How do you approach boosting a vaginal cuff recurrence of cervical cancer with brachytherapy that is tethered to small bowel?

Tough case.

Controlling cancer is important.

MRI guided brachy and making sure GTV is adequately covered even if bowel wall gets that dose.

Warn the patient about bowel obstruction and the need to bypass in future.

An incredibly challenging case!

I would consider using TRUS to guide needles in addition to laparoscopic guidance and would advance the needles into the bowel if necessary to cover the GTV.

I would accept some under coverage of GTV along the bowel/GTV interface as long it's dosed to the tolerance of the bowel. The prescription would depend on histology - it may not need high EQD2 and if you can get up to 75 Gy that would potentially be enough.

Finally, agree with @Dr. First Last that discussing with the patient potential obstruction and or fistula, which may require bowel resection or diversion. But if the tumor is not controlled adequately, then the patient may run into that problem regardless.

If a poor candidate for brachytherapy, can also consider upper vaginectomy with small bowel anastomosis following initial EBRT.

Otherwise, advance needles slowly with interval imaging to get optimal positioning without perforation. Can also consider transrectal US for needle advancement.

The risk of fistula will be higher and should also be explained to the patient.

Depending upon the patient's risk tolerance, age, and performance status, the bowel may need to be underdosed.

I don't do gynecologic brachy anymore, but there once was an expandable applicator that looked something like a squid (can't remember the name of it) due to the multiple catheters extending down from the expandable fluid-filled head, but it allowed more customizable dose to the vaginal mucosa as you could expand it to fill the fornices, then out the wall, and it allowed deeper dose penetration up to a cm. Not sure if that's available or possible in this case.

We would do a laparoscopic attempt to create a plane between the bowel and tumor, then mobilize and suture mesenteric fat into the area to maintain separation. The needle position and depth can also be evaluated/confirmed laparoscopically.
If that separation attempt fails, I would consider placing the needles as well as possible, then proceeding with a curative attempt using a lower dose in the curative range with a larger number of fractions (bid), which would require hospitalizing the patient for a night or two.

The worst complication is an uncontrolled pelvic malignancy!