Skull base chordomas are challenging tumors to treat due to their complex shape and proximity to critical structures. We compared the dosimetry of proton (PR), intensity modulated radiation therapy (IMRT) photon (PH), and combined proton and IMRT photon (PP) irradiation of skull-based chordomas to determine the best technique based on a scoring system comprised of several dosimetric parameters. We acquired the CT simulation scans of five patients who received irradiation to gross residual disease following resection of skull base chordoma. Four plans were generated per patient: an IMRT PH plan with a 1 mm PTV (PH1), used in stereotactic treatment, an IMRT PH plan with a 3 mm PTV (PH3), used in routine treatment, a PR plan with a 3 mm smearing, and a PP plan with 3 mm smearing for proton treatment and 1 mm PTV for the photon segment. All plans were prescribed 74 Gy or CGE to the PTV. In the PP plans, 50 CGE plus a 24 Gy PH boost was prescribed. To facilitate comparison, the primary objective of all plans was ≥95% PTV coverage with the prescribed dose. Plans were optimized to limit dose to organs at risk without sacrificing coverage. Plans were compared via mean values calculated from DVH data of clinically significant parameters. Plans were ranked 1 (best)–4 (worst) for each parameter and rank scores were summated (Table). Target volumes ranged from 4.4–36.7 cc (mean 21.58). The mean % PTV receiving 74 Gy was highest in the PP plan (98.35, range 96.5–99.2), followed by the PR plan (97.3, range 96.1–99.3), the PH1 plan (96.4, range 95.4–97.4), and the PH3 plan (96.1, range 95.1–96.7). The PR plan was the least homogeneous and conformal. Brain mean %V30 Gy was highest for PH3 plans (7.08, range 5.2–9.1) and lowest for PR plans (4.2, range 2.8–6.2). PH3 plans had the highest mean %V of brainstem, chiasm and temporal lobes above tolerance doses. The PH1 plan had the lowest brainstem mean %V67 Gy (2.25, range 0–7.8) and temporal lobes mean %V65 Gy (4.3, range 0.1–7.7). Global evaluation of the plans based on their total ranks scores revealed that PH1 plans and PP plans were more optimal than either single modality PR or PH3 plans (Table). Due to scheduling constraints, some proton centers have used a combination of photon and proton treatment in skull base chordomas. There may be a dosimetric advantage for using PP, as such plans appear to be the most conformal and give the best target coverage. Further investigation of this approach is underway.Tabled 1Dosimetric Parameters and Treatment PlansParameterPH3PH1PPPRPTV74 Gy4312Inhomogeneity Coefficient (D5%-D95%)/D95%2134Conformality Index (V95%/PTV)3214Brain V30 Gy4231Brainstem V67 Gy4132Chiasm V60 Gy4312Temporal Lobes V65 Gy4123Total25131418 Open table in a new tab