A 58-year-old woman presented with progressive right arm pain and hand weakness. She was diagnosed 11 years earlier with right breast carcinoma and positive lymph nodes. Treatment consisted of a modified radical mastectomy, chemotherapy, and adjuvant radiation to the right axilla and chest wall. Six years later metastatic cancer was found in contralateral axillary lymph nodes and she was treated with additional chemotherapy. Beginning 1 year ago (10 years after her initial diagnosis), she noted progressive burning pain in her right hand, arm, and shoulder and tingling in her fingers. She also experienced diffuse weakness and atrophy of the muscles in her right arm. There was no neck pain or left arm symptoms. Examination demonstrated mild weakness of right deltoid, biceps, triceps, and wrist flexors and moderately severe weakness of wrist extensors, finger extensors, finger flexors, interossei, and thenar muscles. The right triceps reflex was mildly reduced, and there was decreased pinprick in palm and dorsum of right hand. The EMG demonstrated the findings shown. Which of the following is the most likely diagnosis?