In a patient with rectal cancer, when would you consider brachytherapy monotherapy or brachytherapy boost after CRT?

Specifically, in a patient with T2N0 low rectal adenocarcinoma (1.5 cm, anterior only involvement, at the level of the sphincter, 4 cm proximal to the anal verge) strongly desiring organ preservation, how can brachytherapy potentially be incorporated? Colorectal surgery recommends against trans-anal local excision.