Answer: Osteonecrosis with fistula tract formation

Histology: The specimen consists of dense cortical bone with osteonecrosis and acute osteomyelitis. The overlying skin is ulcerated, and there is abscess formation with vegetable material in the soft tissues. Against this background, large nests of squamous cells extend through the cortex and spread through the marrow spaces. The squamous cells do not demonstrate significant cytologic atypia.

Discussion: Osteonecrosis is a difficult late complication of radiation treatment of squamous cell carcinomas of the head and neck. Non-vital bone over time can undergo progressive osteomyelitis with extension of the inflammatory process, breakdown of the overlying skin, and fistula tract formation. Clinically, there is often a high index of suspicion of recurrent radiation-resistant carcinoma aggressively invading the mandible or other craniofacial bones. The histologic picture can be equally puzzling, particularly when the fistula tract is lined by squamous epithelium that extends deeply into underlying soft tissues and even bone. The background changes in this case, especially finding of vegetable material within bone, along with the absence of cytologic atypia in the squamous cells, supports the diagnosis of osteoradionecrosis with fistula tract formation.