Thank you for the additional information.
The medical term for this problem is a radiation-induced brachial plexopathy. This is not seen as often as was once seen with better radiation protocols, but can still happen, as your mother has experienced.
Unfortunately, there are no cures for the condition. Some people who develop plexopathy relatively soon after the radiation will improve spontaneously over time, but the likelihood of spontaneous improvement when the plexopathy occurs 17 years after radiation is much lower.
Although there is no cure, there are several medicines that can be used to try to control the symptoms. The gabapentin, an anti-seizure medicine, is one of the commonly used medicines, which works by suppressing the firing in the nerves carrying the pain message, but it may require a higher dose. The 300 mg three times per day is OK if it is effective, but the dose can be increased up to 1800-3600 mg per day, in divided doses, if the current dose is not sufficient.
There are other medicines that can be used to try to control symptoms, including another anti-seizure medicine, pregabalin, and certain antidepressants
, such as amitriptyline, nortriptyline, and duloxetine, which also work by suppressing the firing of the nerves.
In someone with severe, intractable pain, it may also be an option to perform a chemical sympathectomy, a procedure in which the nerves are damaged using a chemical irritant.
The brachial plexus specialist would usually be able to assist her with these options, but if it is difficult to be seen regularly at the Mayo clinic, another consideration would be to see a Pain Management specialist.
If you have any further questions, please let me know.