Hi Mathew sorry to hear about your wife.
I was diagnosed in 2016 with a soft tissue solitary plasmacytoma in the nasal area,5 weeks of radiotherapy,in remission until feb 2018.Then further plasmacytomas .So now re-diagnosed as multiple solitary plasmacytoma.One of the tumours blocked my pancreas, so I had to have insulin as a diabetic.
The chemotherapy I had included dexamethasone a steroid,on the days I took that had to increase the dose of insulin.The diabetic nurses in consultation with my consultant haematoligist decided the dosage.
Luckily the chemo worked , so I am no longer diabetic or need insulin.
In Feb 2019 had a stem cell transplant and currently in remission.All the treatment I have had is Myeloma based.
I am sure your wife consultant will liaise with her diabetic team.It has to be closely monitored with steroids as it put your sugar levels up.
Best wishes Nigel