[Volumetric changes following 125I interstitial brachytherapy of gliomas]

Image fusion permits quantitative analysis of the consequences of 125 Iodine interstitial irradiation of brain tumors. The volume of tumor necrosis, reactive zone and edema can be compared to the dosimetric data.

Nineteen patients with low grade glioma were analyzed on the average 14.5 months following 125 Iodine interstitial irradiation. Dose planning and image fusion were performed with the Target 1.19 (BrainLab) software. The CT/MR images showing the so called "triple ring" (necrosis, reactive ring and edema) developing after the interstitial irradiation of brain tumors were fused with the planning images and the isodose curves. The volume of the three regions was measured. Values at the intersections of isodose curves and necrosis borders were averaged and used for calculation of tumor necrosis. The volume of normal brain tissue irradiated by given dose values, as well as homogeneity and conformality indices were also determined.

The relative volumes of the different parts of the "triple-ring" compared to the reference dose volume were the following: necrosis 54.9%, reactive zone 59.7%, and edema 445.3%. Tumor necrosis developed at 71.9 Gy dose. At the irradiation of an average size glioma with a volume of 12.7 cm3, 5 to 7 cm3 normal brain tissue around the tumor received 60-70 Gy dose. The average homogeneity and conformality indices were 0.24 and 0.57, respectively.

The analysis of changes in the volume of edema, reactive ring and necrosis caused by interstitial irradiation, and their correlation with the dosimetric data using the image fusion method provide useful information for patient follow-up, clinical management and further therapeutic decisions.