Calculated and simulated effects of heterogeneous dose distributions in radiotherapy using the dose volume inhomogeneity corrected biological equivalent dose formula with special reference to prostate cancer.

The aim of this study was to evaluate and estimate the impact on the biological effective dose (BED) of irradiation delivered to a tumour during high dose rate brachytherapy with a heterogeneous dose distribution in the target volume. The calculation of BED in combination with the critical-voxel model and the LQ (linear quadratic) model was used to evaluate the effect of different combinations of heterogeneous dose distribution. The model is called the dose volume inhomogeneity corrected BED (DVIC-BED). Different random and non-random combinations of radiobiological parameters were used to test the model. A simulated clinical treatment of two 10 Gy fractions of brachytherapy was used. In the simulations 0-100% of the target volume was simulated to receive only 80% of the total dose. Different alpha/beta ratios as well as a different alpha value were used. A drastic effect on the outcome was observed especially when the ratio was low and the alpha value was high. The clinical effect is amplified when the tumour is considered to have a step dose respond curve. A 5 Gy decrease in dose corresponds to a reduction in clinical or chemical control < or =10-25% in the interval 65-85 Gy. Random production of different values has basically the same effect as above. The result stresses the importance to have control of the dose and the target volume during brachytherapy of prostate cancer. This is even more important when monotherapy with high dose rate brachytherapy is used and with a low alpha/beta ratio. The advantage of using this formula is that it is based on the LQ/BED formula and that different treatments with different fractions and treatments can be summated independently of the homogeneity of the dose distribution.