Could radiation make brain tumors grow before they start to shrink?

Not actually: Radiation may cause apparent enlargement of brain tumors on imaging and edema (swelling) around the tumor as it dies so what is seen on the scans may look larger but the actual living tumor cells are regressing so the enlargement and even worse local brain function is apparent not real or permanent

Swell: Irradiation can affect tumors with initial swelling from necrosis or killing tumor cells. This can cause tumor to initially swell prior to shrinking.