Abstract
Background and Clinical Significance: Collagenous colitis is an uncommon form of microscopic colitis characterized by chronic watery diarrhea and thickening of the subepithelial collagen layer. While various medications have been implicated in its pathogenesis, paclitaxel-associated collagenous colitis remains exceptionally rare in the literature. Recognition of this adverse event is crucial for appropriate management, particularly in patients receiving dose-modified chemotherapy regimens. This case highlights the importance of considering drug-induced collagenous colitis in cancer patients presenting with severe diarrhea during chemotherapy. Case Presentation: We report a 71-year-old Japanese male with metastatic breast cancer who developed acute-onset collagenous colitis during paclitaxel treatment. His primary tumor was invasive ductal carcinoma with hormone receptor-positive, HER2-negative disease (ER+, PgR+, HER2-, Ki-67 46%) and progressive metastatic disease. Given pre-existing renal dysfunction, paclitaxel was initiated at 60% dose reduction. Sixteen days after treatment initiation, the patient experienced abrupt onset of profuse watery diarrhea with approximately 10 bowel movements daily, necessitating hospital admission. Colonoscopic evaluation demonstrated increased vascular permeability and superficial mucosal erosions. Histopathological analysis revealed diagnostic features of collagenous colitis with a markedly thickened subepithelial collagen band measuring 23 μm. Following immediate cessation of paclitaxel, the patient experienced complete resolution of diarrheal symptoms without subsequent relapse. Conclusions: This case represents a rare manifestation of paclitaxel-induced collagenous colitis. Clinicians should maintain heightened awareness of this potential complication in patients receiving taxane-based chemotherapy who develop significant diarrhea. Prompt recognition and immediate drug discontinuation are essential for favorable outcomes and symptom resolution.