Radiochemotherapy in brain-tumor patients was shown to cause gray matter (GM) volume and cerebral blood flow (CBF) changes. The interaction of these two effects, however, remains unclear. Here, we investigated GM volume and ASL CBF changes and their interaction in the healthy hemisphere of 38 glioblastoma patients undergoing radiochemotherapy with Temozolomide. We found a statistically significant CBF decrease with dependence on the RT-dose. PV-corrected results indicated that, while to a certain extent the apparent CBF decrease measured by ASL is caused by GM atrophy, there still remain significant CBF changes that cannot be explained by structural changes alone.