Objective: To present long- term results after interstitial iodine125 irradiation of adult patients with de- novo World Health Organization (WHO) Grade II astrocytomas and oligoastrocytomas of the insula of Reil with special respect to the treatment- related risk. Methods: 55 consecutively treated patients (from 1979 to 1992) with circumscribed tumors with a diameter < 5 cm (astrocytomas: 46 patients, oligoastrocytomas: 9 patients) were included. The reference dose - calculated to the outer boundary of the tumor - was in the range of 60- 100 Gy and the dose rate was low ( < 10 cGy/h). Progression- free survival, risk of malignant transformation, survival, and the incidence of radiogenic complications were estimated by the Kaplan- Meier method. Prognostic factors were obtained from the Cox- model. Results: Median follow up for the survivors was > 10 years. 5- year (10- year) progression- free survival was 40.7% (20.2% ), and 5- year (10- year) survival 54.6% (28.4% ). Malignant transformation occurred in 42.4 % after 5 years. Neither of the Kaplan- Meier curves showed a leveling off over time. Transient (progressive) radiogenic complications were observed in 13 [4] patients (one- year overall complication rate: 18% ), and were significantly associated with a tumor diameter > 3.5 cm (p < 0.001). No long term side- effects were detected and delayed external beam irradiation (in case of tumor progression) did not enhance the risk. Tumor enhancement on CT was the only unfavorable predictor for survival. Conclusion: A limited overall prognosis for adult patients with insular WHO grade II astrocytomas and oligoastrocytomas was detected. Interstitial 1- 125 irradiation offers a minimal- invasive and low- risk treatment option for circumscribed tumors with a diameter < 3.5 cm. Larger tumors require further evaluation for optimal treatment. Objective: To present long-term results after interstitial iodine 125 irradiation of adult patients with de- novo World Health Organization (WHO) Grade II astrocytomas and oligoastrocytomas of the insula of Reil with special respect to the treatment-related risk. Methods: 55 consecutively treated Patients (from 1979 to 1992) with circumscribed tumors with a diameter <5 cm (astrocytomas: 46 patients, oligoastrocytomas: 9 patients) were included. The reference dose - calculated to the outer boundary of the tumor - was in the range of 60- 100 Gy and the dose rate was low (<10 cGy / h). Progression-free survival, risk of malignant transformation, survival, and the incidence of radiogenic complications were estimated by the Kaplan-Meier method. Prognostic factors were obtained from the Cox 5-year (10-year) progression- free survival was 40.7% (20.2%), and 5-year (10-year) survival 54.6% (28.4) - model. Results: Median follow up for the survivors was> 10 years. %). Malignant transfor Neither of the Kaplan-Meier curves showed a leveling off over time. Transient (progressive) radiogenic complications were observed in 13 [4] patients (one-year overall complication rate: 18%), and No long term side-effects were detected and delayed external beam irradiation (in case of tumor progression) did not enhance the risk. Tumor enhancement on CT was the only unfavorable predictor for survival. Conclusion: A limited overall prognosis for adult patients with insular WHO grade II astrocytomas and oligoastrocytomas was detected. Interstitial 1-125 irradiation offers a minimal- invasive and low- risk treatment option for circumscribed tumors with a diameter <3.5 cm. Larger tumors require further evaluation for optimal treatment.