A radiobiology-based inverse treatment planning method for optimisation of permanent l-125 prostate implants in focal brachytherapy

Treatment plans for ten patients, initially treated with a conventional approach to low doserate brachytherapy (LDR, 145 Gy to entire prostate), were compared with plans for the same patients created with an inverseoptimisation planning process utilising a biologically based objective. The 'biological optimisation' considered a nonuniform distribution of tumour cell density through the prostate based on known and expected locations of the tumour. Using dose planningobjectives derived from our previous biologicalmodel validation study, the volume of the urethra receiving 125% of the conventional prescription (145 Gy) was reduced from a median value of 64% to less than 8% whilst maintaining high values of TCP. On average, the number of planned seeds was reduced from 85 to less than 75. The robustness of plans to random seed displacements needs to be carefully considered when using contemporary seed placement techniques. We conclude that an inverse planning approach to LDR treatments, based on a biological objective, has the potential to maintain high rates of tumour control whilst minimising dose to healthy tissue. In future, the radiobiological model will be informed using multiparametric MRI to provide a personalised medicine approach.