Changes on DWI MRI before and after Radiation Therapy for Nasal Lymphoma in a Cat: Comparison with CT

An 8-year-old, spayed female, Russian Blue cat was presented with a 6-month history of nasal discharge. Computed tomography (CT) revealed a soft tissue structure occupying the bilateral nasal cavities and right frontal sinus, which showed no significant contrast enhancement was indistinguishable from adjacent nasal mucosa and discharge. Magnetic resonance imaging (MRI) characterized the lesion into three distinct areas: 1) marked restricted diffusion on diffusion-weighted imaging (DWI) with mild contrast enhancement, suspected to be a high-cellularity tumor; 2) proteinaceous fluid, hyperintense on T2-weighted (T2W) and T1-weighted imaging without enhancement; 3) an area hyperintense on T2W with distinct enhancement, likely residual mucosa. Cytology confirmed nasal lymphoma, and the cat underwent radiation therapy, resulting in clinical improvement. Follow-up CT and MRI 35 days post-treatment showed significant reduction of the soft tissue area. CT revealed only diffuse, ill-defined opacity, while MRI demonstrated disappearance of the restricted diffusion area. Residual soft tissue in the nasal cavities was hyperintense on T2W and showed contrast enhancement along the mucosa, suggestive of mucosal edema, rhinitis, or post-radiation changes rather than residual tumor. MRI, particularly DWI, proved valuable in differentiating intranasal soft tissue components and monitoring treatment response. The distinct restricted diffusion pattern seen in the tumor pre-treatment and its resolution post-therapy highlight the utility of DWI in assessing feline nasal lymphoma responsiveness to radiation therapy.