Complete response to pembrolizumab in a paediatric patient with Lynch syndrome-associated colorectal cancer: a case report and review of immunotherapy-related bowel obstruction.

Colorectal cancer (CRC) is extremely rare in paediatrics with aggressive histopathology and advanced presentation. Whilst pembrolizumab has shown efficacy in adult microsatellite instability-high or mismatch repair deficiency CRC, paediatric data remain scarce. A 12-year- old boy with Lynch syndrome-associated CRC developed severe cardiac toxicity after initial chemotherapy. Treatment was switched to off-label pembrolizumab, resulting in a complete response after 24 cycles. Despite good tolerability, the patient developed a delayed bowel stricture at the tumour site. After surgery, there was no pathological evidence of residual adenocarcinoma. This case demonstrates the potential of pembrolizumab in paediatric CRC, highlighting the importance of molecular-guided rather than age-restricted therapy selection.