A 10-year-old, male castrated domestic shorthair cat weighing 8.9 kg was presented for a 6 week duration of worsening stridor, wheezing, intermittent coughing and terminal retching exacerbated by exercise and stress. Tracheal auscultation revealed loud referred upper airway sounds. Thoracic radiographs revealed a 3 cm soft-tissue mass at the level of C3-C5 invading and occluding 85-90% of the tracheal lumen. There was a mild bronchial-unstructured interstitial pattern in all lung fields. An esophagram ruled out extraluminal compression by an esophageal mass. Cervical ultrasound showed circumferential involvement of the trachea with intraluminal invasion (Figure 1). Evaluation of fine-needle aspirate samples from the mass was suspicious for a carcinoma. The cat was discharged with directions for hospice care, including piroxicam (2.5 mg PO q24h) and was humanely euthanized 14 days later. Necropsy confirmed the presence of a circumferential tracheal mass with intraluminal invasion (Figure 2). The tumor was submitted for histopathology and subsequent immunohistochemistry (Figures 3 and 4). Samples were fixed in 10% buffered formalin for up to 48 h, trimmed, routinely processed for histology, embedded in paraffin, cut at 5 mum and stained with hematoxylin and eosin.
Immunohistochemistry required additional formalin-fixed, paraffin-embedded sections of the tracheal tumor for staining using anticytokeratin (AE1/AE3) antibodies. These slides were cut at 5 mum on charged slides, air-dried and then heated at 60oC for 15 mins. Slides were then deparaffinized with xylene and rehydrated through graded ethanols to deionized water. Antigen sites were unmasked using proteinase K (Dako) treatment at room temperature for 5 mins. Slides were then rinsed in Tris-buffered saline/Tween (TBST) at pH 7.6 for 10 mins before being loaded onto the Dako Autostainer, where all procedures are performed at room temperature and all slides are rinsed with TBST between the following steps. Endogenous blocks were performed by treating slides with 3% hydrogen peroxide for 5 mins followed by a serum-free protein block (Dako) for 5 mins. Mouse cytokeratin AE1/AE3 primary antibodies (1:800) were applied and incubated for 30 mins. A horseradish peroxidase-labeled polymer system (Dako EnVision+System) was applied for 30 mins followed by a 10 min application of 3,3'-diaminobenzidine chromogen (Dako). All slides were then taken off the Dako Autostainer, rinsed in deionized water, then counterstained with hematoxylin for 5 s. Slides were blued in ammonia water, then dehydrated through ethanol, cleared with xylene and coverslipped. Negative control slides were processed Universal Negative Control+ mouse serum instead of primary antibodies. Cytokeratin (AE1/AE3) was validated by using respiratory epithelium and the mixed tracheal glands as in internal control within the examined slide.
The neoplasm in this case was a carcinoma representative of epithelial origin with defined cellular nests but no definitive glandular structures to further categorize it as an adenocarcinoma. It is possible that this neoplasm was a poorly differentiated adenocarcinoma or may represent a malignant transformation of tracheal epithelium consistent with a primary tracheal carcinoma. The latter would represent a unique tumor described in the cat.