Abstract
A nature of inverse physical depth-dose profile of heavy charged particles is biologically enhanced in carbon ions. The characteristic is considered to be beneficial when treating deep-seated tumour. Radiotherapy with those heavy charged particles was first initiated at Lawrence Berkeley Laboratory (LBL) in the United States, then at present followed with carbon ions at National Institute of Radiological Sciences (NIRS) and Hyogo Ion Beam Medical Center in Japan, and Gesellschaft für Schwerionenforschung mbH (GSI) in Germany.
Biological or clinical effects of conventional X-rays are regarded as solely proportional to absorbed dose while those of carbon ions are known to be affected not only with dose but also radiation quality. Then, in order to achieve uniform tumour control in actual therapy, a model is required which converts an absorbed dose distribution of carbon ions into a distribution of corresponding clinical effect. Here RBE, relative biological effectiveness, is practically used for that purpose and clinical dose is given in terms of GyE derived by multiplying absorbed dose with the RBE.
The enormous complexity of the RBE determination hinders itself from being understood even at this moment. In Japan, RBE is estimated from a combination of cell-survival response of human salivary gland tumour cell line (HSG) against carbon ions and clinical experiences in fast neutron therapy at NIRS. GSI, on the other hand, calculates RBE from a combination of tumour response against X-rays and microscopic dose distribution formed by individual ion in the beam. The difference in model or endpoint may result in different quantity in clinical dose between two facilities though apparently expressed with the same unit, GyE. In this talk, the difference in model and resultant clinical dose distribution is shown through an intercomparison on actual treatment planning. Clinical outcome (tumour control probability, TCP) is also analyzed by a TCP model in order to verify the RBE model as well as to clarify unique characteristics of carbon ion radiotherapy.
