Tissue characterization of brain tumors during and after pion radiation therapy.

Negative Pi-mesons (pions) are applied at the Paul Scherrer Institute in the radiotherapy of highly malignant gliomas using a dose escalation program. The therapy effects of 7 randomly selected patients were followed up by 62 MRI examinations. The quantification of the effects is based on the relaxation times T1 and T2, which are acquired by a new designed multi-echo multiple saturation recovery imaging technique. As a summary of the results, roughly two reaction types are observed. For both types the relaxation times increase up to two to three months after the radiation therapy. Then in one type (two patients) the T1 and T2 values of the tumors, and of the edemas surrounding the tumors, further increase, indicating an unfavorable prognosis. In the other type (five patients) the relaxation times drop down towards, or even below, their initial values, reflecting the onset of the reparation processes in the tissue. This later behaviour reflects an at least temporary control of the disease; that is, the short term prognosis for these patients is more favorable. It further can be concluded, with respect to our MR parameters, that the radiotolerance of healthy brain tissue is much higher than that of malignant glioma tissue, despite the fact that these tumors are very seldom definitively radiosensible.