Accuracy and conformity of stereotactically guided interstitial brain tumour therapy using I-125 seeds.

OBJECTIVE To assess the accuracy of the stereotactic implantation procedure of catheters containing I-125 seeds in brain tumours and investigate the effect of catheter deviations on the dose distribution in patients. METHODS A randomised sample (n = 37) of all patients treated with I-125 seeds in our department between 6/1994 and 2/2002 was examined. Intraoperative X-ray images were used to measure deviations of implanted I-125 seed catheters from their planned positions and the influence on dose conformity, tumour surface dose and dose burden of surrounding healthy brain tissue was determined. RESULTS The mean spatial target point deviation was 2.0 mm (maximum 4.1 mm, SD 0.9 mm) and in 54.1% of the cases, reduction of the planned dose was greater than 5%. Target point deviations less than 1.5 mm have only minor influence on surface dose and conformity. Results indicated that in 10.8% of the cases the realized dose distribution showed a 'slight deviation', according to the guideline criteria for external radiosurgery of the Radiation Therapy Oncology Group. In 89.2% of the patients the applied dose conformed to the target volume. CONCLUSIONS Stereotactically guided interstitial irradiation with I-125 seeds can be used to treat brain tumours and metastases with high conformity comparable to radiosurgery. The observed deviations of the stereotactically implanted I-125 seed catheters from their planned target points were smaller when compared to frameless procedures. In order to maintain the required spatial accuracy of 1.5 mm in interstitial therapy using I-125 seeds, it appears necessary to optimise stereotactic instruments further.