Case report: Clinical utility and limitations of high-frequency ultrasound for monitoring sonidegib response in nodular basal cell carcinoma: a three-case series

Background Sonidegib, a Hedgehog pathway inhibitor, can induce clinical and imaging remission in locally advanced basal cell carcinoma (BCC). Residual deep tumor may persist after malignant dermoscopic and ultrasound features disappear. This case series assessed the utility and limitations of 20–30 MHz high-frequency ultrasound (HFUS) for confirming complete imaging remission after sonidegib. Cases Presentation Three patients with nodular BCC received sonidegib 200 mg daily (October 2024–May 2026). Two achieved complete imaging remission, defined as resolution of malignant imaging features with residual non-malignant hypoechoic areas allowed, and underwent surgical resection with histopathological correlation; the third declined surgery. Baseline ultrasound was unavailable, so findings were compared with the earliest 3-4-month follow-up. Results Response involved lesion shrinkage and loss of intratumoral blood flow. Serial imaging showed gradual resolution of hypoechoic nodules with vascularity on HFUS and blue-gray nests with arborizing vessels on dermoscopy, leaving avascular scar-like structures. Preoperative HFUS residual hypoechoic areas were correlated with histopathology, which showed chronic inflammatory infiltration (case 1) and mild dermal fibrosis (case 2), with no residual tumor. The third patient’s malignant features regressed after six months. Conclusions In this small case series, HFUS monitored deeper tissue changes after targeted therapy. In two histopathologically confirmed patients, complete imaging remission on HFUS plus dermoscopy corresponded to pathological complete response; however, given the small sample and histopathological confirmation in only two patients, this is a possible correspondence rather than evidence of true predictive capability. Post-treatment hypoechoic areas may reflect inflammation, fibrosis, or residual tumor, limiting specificity. Imaging-based complete response should be confirmed by biopsy or excision when feasible. Surgery remains the gold standard; non-surgical patients require close follow-up.