherapy (IMRT) normal lung avoidance treatment plans that incorporate hyperpolarised helium-3 magnetic resonance imaging (³He MRI) ventilation data. Eight non-small cell lung cancer patients had pre-treatment ³He MRI that was registered to inspiration breath-hold radiotherapy planning computed tomography. IMRT plans that minimised the volume of total lung receiving  ⩾20 Gy (V₂₀) were compared with plans that minimised ³He MRI defined functional lung receiving  ⩾20 Gy (fV₂₀). Coplanar IMRT plans using 5-field manually optimised beam angles and 9-field equidistant plans were also evaluated. For each pair of plans, the Wilcoxon signed ranks test was used to compare fV₂₀ and the percentage of planning target volume (PTV) receiving 90% of the prescription dose (PTV₉₀). Incorporation of ³He MRI led to median reductions in fV₂₀ of 1.3% (range: 0.2-9.3%; p  =  0.04) and 0.2% (range: 0 to 4.1%; p  =  0.012) for 5- and 9-field arrangements, respectively. There was no clinically significant difference in target coverage. Functionally-guided IMRT plans incorporating hyperpolarised ³He MRI information can reduce the dose received by ventilated lung without comprising PTV coverage. The effect was greater for optimised beam angles rather than uniformly spaced fields.