A 22-year-old woman with moderate caries risk has a distal radiolucency on tooth 15. On bitewings 18 months ago it reached the inner half of enamel; it now just crosses the enamel-dentine junction into the outer third of dentine. She has used fluoride toothpaste twice daily and received varnish at every recall. Temporary elastic separation shows an intact, non-cavitated proximal surface. What is the most appropriate management?