Radiation injury versus malignancy after stereotactic radiosurgery for brain metastases: impact of time-dependent changes in lesion morphology on MRI.

Background: We sought to determine whether radiation-induced injuries could be distinguished from malignancy after stereotactic radiosurgery (SRS) by analyzing time-dependent changes in lesion morphology on sequential MRI for up to 55min.

Methods: In 31 consecutive patients treated with SRS for brain metastases, the time-dependent changes in lesion morphology were analyzed on MRI 2min, 15min, and 55min after contrast administration and on subtraction images. A simultaneous, matched-pairs approach was used for quantitative region of interest analysis of the area of the lesion. Qualitative analysis comprised the shape of the border, the structure of the interior area, the presence of leptomeningeal enhancement, and feeding vessels. The signal intensity changes of the border and the interior area of the lesions over time were assessed visually. The time-dependent changes in the 2 entities were compared.

Results: Twenty radiation-induced injuries and 21 malignancies were analyzed. A significant interaction effect between time point and diagnosis (P<.001) was found for the time-dependent changes of the margin of the lesion for 2min to 15min and in signal intensity differences of the rim and interior area as well as of the size of the interior area for up to 55min. All radiation-induced injuries showed a black interior area on the subtraction images for 15min minus 55min, whereas all malignant lesions had white components (P<.001).

Conclusions: Analysis of time-dependent changes in lesion morphology on sequential MRI for up to 55min is a reliable tool to distinguish radiation-induced injuries from malignancy after SRS.