Hypofractionated stereotactic radiation therapy for skull base and upper cervical chordoma and chondrosarcoma: preliminary results.

INTRODUCTION

Chordoma and chondrosarcoma are rare tumors of the base of the skull and are difficult candidates for surgical treatment. They are also usually resistant to conventional radiation therapy. We report preliminary results of hypofractionated stereotactic radiation therapy (SRT) using the Cyberknife system (Accuray Inc., Sunnyvale, Calif., USA) for primary and recurrent chordomas and chondrosarcomas of the skull base and upper cervical region.

MATERIAL AND METHODS

Nine pathologically proven chordoma/chondrosarcoma patients underwent Cyberknife treatment, and in 4 patients Cyberknife was performed as a primary adjuvant treatment after operation. Remainder of the patients had previously received conventional radiotherapy except 1 who had received Gamma Knife treatment. The prescribed tumor dose ranged from 21 to 43.6 Gy in three to five fractions. The dosimetric characteristics were evaluated for conformity and coverage indices. Dose volume histograms of both the tumor and the critical structure were obtained, and the dose delivered to a specific volume (25, 50 and 100%) of the critical structure was calculated in each case. Mass response was measured on follow-up MRI scans. Total tumor doses of different fractionation numbers were converted into single session equivalent doses and linear quadratic equivalent doses of conventional radiation for comparison among patient groups.

RESULT

No significant complications were observed during the treatment and early follow-up periods except one instance of transient esophagitis and one instance of otitis. All treatment plans met the criteria for standard protocol of radiosurgery suggested by the Radiation Therapy Oncology Group, specifically in terms of conformity index, which ranged from 1.01 to 1.83. Three plans had a coverage index that was rated as a minor acceptable deviation. All patients were followed from 11 to 30 (median 24) months following the treatment with regular magnetic resonance images, and 4 patients showed mass reduction. Disease progression was not noted in any patient during the above follow-up period except 1 patient who showed asymptomatic recurrence on 27-month follow-up MRI. Dose volume histograms revealed that the relative dose to volume percent of critical structure, measured at 25, 50 and 100%, was apparently lower in the pretreatment surgical decompression group than in the nondecompression group. Two patients developed radiation-induced myelopathy. The delivered radiation dose to the critical structure calculated using the linear-quadratic formula was within the acceptable range in one case and exceeded 70 Gy at 50% volume of the spinal cord in the other.

CONCLUSION

The hypofractionated Cyberknife SRT is effective in generating therapeutic response in these radioresistant tumors, with minimal toxicity during the procedure and early follow-up period. Repeated radiosurgical treatment may also be feasible for tumor recurrences but deserves great caution with respect to the biological effects of the accumulated dose on the adjacent critical structures. Cyberknife SRT may be a potentially valuable treatment option once the long-term results and appropriate dose calculators are optimally defined.