Isoeffective dose: a concept for biological weighting of absorbed dose in proton and heavier-ion therapies

When reporting radiation therapy procedures, International Commission on Radiation Units and Measurements (ICRU) recommends specifying absorbed dose at/in all clinically relevant points and/or volumes. In addition, treatment conditions should be reported as completely as possible in order to allow full understanding and interpretation of the treatment prescription. However, the clinical outcome does not only depend on absorbed dose but also on a number of other factors such as dose per fraction, overall treatment time and radiation quality radiation biology effectiveness (RBE). Therefore, weighting factors have to be applied when different types of treatments are to be compared or to be combined. This had led to the concept of 'isoeffective absorbed dose', introduced by ICRU and International Atomic Energy Agency (IAEA). The isoeffective dose D(IsoE) is the dose of a treatment carried out under reference conditions producing the same clinical effects on the target volume as those of the actual treatment. It is the product of the total absorbed dose (in gray) used and a weighting factor W(IsoE) (dimensionless): D(IsoE)=D×W(IsoE). In fractionated photon-beam therapy, the dose per fraction and the overall treatment time (in days) are the two main parameters that the radiation oncologist has the freedom to adjust. The weighting factor for an alteration of the dose per fraction is commonly evaluated using the linear-quadratic (α/β) model. For therapy with protons and heavier ions, radiation quality has to be taken into account. A 'generic proton RBE' of 1.1 for clinical applications is recommended in a joint ICRU-IAEA Report [ICRU (International Commission on Radiation Units and Measurements) and IAEA (International Atomic Energy Agency). Prescribing, recording and reporting proton-beam therapy. ICRU Report 78, jointly with the IAEA, JICRU, 7(2) Oxford University Press (2007)]. For heavier ions (e.g. carbon ions), the situation is more complex as the RBE values vary markedly with particle type, energy and depth in tissue.