Lump biopsy

Hi ,, I want some advice from others that may have gone through what I am. My autistic daughter showed me a small lump in her arm in August. I didn't think nothing of it coz she told me she banged her arm on the side of the pool so put it down to that. In Nov she had a doc appointment for something else and she just showed her her arm. Doctor wasn't concerned but said let's do a xray. Xray was fine so referred for a ultrasound, I payed private as I have health anxiety and couldn't wait. Sonographer didn't seem worried. He said its not attached and has no blood going through it or anything. It measured 1.4cm. 

Doctor then spoke to radiologist which then said let's do a mri to be safe. Mri results came back still don't know what it is, the radiologist was going to do a biopsy but the doc said due to her age being 18 amd mum's anxiety let's refer to sarcoma clinic to be safe?. Mri said it was measuring 1.1cm and in middle was either scar tissue or fat tissue. 

It went to mdt meeting on new years eve and as its so small and it's more like a area than a lump they said let's do a biopsy. I am grateful their being safe but as u can imagine my daughter is oblivious and I'm taking on the strain as a mum and if it's happening to me.( if this makes sense) I feel guilty I didn't take her to GP straight away in August and left until Nov. I don't know what I'm asking for. The mdt lady from clinic did say it didn't look scary but need to be 100% if it was any form of cancer I'm guessing they have to tell u they suspect it as I did ask them and they said noone has suggested it on the forms but they need ti be sure?? Any advice is helpful.