SPECIFICATION OF ABSORBED DOSE AND RADIATION QUALITY IN HEAVY-PARTICLE THERAPY

The introduction of heavy particles (hadrons) into radiation therapy a ims at improving the physical selectivity of the irradiation (e.g. pro ton beams), or the radiobiological differential effect (e.g. fast neut rons), or both (e.g. heavy ion beams). Each of these new therapy modal ities requires several types of information before prescribing doses t o patients, as well as for recording and reporting the treatments: (i) absorbed dose measured in a homogeneous phantom in reference conditio ns; (ii) dose distribution computed at the level of the target volume( s) and the normal tissues at risk; (iii) radiation quality from which an evaluation on the RBE could be predicted; and (iv) RBE measured on biological systems or derived from clinical observation. The ICRU has published recommendations for fast neutrons and a similar report is in preparation for proton beams. These recommendations are now universal ly applied. The single beam isodoses and thus the dose distributions a re similar in neutron and photon therapy. Similar algorithms can then be used for treatment planning and the same rules can be followed for dose specification for prescribing and reporting a treatment In hadron therapy, the RBE of the different beams raises specific problems. For fast neutrons, the RBE varies within wide limits (about 2 to 5) depen ding on the neutron energy spectrum, dose, and biological system. For protons, the RBE values range between smaller limits (about 1.0 to 1.2 ). A clinical benefit is thus not expected from RBE differences. Howev er, the proton RBE problem cannot be ignored since dose differences of about 5% can be detected clinically in some cases. The situation is m ost complex with heavy ions since the RBE variations, as a function of particle type and energy, dose and biological system, are at least as large as for fast neutrons. In addition, the RBE varies with depth. R adiation quality thus has to be taken into account when prescribing an d reporting a treatment. This can be done in different ways: (a) descr iption of the method of beam production; (b) computed LET spectra and/ or measured microdosimetric spectra at the points clinically relevant; (c) RBE determination. The most relevant data are those obtained for late tolerance of normal tissues at 2 Gy per fraction ('reference RBE' ). The 'clinical RBE' selected by the radiation oncologist when prescr ibing the treatment will be close to the reference RBE, but other fact ors (such as heterogeneity in dose distribution) may influence the sel ection of the clinical RBE. Combination of microdosimetric data and ex perimental RBE values improves the confidence in both sets of data.