Optimal Treatment Planning for Skull Base Chordoma: Photons, Protons, or a Combination of Both?

Purpose: We compared dosimetry of proton (PR), intensity modulated radiation therapy (IMRT) photon (PH), and combined PR and IMRT PH (PP) irradiation of skull base chordomas to determine the most optimal technique. Methods and Materials: Computed tomography simulation scans of 5 patients with skull base chordoma were used to generate four treatment plans: an IMRT PH plan with 1-mm planning target volume (PTV; PH1) for stereotactic treatment, an IMRT PH plan with 3-mm PTV (PH3) for routine treatment, a PR plan with beam-specific expansion margins on the clinical target volume, and a PP plan combining PR and PH treatment. All plans were prescribed 74 Gy/Cobalt Gray equivalents (CGE) to the PTV. To facilitate comparison, the primary objective of all plans was 95% or greater PTV prescribed dose coverage. Plans then were optimized to limit dose to normal tissues. Results: PTVs ranged from 4.4 to 36.7 cc in size (mean, 21.6 cc). Mean % PTV receiving 74 Gy was highest in the PP plans (98.4%; range, 96.5-99.2%) and lowest in the PH3 plans (96.1%; range, 95.1-96.7%). PR plans were the least homogeneous and conformal. PH3 plans had the highest mean % volume (V) of brain, brainstem, chiasm, and temporal lobes greater than tolerance doses. The PH1 plans had the lowest brainstem mean % V receiving 67 Gy (V_67Gy; 2.3 Gy; range, 0-7.8 Gy) and temporal lobe mean % V_65Gy (4.3 Gy; range, 0.1-7.7 Gy). Global evaluation of the plans based on objective parameters revealed that PH1 and PP plans were more optimal than either single-modality PR or PH3 plans. Conclusions: There are dosimetric advantages to using either PH1 or PP plans, with the latter yielding the best target coverage and conformality.