Examination of GyE system for HIMAC carbon therapy

A retrospective analysis was made to examine appropriateness in the estimation of the biologic effectiveness of carbon-ion radiotherapy using resultant data from clinical trials at the heavy-ion medical accelerator complex (HIMAC) at the National Institute of Radiological Sciences in Chiba, Japan.

At HIMAC, relative biologic effectiveness (RBE) values of therapeutic carbon beams were determined based on experimental results of cell responses, on values expected with the linear-quadratic model, and based on experiences with neutron therapy. We use fixed RBE values independent of dose levels, although this apparently contradicts radiobiologic observations. Our RBE system depends only on LET of the heavy-ion radiation fields. With this RBE system, over 2,000 patients have been treated by carbon beams. With data from these patients, the local control rate of non-small-cell lung cancer was analyzed to verify the clinical RBE of the carbon beam. The local control rate was compared with rates published by groups from Gunma University and Massachusetts General Hospital. Using a simplified tumor control probability (TCP) model, clinical RBE values were obtained for different levels of TCP.

For the 50% level of the clinical TCP, the RBE values nearly coincide with those for in vitro human salivary gland cell survival at 10%. For the higher levels of clinical TCP, the RBE values approach closer to those adapted in clinical trials at HIMAC.