Hello and welcome to HCM
A lump on the left back can be due to enlargement of the structures/ organs in that region, which are:
, soft tissue
You should get an ultrasound
As you have mentioned that your husband had had a stone in the same kidney, there can be a recurrence of the stone.
a large stone can obstruct the passage of urine from the kidney and cause its enlargement, which can be palpable.
Development of cysts in kidney, however this is not very common after kidney stones.
A kidney tumor can also form a palpable mass.
Other soft tissues in the region like fat, muscle, fibrous tissue can also form a palpable mass.
Lymph nodes in the region can get enlarged due to inflammation or due to maliganancy.
An ultrasound of pelvis along with urine examination is needed as soon as possible/
Thanks and take care
Dr Shailja P Wahal