Dose variation during hypofractionated image-guided radiotherapy for prostate cancer: planned versus delivered

To determine variation in the actual doses delivered to the organs at risk and the target in patients treated for localized carcinoma of the prostate using image-guided radiotherapy.

Ten patients treated with helical TomoTherapy underwent daily target localization with megavoltage CT, on which the prostate, rectum and bladder were recontoured. The planned adaptive module was used for dose recalculation. The study endpoints were to analyse the variations in certain dose-volume parameters of the rectum and bladder (BD 2cc , RD 2cc , BV 100% , BV 70% , RV 100% , RV 70% ), the maximum anteroposterior (AP) and lateral rectal diameters, the volume of the CTV receiving 100% of the prescription dose (CTV V 100% ) and the dose to 100% of the CTV (CTV D 100% ).

The difference between the planned and delivered target doses (CTV V100% and CTV D100%) was small and clinically insignificant indicating adequate target coverage during treatment. There was a large variation in the AP and lateral rectal diameters, with no particular trend or correlation to dose parameters being noted during the course of treatment. The mean AP diameter during treatment was significantly less than the planned diameter (P < 0.05). The percentage of fractions where the delivered BV100%, BV70%, RV100%, and RV70% was more than the planned values were 42.8%, 17.1%, 45.4%, and 44.4%, respectively. The delivered BD2cc and RD2cc were similar to their planned values.

This study demonstrates the usefulness of daily soft tissue image guidance in negating the effects of physiological variation of the rectum and bladder on the dose delivered to the prostate.