Thoracolumbar spinal cord tolerance to high dose conformal proton-photon radiation therapy

To evaluate and understand the tolerance of the thoracolumbar spinal cord using equivalent uniform dose (EUD) and dose volume histogram (DVH) analysis after combined high dose photon-proton radiotherapy.

A total of 68 patients were identified as having high dose radiotherapy, ⩾5900cGy (RBE) in the region of the thoracolumbar spinal cord, defined as extending inferiorly to L2. Pathological diagnosis for patients in this review included chordoma (50 patients, 53.1%), chondrosarcoma (28 patients, 29.8%), osteosarcoma (3 patients, 3.2%), other sarcoma (11 patients, 11.7%), and other (2 patients, 2.1%). Patient data were reviewed retrospectively, detailed dose volume histogram data (DVH) were available for 23 patients. Composite plans and DVH were constructed for both pre-operative and post-operative radiation therapy courses in MIM-Vista software, as available. Dose constraints to the center and surface of the cord were 5400cGy (RBE), and 6300cGy (RBE) respectively, and patients receiving concurrent chemotherapy received an eight percent dose reduction. Spinal cord toxicity was recorded using the RTOG/EORTC late effects scoring system.

Clinical and dosimetric data for each patient were analyzed. Median prescription dose was 7020cGy (RBE), range (5940-7820cGy (RBE)). Median follow-up was 12.9months. Five-year overall survival for all patients in this group was 88.7%, 95%CI (74.7-95.2). One patient suffered from transient paralysis following stem cell transplant for treatment of myelodysplastic syndrome. Other reasons for spinal cord injury following treatment included: local disease progression, noted in 7 patients (10.3%), and direct result of surgery, noted in 8 patients (11.8%). Freedom from neurological injury (RTOG Grade 2 or higher) at 5years was 92.9%(95%CI: 74.6-98.2), at 6years was 80.9%(95%CI: 55.3-92.7), and at 8years 80.9%(95%CI: 55.3-92.7).

Our clinical and dosimetric data suggest that the noted dose constraints are safe and acceptable with regard to spinal cord complications. Pre-existing disease characteristics, surgical complications, as well as tumor progression, appear to be more important factors when it comes to spinal cord toxicity.