Demineralized root dentin and cementum is made up of mostly collagen that shrinks significantly upon dehydration or drying with air. During remineralization mineral is deposited on the outside of the lesion creating a highly mineralized surface layer that inhibits diffusion, arrests the lesion and prevents shrinkage. Previous studies suggest that active root caries lesions manifest shrinkage, while arrested lesions no longer manifest shrinkage upon dehydration. The purpose of this study was to demonstrate that the shrinkage of root caries lesions can be monitored during dehydration using an optical coherence tomography probe suitable for clinical use. In this in vitro study the shrinkage of simulated and natural root caries lesions on extracted teeth was measured using a cross polarization optical coherence tomography (CP-OCT) system and a 3D printed appliance with an integrated air nozzle suitable for clinical use. Two methods were employed to assess shrinkage, changes in the position of the lesion surface and changes in the thickness of the lesion. CP-OCT was successful in measuring a significant (P < 0.05) contraction of the lesion surface, significant decrease in the lesion thickness and increase in the reflectivity per micron upon drying natural lesions on extracted teeth. In this preclinical study, we have demonstrated that a CP-OCT handpiece modified for infection control with an attached air nozzle suitable for in vivo use can be used to monitor the shrinkage of root caries lesions. In addition, we have developed a new approach to measuring lesion shrinkage with OCT, namely monitoring changes in the lesion thickness as opposed to the position of the lesion surface, that does not require an initial reference position and is more easily implemented in vivo. Copyright © 2019 Elsevier Ltd. All rights reserved.