CT-MRI Fusion for Radiation Treatment Planning and Serial Imaging of a Canine Maxillofacial Osteosarcoma.

A 10-year-old poodle with left maxillofacial osteosarcoma received volumetric modulated arc therapy delivering 32 Gy in four fractions. Treatment planning was guided by computed tomography-magnetic resonance imaging (CT-MRI) fusion for precise target delineation near the globe. Serial CT and MRI up to 12 months after treatment showed transient enlargement of intratumoral cysts with fluid-fluid levels immediately after treatment, indicating pseudoprogression, followed by marked tumor regression. Progressive cortical remodeling and re-ossification restored normal bone contours. These findings present the value of multimodal imaging in radiotherapy planning and follow-up of head and neck tumors to distinguish pseudoprogression from tumor growth and identify postradiation bone regeneration.