Approximately how much demineralisation must occur before a carious lesion becomes detectable on a conventional radiograph?

Correct answer: D. 40–50% mineral loss

• A. 5–15% loss is far below the threshold of radiographic detection — such a lesion is at most an early white-spot change.
• B. 20–30% loss still falls below the reliable detection threshold on conventional film or sensor.
• C. 70–80% loss would represent gross cavitation, well past the point of first radiographic visibility.
• D. CORRECT. A lesion generally must lose in the region of 40–50% of its mineral content before it produces enough change in radiographic density to be visible. This is the single most important limitation of radiographic caries diagnosis: radiographs are highly SPECIFIC but poorly SENSITIVE, and a lesion always extends further histologically than it appears on the film.

Note: Two practical consequences. First, a lesion that merely 'touches' the DEJ radiographically is usually already into dentin histologically. Second, radiographs must be combined with visual and tactile examination, transillumination and, where available, laser fluorescence — never used alone.