Figure 6.
Two intensity-modulated proton therapy plans for a patient with an ependymoma in whom the target volume involves parts of the brainstem. The patient was treated with 3 posterior oblique beams. The left panel shows the 3 fields as prescribed by the planning system (created based on a 2 mm clinical target volume to planning target volume expansion). The right panel shows the 3 fields as prescribed based on LET optimization obtained after minimizing (LETd × dose × empirical constant (0.04 μm/keV)) in the brainstem while constraining the dose distribution to remain close to the conventional plan. The fourth row shows LETd × dose × empirical constant (0.04 μm/keV) for all 3 fields clearly illustrating the reduction in biological effect in the brainstem. See reference [84] for more details. Abbreviation: LETd, dose-averaged linear energy transfer.