Spinal cord tolerance to high-dose fractionated 3D conformal proton-photon irradiation as evaluated by equivalent uniform dose and dose volume histogram analysis

Abstract

Purpose To evaluate cervical spinal cord tolerance using equivalent uniform dose (EUD) and dose volume histogram (DVH) analysis after proton-photon radiotherapy. Methods and material The 3D dose distributions were analyzed in 85 patients with cervical vertebral tumors. Mean follow-up was 41.3 months. The mean prescribed dose was 76.3 Cobalt Gray Equivalent (CGE = proton dose × RBE 1.1). Dose constraints to the center and the surface of the cervical cord were 55–58 CGE and 67–70 CGE, respectively. Dose parameters, DVH and EUD, were calculated for each patient. The spinal cord toxicity was graded using the European Organization for Research and Treatment of Cancer (EORTC) and Radiation Therapy Oncology Group (RTOG) late effects scoring system. Results Thirteen patients experienced Grade 1–2 toxicity. Four patients had Grade 3 toxicity. For the dose range used in this study, none of the dosimetric parameters was found to be associated with the observed distribution of cord toxicities. The only factor significantly associated with cord toxicity was the number of surgeries before irradiation. Conclusion The data and our analysis suggest that the integrity of the normal musculoskeletal supportive tissues and vascular supply may be important confounding factors of toxicity at these dose levels. The results also indicate that the cervical spinal cord dose constraints used in treating these patients are appropriate for conformal proton-photon radiotherapy.