Keloid Treatment by External Beam Radiotherapy – Optimisation of BED Values and Fractionation
Regimens
Henry Weatherburn1 PhD and John Glees1 MD, FRCR, DMRT and Kamal Malaker2 MD, PhD
BACKGROUND
The biologically effective dose (BED) is a radiobiological method for comparing radiotherapy dose-fractionation schedules by converting total dose and dose per fraction into a common biological effect scale. This is based on the linear–quadratic (LQ) model for cell survival, from which an “α/ ratio” (i.e. the dose, in Gy, where the linear/single-hit and quadratic /multi-hit cell killing components of dose are equal) is determined for the tissue under consideration. The LQ model has proven to be robust across many tissues but uncertainty persists in keloid radiotherapy, both because fibroblasts are involved and because BED values for keloid treatment have frequently been reported without specifying whether they are calculated using α/ =10 (BED10), as employed in early radiobiological analyses, or 2.5 (BED2.5), as increasingly argued to be more appropriate for the post-excisional keloid bed. This ambiguity complicates interpretation of dose– response relationships and comparison between treatment regimens.
Further controversy concerns whether outcome is determined primarily by dose, radiation modality, fractionation schedule, timing of postoperative irradiation, or whether these factors converge within an optimal therapeutic BED window, consistent with Dale’s funnel model of radiobiological optimisation.
CONCLUSION
Systematic review of available outcome data supports a model of BED guided optimisation, rather than simple dose escalation (e.g. up to a historically proposed BED10 = 30 Gy) or fraction-number selection, as a rational framework for post-excisional keloid radiotherapy. Current evidence suggests an effective therapeutic BED window may exist in which durable recurrence control can be achieved while
minimising toxicity, consistent with Dale’s funnel model.
A single 10 Gy fraction delivered within 24 hours appears to lie within this effective window between BED2.5 = 40 Gy and BED2.5 = 60 Gy and compares favourably with more fractionated regimens, when biological dose and follow-up maturity are considered. The evidence further suggests that ambiguity surrounding α/ assumptions has contributed substantially to confusion in the literature and that BED values should always be explicitly reported as BED2.5 or BED10.
While prospective comparative studies remain needed, present evidence increasingly supports the hypothesis that achieving an optimal BED window and treating early may matter more than fraction number per se.
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