A 51-year-old man presented with a low-grade chondrosarcoma of the L1 vertebral body, which had been completely resected. Radiotherapy was indicated; however, the close proximity of critical structures such as the spinal cord and the kidneys complicated the delivery of the high radiation dose required. In this paper we present the clinical and technical aspects of the radiotherapy technique used to treat this patient; we also describe the patient immobilization and radiation dose verification techniques used. The patient was treated with 18 MV photons using multiple field arc rotations with fields asymmetric with respect to the rotational axis of the collimator head. The spinal cord was aligned along the horizontal axis of the linear accelerator isocenter, and the patient was immobilized using a 10 degrees styrofoam wedge combined with an Alpha Cradle mold. In this manner, the patient was stabilized in a comfortable position, which facilitated the daily check of the isocenter position within the patient. Radiation dose verification was carried out with films in an anthropomorphic phantom and with an ionization chamber in a water phantom. These measurements confirmed the accuracy of the isodose distributions calculated for the treatment with asymmetric fields in the arc therapy mode. The use of this technique in conjunction with the positioning device permitted the delivery of 64 Gy to the L1 vertebral body with relatively low doses to the critical structures, amounting to 32 Gy at the surface of the spinal cord and less than 16 Gy to half of both kidneys.