Dosimetric comparison of conventional helmet-field whole-brain irradiation with three-dimensional conformal radiotherapy: dose homogeneity and retro-orbital area coverage

The aim of this study was to compare the dosimetric data from conventional two-dimensional (2D) helmet-field whole-brain irradiation (WBI) with those from three-dimensional conformal radiotherapy (3D-CRT), and to investigate the potential benefits of 3D-CRT as regards both dose coverage of the brain and retro-orbital (RO) area and ocular lens protection. Simulation CT scans of 30 patients were used. In 3D-CRT planning, the brain, optic nerves, ocular lenses and RO areas were contoured. Two opposed lateral fields were used and matched non-divergently behind the ocular lenses. The nominal prescribed photon beam dose was 1800 cGy in 10 fractions using 6 MV photons. Brain and RO areas were covered by at least 95% and 90% of the prescribed dose, respectively, and the maximum doses were limited to 110% of prescribed dose. In conventional 2D planning, the same field centre and angles were used as for the 3D-CRT plans. Fields were created using digital reconstructed radiographs and bony reference marks without information on the contour of structures. Brain, ocular lens and RO area doses were compared using cumulative dose-volume histograms. The mean value of minimum brain doses was significantly higher (p = 0.008) for 3D-CRT plans: all patients received a minimum of 95% of the prescribed dose. Mean doses of the left (p = 0.004) and right (p = 0.003) RO areas were also higher for 3D-CRT plans than for conventional 2D plans: all patients received a minimum of 90% of the prescribed dose. Furthermore, the mean values of minimum doses for both RO areas were statistically higher for 3D-CRT (p<0.001). 3D-CRT planning significantly improved the coverage of the RO areas and the dose homogeneity in WBI while protecting the ocular lenses.