Mum has stage 2 uterine cancer and they aren't performing a hysterectomy- I'm confused and concerned

Hello, I'm new here and feeling a bit confused by my mum's diagnosis and treatment.
Around 10 days ago mum hemorrhaged and went to emergency where she was admitted and a biopsy was performed. A few days later she got the news that she had uterine cancer but, as it was her first sign of symptoms, we assumed it would be stage 1. Well, today we found out that it's stage 2, which isn't the worst news but it's worse than we were expecting.
What I don't understand is that they are not going to perform a hysterectomy. Everything I have read about uterine cancer says that surgery is the first line of treatment so why would they not do this? Mum doesn't fit any of the high risk categories so I don't understand the reasoning behind this decision.
Instead of a hysterectomy they are going with radiation therapy and chemo.
I wasn't there when mum met the oncologist (she's being treated at the Royal Melbourne hospital) but apparently he said the tumor is quite large and in an area that rules out surgery- it's uterine cancer, it's in the uterus so you remove the damn uterus and that also removes the tumor... I am really confused by this whole situation. And they haven't done a CT or anything so how does he even know where the tumor is and how large it is?
Mum is having an MRI tomorrow to check for "hot spots" and to see exactly what they're dealing with but, again, surgery is usually how they determine a lot of this.
Mum isn't overweight (she's quite slim), she doesn't smoke or have any diseases like diabetes etc, she's actually pretty healthy (except for the obvious) so again, she's not in a high risk group in terms of surgical complications so what is going on?
If anyone has any experience with this, or ideas about this, I'd really appreciate any information you might have. I have googled but every site I have visited talks about hysterectomies and the like as the first choice in treatment, so I am not having any luck finding stuff out myself.
TIA
Emma
x posted to the general treatment forum