Thanks for the query.
Renal cysts are often seen in otherwise healthy individuals and their incidence increases with age. With a single cyst of 8 mm, all we need to do is monitor its size by doing an ultrasound
every 1-2 years. It does not require any other treatment.
The cyst is not the cause of low back pain
I suggest that you take the opinion of an orthopaedic surgeon
for your mother's symptoms.
Hope this helps