Pharmacokinetic MRI for assessment of malignant glioma response to stereotactic radiotherapy: Initial results

The purpose of this study was to assess the value of dynamic, contrast-enha nced MRI in patients with malignant glioma (a) to predict before stereotact ic radiotherapy local tumor control, (b) to investigate temporal changes in tumor microcirculation after stereotactic radiotherapy, and (c) to analyze whether malignant glioma response may be predicted earlier by alterations in the tissue pharmacokinetics rather than in terms of tumor volume. Ninety MRI studies were performed of 18 patients with malignant glioma before and 6, 18, 26, 52, and 72 weeks after the end of stereotactic radiotherapy. Th e signal time courses of the contrast-enhanced tumors were analyzed using a pharmacokinetic two-compartment model that calculates for the parameter A, reflecting the degree of MRI signal enhancement [no units] and the exchang e rate constant k(21) [min(-1)], Before radiotherapy, the amplitude A was s ignificantly (P <.05) lower in patients with subsequent local tumor control (n = 8; mean A =.34 +/- .15) compared to patients without subsequent local tumor control (n = 10; mean A =.94 +/- .71). In the local tumor control gr oup, early after stereotactic radiotherapy (at 6-18 weeks), there was a sig nificant (P <.05) time-dependent decrease in the parameter k(21), whereas t here was still no alteration in the tumor volume. A low amplitude A before radiotherapy, combined with an early drop of k(21) after stereotactic radio therapy, reliably characterized the group of patients with subsequent tumor volume decrease. Our preliminary results suggest that two contrast-enhance d dynamic MR studies, one before and one early after stereotactic radiother apy, offer important information on local tumor control within the first 6 to 18 weeks after stereotactic radiotherapy. Moreover, this response may be evidenced before tumor volume changes and provides a therapeutic window to broaden treatment options and to improve treatment outcome.