Case Report: Pyogranulomatous Panniculitis Mimicking an Invasive Subcutaneous Mass with Muscular Pseudo-Invasion in a Dog.

This report describes an atypical presentation of pyogranulomatous panniculitis and perifolliculitis in an 11-year-old mixed-breed dog. The patient presented with a firm, warm cervical mass that progressed with unusual aggression over a matter of days. While advanced diagnostics, including Computed Tomography (CT) and Ultrasonography (US), provided critical data regarding the lesion's extent, they proved deceptive in determining its etiology; the imaging demonstrated apparent continuity between the mass and the adjacent musculature, strongly favoring a diagnosis of invasive neoplasia. Due to this diagnostic ambiguity and the mass's rapid expansion, the lesion was surgically excised for definitive analysis. Surprisingly, histopathological examination contradicted the clinical and radiographic suspicion of malignancy, identifying instead a severe multifocal-to-coalescing inflammatory process. The patient recovered rapidly postoperatively with no evidence of recurrence. This case highlights that while imaging is an invaluable tool for surgical planning, it cannot reliably differentiate between aggressive inflammatory processes and invasive masses. The patient's rapid recovery with no recurrence reinforces this limitation. Consequently, histopathological examination remains the gold standard for definitively diagnosing and guiding the management of rapidly expanding lesions.