POTENTIAL ADVANTAGES OF PROTONS OVER CONVENTIONAL RADIATION BEAMS FORPARASPINAL TUMORS

Background and purpose: Conformal treatment planning with megavoltage X-rays and protons was studied in an attempt to determine if there are advantages of boost therapy with protons instead of X-rays for a pati ent with a tumour growing around the cervical spinal cord. Materials a nd methods: A patient with a Ewing sarcoma was selected for the model study. The proton boost plan was realised with a six beam patched tech nique. Several X-ray boost techniques were planned, some not yet pract ically realisable. The techniques giving the best dose distributions a nd the best tumour control probabilities in the absence of significant late toxicity were looked for. The boost techniques were added to two large lateral X-ray beams covering the planning target volume (PTV) a nd the main risk organ, the spinal cord. The evaluation was made with two biological models, i.e. the tumour control probability (TCP) model , proposed by Webb and Nahum (Webb, S. and Nahum, A.E. A model for cal culating tumour control probability in radiotherapy including the effe ct of inhomogeneous distributions of dose and clonogenic cell density. Phys. Med. Biol. 38: 653-666, 1993), and the normal tissue complicati on probability (NTCP) model, first derived by Lyman (Lyman, J.T. Compl ication probability as assessed from dose-volume histograms. Radiat. R es. 104: s13-s19, 1985). Results: The comparison showed small but clea r advantages of protons for the boost. At 1% NTCP in the spinal cord, the calculated TCP was on average 5% higher. However, depending on the values of the parameters chosen in the biological models, the gain fo r protons varied from 0-10%. The smallest gains were seen in radiosens itive tumours for which the TCP was close to 100% with any of the tech niques and in radioresistant tumours for which neither technique resul ted in any appreciable probability of local cure. Conclusion: Protons appear to have therapeutic advantages over conventional radiotherapy i n tumours with relatively high radiosensitivity situated close to the spinal cord. (C) 1997 Elsevier Science Ireland Ltd.