Would you recommend the addition of radiation to neoadjuvant chemotherapy in unresectable cT4b sigmoid adenocarcinoma (at or above the peritoneal reflection) adherent to the bladder?

Yes. It’s unresectable in the pelvis. This is not an uncommon scenario. I haven’t found it any more problematic than treating a rectal cancer invading the bladder.

Please see an old series from Tufts University on sigmoid cancer and radiotherapy from Cancer (May 1983) which I wrote.

A much greater MGH and Mayo experience over decades of patients was from Dr. @Dr. First Last, in many articles and book chapters.