The normal tissue sparing potential of adaptive strategies in radiotherapy of bladder cancer

Background and purpose. To improve the outcome in bladder radiotherapy by improving treatment conformation, we have investigated various adaptive treatment strategies involving on-board tumour/target visualisation for the bladder. The strategies are compared in terms of the amount of normal tissue enclosed within the PTV and the percentage volume overlap between the CTVs (i.e. bladders) on planning and repeat CT scans. Materials and methods. Five male bladder cancer patients having a planning and either 7 or 8 repeat scans during treatment were included in this study. Tumour positions were simulated on the sup/inf/ant/post/left/right wall and were identified in the repeat scans based on a reference coordinate system. The reference origin was positioned on an axis joining the centres of mass of the prostate and the bladder at a point mid way between the centre of mass of the bladder and the inferior bladder wall. Tumour positions on the repeat CTVs were overlapped using translation with corresponding positions on the planning CTV, after which the required isotropic and anisotropic margins were determined using a previously published margin calculation algorithm. Calculations were performed to find the margins required to enclose the envelope covering all repeat CTVs and those enclosing the repeat CTVs one by one, i.e. simulating adaptation on daily basis. These results were compared to optimising the margins for all repeat scans, firstly without any positional correction of the CTV and secondly after applying the optimum translation for the whole bladder. Results. Compared with optimisation of all scans, daily adaptation increased the average percentage volume overlap by 20% to 79-82% for the various tumour positions. The volume overlap achieved was similar for no translation of the isocentre (average 79%), and slightly higher with optimal translation (average 85%). Conclusion. Translation of the isocentre according to tumour position did not compromise the normal tissue irradiation compared with no translation of the isocentre. Optimal translation of the isocentre is superior in terms of normal tissue sparing.