By applying the Mean Distance to Conformity (MDC) index to the dose distribution of a radiotherapy plan, the conformity of the 95% isodose line to the Planning Treatment Volume (PTV) can be measured, allowing both overdosing of normal tissue and underdosing of the target volume to be measured by OverMDC and UnderMDC respectively. The aim of this study was to analyse this aspect of treatment planning, in addition to volume of PTV and the treatment delivery method (3D conformal or Intensity-Modulated Radiotherapy), and relate to patient outcome within the context of clinical trial.The OverMDC and UnderMDC of the 95% isodose line (50Gy prescribed dose) to the PTV was calculated for 97 patients from the UK SCOPE 1 trial on definitive chemoradiotherapy in oesophageal cancer. Data preparation was performed in CERR, Kaplan-Meier and multivariate analysis undertaken in EUCLID and further tests in Excel and SPSS. A statistically significant breakpoint in the overall survival data, independent of cetuximab, was found with OverMDC (0.44cm, p<0.05). This was not the case with UnderMDC. There was a statistically significant difference in PTV volume either side of the OverMDC breakpoint (Mann Whitney p<0.001) and in OverMDC value dependent on the treatment delivery method (mean IMRT=0.21cm, mean 3D-CRT=0.41cm Mann Whitney p<0.001). Re-planning the worst performing patients according to OverMDC from 3D-CRT to VMAT resulted in a mean reduction in OverMDC of 0.28cm (0.16–0.40cm). OverMDC was not significant in multivariate analysis that included age, sex, staging, tumour type, and position.We have shown in univariate analysis that a patient's OverMDC is correlated with overall survival. OverMDC is strongly related to IMRT and to a lesser extent with PTV volume. We recommend that VMAT planning should be used for oesophageal planning when available and that attention should be paid to the conformity of the 95% to the PTV.
Rhys Carrington's PhD is funded by Cancer Research Wales (Grant Number: 2445)