Can 5-FU be substituted for adjuvant capecitabine for a dialysis dependent patient with triple negative breast cancer s/p neoadjuvant chemotherapy followed by mastectomy showing small residual disease?

I would not substitute 5-FU in this situation.

Although the CREATE-X trial showed a survival benefit for adjuvant capecitabine in women who do not achieve pCR with neoadjuvant chemotherapy in a subset analysis of women with triple negative breast cancer, there are reasons to be concerned about these results.  Other studies, such as the GEICAM/CIBOMA trial and the FINXX, have failed to show a benefit of adding capecitabine to the adjuvant therapy of women with triple-negative breast cancer.  A meta-analysis of available trials suggested and OS benefit but not a RFS benefit for capecitabine in the adjuvant setting in TNBC, which is a curious and difficult to explain discrepancy. Concerns about conflicts of interest in the CREATE-X trial have also been raised. (Sci Eng Ethics. 2018 Dec;24(6):1809-1811)  Thus, although I give women who have TNBC who do not achieve pCR capecitabine, I do not feel obligated to recommend it when there are contraindications.

Although it is logical that 5-FU would have a similar effect to capecitabine, there is not evidence to support 5-FU monotherapy in triple negative breast cancer after standard neoadjuvant chemotherapy.  Indeed, as the GIM group demonstrated, 5-FU does not improve survival when added to standard anthracycline and taxane based adjuvant therapy.  

In the absence of a trial showing benefit and a known optimal dose for 5-FU in this situation, I would not recommend it.