Calculated effects of displacement errors in external beam radiotherapy of prostatic adenocarcinoma

In order to evaluate the impact on the biological effective dose (BED) of irradiation delivered to a tumour with large displacement errors (LDE) and to estimate the effect on local control, simulated treatment of prostatic adenocarcinoma was performed. The calculation of BED in combination with the critical-voxel model and the LQ model was used to evaluate the effect of different combinations of LDEs. The model is called the Dose Volume Inhomogeneity Corrected BED (DVIC-BED) model. The dose-response curve was assumed to follow Poisson statistics. Different combinations of radiobiological parameters were used to test the model. A simulated clinical treatment with a dose of 66-80 Gy in 2 Gy fractions was carried out to evaluate displacement errors and non-optimal dose distributions. Five random LDEs excluding 33% of the target volume corresponded to an overall dose reduction of 3-5 Gy compared with a 10 Gy reduction if 100% of the target is missed five times. A 5 Gy decrease in dose corresponds to a reduction in clinical or chemical control up to 10-25% in the interval 65-85 Gy. LDEs in different directions are less deleterious than errors occurring in the same direction. Different alpha/beta-ratios (3-15) had little effect on the DIC-BED, but the effect of different alpha values (0.05, 0.2 and 0.5) was large. However, the results depend on radiobiological parameters for prostatic adenocarcinoma, which not are well known, and further studies in the field should be encouraged.