Changes in the liver parenchyma after proton beam radiotherapy: evaluation with MR imaging

The objective of this study was to describe magnetic resonance (MR) findings with a 1.5T imager for hepatic parenchymal changes after proton beam radiotherapy. Thirty-two patients who received proton radiotherapy with doses of 50–87 Gy underwent MR imaging 1–75 months (mean 22 months) after the start of irradiation. Axial T2, T1-weighted imaging, and a dynamic study after a gadolinium injection were performed. The irradiated areas showed hypointense in T1-weighted images, hyperintense in T2-weighted images, and intense and prolonged enhancement on the dynamic study (maximum relative enhancement 441.8% ± 263.3 vs. surrounding liver 145.6% ± 67.7, p < 0.0001). T2 values of the irradiated areas were 50.6 to 65.8 msec greater than in the surrounding liver (p < 0.005). The values increased with time, being significantly greater 13 months or longer after the beginning of the therapy than after a period of less than 3 months (p < 0.05). Pathologic examinations (n = 3) indicated that the irradiated areas were composed of collapsed lobules with hepatic small vein occlusions, and rich extracellular matrices which retained extracellular fluid. MR imaging can demonstrate hepatic parenchymal changes after proton beam radiotherapy, and show the changes are irreversible.