SPECIFICATION OF THE DOSE TO ORGANS AT RISK IN EXTERNAL-BEAM RADIOTHERAPY

Reporting of the clinical relevant dose to organs at risk (OR) and oth er normal tissues is crucial in trials and protocols where the aim is to assess late complications and to increase the therapeutic ratio for external beam radiotherapy. The dose distribution in normal tissues a nd ORs are, however, most often heterogeneous, at least when more than two opposing beams are applied. To decide the most clinical relevant dose with respect to late occurring complications is therefore not a s traight forward problem. In this work we discuss what parameters chara cterise the dose-volume-histogram (DVH) best by calculating normal tis sue complication probabilities (NTCPs) by using the Lyman model and va rious sets of statistical parameters drawn out from the DVHs. These NT CPs are compared to NTCPs calculated from the full DVHs, when the sets of parameters are evaluated. Our calculations indicate that the NTCP based on the Lyman model is best correlated to the D-max value, for se rially organised tissues such as the spinal cord. For organs, describe d largely as tissues organised in parallel, the D-median or D-mean of the DVH may be applied. Our calculations reveal that D-mean is the par ameter of choice when D-median is quite small, but when the two parame ters approach each other, D-median will be a better choice, using a un ity volume fraction. For ORs characterised by a mixed serial and paral lel functional structure, as the heart, neither D-max, D-median, nor D -mean may predict the actual NTCP.