Mixed modality intensity-modulated radiation therapy treatment planning for intracranial lesions

Purpose: Intensity-modulated radiotherapy may be improved by incorporating limited-range electrons into photon beam therapy. We examined the feasibility of inverse treatment planning with intensity-modulated photon fields, either alone or combined with uniform high-energy electron fields, for intracranial tumors. Our goal was to generate dose distributions superior to those generated with conventional three-dimensional conformal techniques. Materials and Methods: Optimized three-dimensional treatment plans were compared to intensity-modulated photon plans with and without unmodulated high-energy electron fields for a cohort of previously treated brain tumors. Our in-house optimization system employed an iterative conjugate gradient search algorithm for cost function minimizing. Each set of plans was constrained to identical dose volume limits for critical non-target structures and dose prescription specifications for the planning target volume. In addition, each set used almost identical photon beam orientations to facilitate comparisons (for intensity-modulated plans, parallel opposed fields were slightly off-set to more effectively utilize the dosimetric advantages of inverse planning). Dosimetric comparisons were performed by examining planar and volumetric isodose distributions as well as dose-volume histograms. In particular, differences in integral dose to non-target brain tissue were evaluated. All plans were designed for implementation on a standard Varian 2100C with dynamic multileaf capability. Results: Peripheral targets demonstrated the greatest benefit from mixed modality intensity-modulated treatment planning. The principle dosimetric advantage was a decreased integral dose to the normal brain when calculated by taking a first moment integral of a differential dose volume histogram of normal brain tissue. The majority of this benefit was typically achieved through at least a 50% reduction in the volume of normal tissue receiving more than 80% of the prescription dose. For centrally located tumors, intensity-modulated photon planning was superior to three-dimensional planning, but incorporation of electron fields was detrimental to normal brain sparing. Conclusion: For peripheral brain tumors, inverse planning with intensity-modulated photon fields combined with uniform electron fields permits further reductions in integral non-target brain dose when compared to conventional three-dimensional and intensity-modulated planning. This dosimetric improvement is accomplished by exploiting the rapid dose fall-off of electrons and the capacity of intensity-modulated photons plans to the conform to target regions underdosed by electrons. Since a lower dose of photons is required for mixed modality therapy, the integral dose to non-target brain is decreased compared to single modality plans