THE RISK OF INTERSTITIAL RADIOTHERAPY OF LOW-GRADE GLIOMAS

Background and purpose: The risk of side effects of low activity (i.e. < 20 mCi) Iodine-125 (I-125) interstitial radiotherapy was analyzed i n patients with low-grade gliomas. Materials and methods: Permanent (2 47 patients) or temporary I-125-implants (268 patients) were used with a median reference dose of 60 Gy and 100 Gy, respectively, which was calculated to the outer rim of the tumour. The mean dose rate for temp orary implants was low (median, 10 cGy/h). Risk factors were obtained from the multivariate proportional-hazards model. Results: Radiogenic complications occurred in 39/515 patients (28 patients with transient symptoms and 11 patients with progressive symptoms). The most importan t risk factor was the volume of the intratumoural 200 Gy isodose. Avai lable experimental data have associated a high dose zone in this range with the size of the treatment induced radionecrosis; Rapid tumour sh rinkage (decrease of the tumour volume greater than or equal to 50%) w ithin the first 6 months with subsequent centripetal movement of non-p athologic tissue into the high dose zone and a reimplantation were add itional risk factors. Radiation injury after rapid tumour shrinkage:co uld be better avoided with temporary implants. A 200 Gy isodose volume < 4.5 ml corresponded to an-estimated risk of radiogenic complication s < 3%. There was a steep increase of the risk beyond this limit, Tran slation of the 200 Gy isodose volume in terms of the treatment volume and the reference dose allows rational treatment planning. The estimat ed risk of a temporary implant with an applied reference dose of 60 Gy and a treatment volume less than or equal to 23 ml was < 3%. Conclusi ons: The intratumoural necrotizing effect of a low activity I-125 impl ant limits its application to small treatment volumes. Radiation injur y outside the treatment volume can be better avoided with temporary im plants in the case of rapid tumour shrinkage. (C) 1997 Elsevier Scienc e Ireland Ltd.