Dad - Targeted Radiotherapy Effects 2 months later

Good Evening Everyone 

I wonder if anyone has any similar experience or advice please. 
in February 2021 my Dad received targeted knife radiotherapy on a small tumour in his sacrum, lower back. This was the last known node that had gone astray from his primary cancer site (in his bowel). we are yet to find out if the radiotherapy has cleared the tumour or not. Awaiting scan results. He also had a bone strengthening injection too. 

He seemed to sail through the 5 sessions of radiotherapy and has been active ever since.
In the last 2 weeks he has all of a sudden suffered terrible eyesight, blurred vision, forgetfulness, trouble gathering his words and it has knocked him for six. Now I was just wondering if this is a known side effect post radiotherapy or whether we need to worry something else is going on? Is there anything we can do to help him? 
thanks in advance 

xx