Extensive-Disease Small-Cell Lung Cancer With Severe Immune-Related Adverse Events Due to Atezolizumab Maintaining a Complete Response for Two Years: A Case Report

A 75 -year -old male with diabetes mellitus was referred to our hospital with an abnormal shadow on chest radiography, based on which he was diagnosed with extensive -disease small -cell lung cancer (ED-SCLC; cT2bN2M1a). The first -line therapy comprised atezolizumab, carboplatin, and etoposide. After four cycles, the patient achieved complete response (CR), and maintenance therapy was initiated with atezolizumab. However, even though CR was maintained, maintenance therapy was discontinued after 16 cycles due to persistent grade 2 anorexia and fatigue. Simultaneously, the HbA1c decreased to 5.5%, and antidiabetic therapy was discontinued. Six months after the last dose of atezolizumab, the patient visited the emergency room because of anorexia, dry mouth, and fatigue. Laboratory findings were as follows: blood glucose was 668 mg/dL, glycated hemoglobin (HbA1c) was 8.8%, urine ketone was 2+, sodium (Na) was 127 mmol/L, potassium (K) was 6.5 mmol/L, creatinine (Cre) was 1.43 mg/dL, and arterial pH was 7.29. Based on these findings, his presentation was consistent with fulminant type 1 diabetes mellitus (T1DM) complicated by diabetic ketoacidosis (DKA). Regular continuous insulin and saline administration was initiated in the intensive care unit, and acidosis and electrolyte abnormalities were corrected. His C -peptide was <0.03 ng/mL. His insulin secretory capacity was considered to be depleted, and he required continuous subcutaneous insulin injections. Glutamic acid decarboxylase and insulin autoantibodies were absent. The complete response persisted without further therapy until two years since the event.