Radiation-induced enterocolitis after combination therapy with palliative radiotherapy and immune checkpoint inhibitors in patients with metastatic lung cancer

The impact of immune checkpoint inhibitors (ICIs) on radiation-induced enterocolitis (RIE) after palliative radiotherapy (PRT) to the bowel has remained to be fully investigated. The aim of the present study was to investigate whether ICIs affect RIE after PRT. For this purpose, 32 lesions (vertebral bone, 13; pelvic bone, 12; adrenal gland, 3; lymph node, 3; liver, 1) in 28 patients with metastatic lung cancer who were treated with both PRT involving the bowel (8-48 Gy; typically 30 Gy in 10 fractions or 20 Gy in 5 fractions) and ICIs between December 2015 and June 2021 were retrospectively reviewed. A total of 12 lesions were treated with ICIs only prior to PRT, 16 received ICIs only after PRT and the remaining 4 received ICIs both prior to and after PRT. The 1-year overall survival rate was 53%. The median PRT dose was 30 Gy (range, 8-48 Gy) in 10 fractions (range, 1-24 fractions). The median interval between PRT and the closest administration of ICIs was 20.5 days (range, 1-212 days). Combination therapy with PRT and ICIs was well tolerated by the majority of patients. However, grade 2 or higher RIE occurred in 6.3% of the patients. In these patients, ICIs were administered within 7 days after completing PRT with 3.6 Gy or a higher-fraction dose (evaluated at the isocenter). There were significant differences in the incidence of RIE between administration of ICIs <7 days after PRT completion and ≥7 days (P=0.05), between <3.6 Gy per fraction and ≥3.6 Gy (P=0.04), and between maximum dose to 2 cc (D2cc) of large bowel <3.3 Gy and D2cc of large bowel ≥3.3 Gy (P=0.02). There was no clear association between the incidence of RIE and any other factors. These results suggest that the administration of ICIs soon after PRT completion and a comparatively high fraction dose may potentially increase the risk of grade 2 or higher RIE.