3D photogrammetry quantifies the size of basal cell carcinoma lesions with submillimeter accuracy: high correlation with lesion response to photodynamic therapy.

Significance: Noninvasive imaging to accurately measure subtle changes in tumor size is underutilized when assessing therapeutic responses in the skin. During photodynamic therapy (PDT) for basal cell carcinoma (BCC), a better definition of the tumor size threshold for PDT responsiveness is needed.

Aim: We aim to quantitatively demonstrate the first clinical evidence of tumor shrinkage after multiple rounds of PDT using a robust measurement and analysis approach.

Approach: Tumors were monitored experimentally using a 3D camera and software system (stereo photogrammetry). A total of 122 BCC tumors in 35 patients were treated with PDT (5-ALA and blue light) in three sessions. Calculated volumes and heights were used to plot changes in tumor size.

Results: In total, 70% of BCC cleared completely. Measured heights correlated with histological tumor depth; average heights were $\sim 10\%$ to 20% of actual tumor depth. From photogrammetry at baseline, an average height of $<0.15\mathrm{mm}$ was found to predict a complete therapeutic response. Thus, our 3D morphometric technique provides a surrogate measure of BCC tumor depth that predicts PDT response and is accurate to well below the millimeter level.

Conclusions: 3D photogrammetry can inform the selection of BCC tumors for PDT with exceptionally high spatial accuracy, dramatically better than can be quantified by a clinician.