The role of biologically effective dose (BED) in clinical oncology

There are many clinical situations in which radiobiological considerations can be usefully applied and all clinicians should be aware of the potential benefits of developing a quantitative radiobiological approach to their pr actice. The concept of biologically effective dose (BED) in particular is u seful for quantifying treatment expectations, but clinical oncologists shou ld recognize that careful interpretation of modelling results is required b efore clinical decisions can be made and that there is a lack of reliable h uman parameters for application in some situations. Correct use of the BED concept will, in more complex treatment situations, sometimes involve the u se of multiple parameters and BED calculations. Examples include, 1. Where the dose per fraction is being altered and it is possible that nor mal tissue tolerance may be compromised, calculations should include two or more alpha/beta ratio values, some being less than 3 Gy, in order to estim ate the 'worst case scenario'. 2. A single one-point BED calculation will not be representative of the bio logical effect throughout a large planning target volume where there are si gnificant 'hot spots'. Multiple BED evaluations are then indicated. 3. Where there are combinations of radiotherapy treatments or phases of tre atments, these can be quantitatively assessed by the addition of BEDs, alth ough the volume of tissue is not inherently included in the BED calculation and any high-dose region needs to be separately assessed as in point 2, 4. Allowance for tumour clonogen repopulation during therapy is required fo r some tumour types. 5. Different histological classes of cancers require the use of different a lpha/beta ratios. Where there is reasonable doubt regarding this parameter, a suitable range should be used. The principles involved are illustrated by worked examples. Attention to de tail and the examination of ranges of possible results should offer a safer guide to alternative dose fractionation schedules, although the ultimate c hoice will be tempered by clinical circumstances.