Welcome to HCM,
With little history, I feel you had lichen planus
at the age of 12.
Now you at 52. And developed same type of discolored rash on back.
The rash on back may be lichen planus or lichenoid dermatitis
Or it may be neurodermatitis
Difficult to diagnose... You consult dermatologist
Rule out DM or other systemic diseases by blood tests.
Apply steroid cream..
Take antihistaminics ,if you have itching.
Take steroid in tappering dose, if not contraindicated.
You may be benefitted ..
Dr. Ilyas Patel MD