Biological characteristics of carbon-ion therapy

Radiotherapy using charged and/or high-linear energy transfer (LET) particles has a long history, starting with proton beams up to now carbon-ions. Radiation quality of particle beams is different from conventional photons, and therefore the biological effects of high-LET irradiation have attracted scientific interests of many scientists in basic and clinical fields. A brief history of particle radiotherapy in the past half-century is followed by the reviewed biological effectiveness of high-LET charged particles.

The latter includes 54 papers presenting 506 RBE (relative biological effectiveness) values for carbon ions and a total of 290 RBE values for other ions identified from 48 papers. By setting a selection window of LET up to 100 keV/microm, we fitted a linear regression line to an LET-RBE relation. The resulting slope of the regression line had a dimension of microm/keV, and showed different steepness for different cells/tissues and endpoints as well. The steepest regression was found for chromosome aberration of human malignant melanoma while the shallowest was for apoptosis of rodent cells/tissue. Both tumour and normal tissue showed relatively shallower slopes than colony formation.

In general, there is a large variation of slope values, but the majority (25 out of 29 values) of data was smaller than 0.05 microm/keV.