How would you treat an early-stage dMMR rectal squamous cell carcinoma in an otherwise healthy elderly patient?  

With no squamous cells in the actual rectum, I typically treat this as I would anal cancer—definitive chemoradiotherapy, ideally with capecitabine/5-FU-mitomycin (Day 1 only).

Squamous cell carcinoma of the rectum is a rare histologic subtype, accounting for approximately 0.3% of rectal cancers, with anatomic and histologic similarities to anal squamous cell carcinoma. (Copur et al., PMID 29447418) It has molecular profiling distinct from rectal adenocarcinoma with a high...

I have only seen one MSI-H anal SCC; it was metastatic, and I treated it with chemo-immunotherapy. Hard to know the contribution of components. Your rectal SCC is likely anorectal (low rectal tumor) and should be treated like an early-stage anal cancer. I would recommend CRT. While it's attractive t...