Deciding when to treat a tooth with a radiographically visible defect is a controversial subject.
E1 defects are not usually treated, since these are rarely cavitated and may become inactive with improvements in diet and home care. Note that remineralization of enamel is effective, but mostly a surface phenomenon. Therefore an inactive lesion will not look different radiographically from an active lesion.
E2 lesions have traditionally been recommended for restoration (and they are acceptable for use in WREB Exams.) It has been shown, though, that E2 lesions are cavitated only about half of the time. There are academic authors who claim that uncavitated lesions should never be treated before remineralization efforts have been attempted. However, if these attempts are unsuccessful (i.e.. patient compliance is inadequate) the damage within to the tooth will be significant before radiographic evidence is likely to indicate changes. The restoration in that event becomes more aggressive and less reliable.
D1 lesions should be recommended for treatment, although some may not yet be cavitated. Dentin involvement is always present. A direct restoration is usually adequate, but lesions might be much more extensive than radiographically evident.
D2 lesions always need treatment; direct restoration may be inadequate. Care needs to be taken to avoid pulp exposure. Pulp test before anesthesia.
D3 lesions will nearly always lead to pulp exposure if carious dentin is aggressively excavated. Destruction of coronal tooth structure will be extensive. Pulp testing and careful history taking should precede any treatment on such teeth.