Advantage and limitations of weighting factors and weighted dose quantities and their units in boron neutron capture therapy

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Abstract
Defining the parameters influencing the biological reaction due to absorbed dose is a continuous topic of research. The main goal of radiobiological research is to translate the measurable dose of ionizing radiation to a quantitative expression of biological effect. Mathematical models based on different biological approaches (e.g., skin reaction, cell culture) provide some estimations that are often misleading and, to some extent, dangerous. Conventional radiotherapy is the simplest case because the primary radiation and secondary radiation are both low linear energy transfer (LET) radiation and have about the same relative biological effectiveness (RBE). Nevertheless, for this one‐dose‐component case, the dose–effect curves are not linear. In fact, the total absorbed dose and the absorbed dose per fraction as well as the time schedule of the fractionation scheme influence the biological effects. Mathematical models such as the linear‐quadratic model can only approximate biological effects. With regard to biological effects, fast neutron therapy is more complex than conventional radiotherapy. Fast neutron beams are always contaminated by γ rays. As a consequence, biological effects are due to two components, a high‐LET component (neutrons) and a low‐LET component (photons). A straight transfer of knowledge from conventional radiotherapy to fast neutron therapy is, therefore, not possible: RBE depends on the delivered dose and several other parameters. For dose reporting, the European protocol for fast neutron dosimetry recommends that the total absorbed dose with γ‐ray absorbed dose in brackets is stated. However, boron neutron capture therapy (BNCT) is an even more complex case, because the total absorbed dose is due to four dose components with different LET and RBE. In addition, the terminology and units used by the different BNCT groups is confusing: absorbed dose and weighted dose are both to be stated in grays and are never “photon equivalent.” The ICRU/IAEA made proposals, which should be followed by all BNCT groups, to report always the four absorbed dose components, boron dose proton dose γ‐ray dose and neutron dose as well as the sum of all components, as total absorbed dose, together with the total weighted dose (to be used only for internal purposes, indicating the used weighting factors) at all points of interest and the treatment conditions.