Improved dose localization with dual energy photon irradiation in treatment of lateralized intracranial malignancies

Dual energy photon irradiation (6 MV and 20 MV) was compared to conventional treatment planning with 6 MV photons in a lateralized intracranial malignancy. Dose volume analysis was performed of both the tumor plus a 2 cm margin (target volume, TV) and normal tissues (NT). Parallel opposed treatment using weightings of 1:1, 1.5: 1, and 2:1 were compared for 6 MV photons alone or in combination with 20 MV photons. Uniform treatment of the TV was accomplished within the 60 Gy isodose. Significant differences were observed, however, in NT volumes receiving ≥60 Gy and 45-59 Gy. Dual photon energy reduced treatment of NT volumes to ≥60 Gy by 13% (177 cm 3 vs 204 cm³ in 2:1 weighting) to 70% (147 cm³vs 498 cm³ in 1:1 weighting) for comparable plans. Dose optimization was also performed for both 6 MV alone or in combination with 20 MV photons. Usual approaches to achieve dose lateralization with conventional isocentric techniques were applied including parallel opposed 6 MV photons ipsilaterally weighted 3.4:1 (POP), and a 110° arc rotational field used to limit treatment to the eye (ARC). Dual energy photon optimized plans included a three beam parallel opposed plan (TOP) and a mixed photon ipsilateral (IPSI) approach. The technique using parallel opposed 20 MV photons and ipsilateral 6 MV photons (TOP) used beam weightings of 1.1 (contralateral 20 MVX): 1.6 (ipsilateral 6 MVX): 1 (ipsilateral 20 MVX) to achieve dose optimization. The ipsilateral approach with 6 MVX and 20 MVX (IPSI) used beam weightings of 1:1.4, respectively. All optimized plans demonstrated a 41% (120 cm³; POP) to 53% (95 cm³; TOP) improvement over parallel opposed 6 MV photons weighted 2:1 (204 cm ³) in NT volume receiving ≥60 Gy. Comparison of optimized treatment showed the IPSI plan to be superior, treating 12% of NT volume to ≥60 Gy and 38% to 45-59 Gy; the 6 MV POP plan resulted in NT volumes of 15% and 51%, respectively, for those dose levels. Dual photon energy irradiation of lateralized intracranial malignancies allows reduction of dose to normal tissue volumes while achieving excellent coverage of the target volume. Treatment planning should be performed in all lateralized intracranial lesions to achieve dose optimization exploiting depth dose characteristics.