I would be inclined to go this route:
At some point, I assume, there would need to be an office visit, post chemo, in which the provider evaluates if continuation of the expanders is okay to be resumed. The provider gives the green light and the procedure takes place.
9921X-25 + XXXXX for expander. My thought here is that a "new" decision is made given the extenuating circumstance of the chemo, therefore an office visit should be coded. I would also make sure to tack on a DX for the condition that prompted the chemo and its current status for the office visit.
The subsequent visits would be only the procedure code, UNLESS a new issue arises that influences the continuation of the expanders. The procedure should get paid.
Now, if this is a matter where the patient had a mastectomy, any restoration or reconstruction procedures would never be bundled into the mastectomy by law, even if it's still in the global, but again, no office visit would be billed unless it's unrelated, such as complaint of a fever and cough.