Severe Radiation-Induced Brachial Plexopathy: A Case Report on Radiation Toxicity in a Patient With Invasive Ductal Carcinoma

The ataxia-telangiectasia mutated (<i>ATM</i>) gene is an important regulator of cell checkpoint signaling and the repair of double-stranded breaks. When the <i>ATM</i> gene is mutated or damaged, cells are less capable of responding to damage induced by radiation therapy (RT). Here, we present a case of a 50-year-old woman with stage IIIA invasive ductal carcinoma of the left breast who had genetic testing revealing pathogenic <i>ATM</i> mutations (c.5290del and c.4396C>G) and a <i>PALB2</i> mutation (c.1619dup). While guidelines suggest that adjuvant radiation therapy is safe for patients with <i>ATM</i> mutations, this patient experienced severe radiation-induced toxicities, including brachial plexopathy. These <i>ATM</i> mutations have not previously been described as imparting severe radiation-associated toxicities.