Quantification of cytotoxic chemotherapy effects within the linear-quadratic model of radiotherapy

The use of chemoradiotherapy (CRT) is widespread but quantitative evaluation of the benefit accruing from concomitant use of cytotoxic drugs is difficult since radiation and chemotherapy “doses” are currently expressed in different and incompatible ways. As the biological impact of any radiotherapy schedule may be expressed in terms of a biologically effective dose (BED) it is shown that chemotherapy effects can be similarly quantified, allowing drug contributions to be expressed in terms of a radiation-equivalent quantity which is familiar to radiation oncologists. This paper discusses various ways in which chemotherapy can enhance radiation effect and derives the associated BED equations. The availability of such equations can potentially help inform clinical judgement in relation to the true value of CRT treatments and also allows derivation of alternative, radiation-only, schedules.