Heterotopic Ossification in an Appendiceal Sessile Serrated Lesion With Synchronous Neuroendocrine Tumor

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.