Abstract
Heterotopic ossification (HO) in colorectal polyps is rare and scarcely reported in serrated lesions. We report an appendiceal sessile serrated lesion (SSL) with mature bone and a contiguous well-differentiated neuroendocrine tumor. A 64-year-old woman had a firm appendiceal-orifice polyp biopsied in 2012 showing calcification; SSL was identified in 2019. In 2024, an 8-mm lesion was removed en bloc by underwater cold snare; histology showed SSL with HO. Laparoscopic appendectomy confirmed residual SSL with HO and an adjacent 4.5-mm grade-1 neuroendocrine tumor (INSM1+, Ki-67 <3%) with negative margins. HO recognition should prompt en bloc excision and attentive pathology.