What is your diagnosis? Peritoneopericardial diaphragmatic hernia.

A 15-year-old castrated male domestic shorthair cat was referred for evaluation of uncontrolled hyperthyroidism and for possible treatment with radioactive iodine. The owner reported that the diagnosis of hyperthyroidism had been made approximately 2 years earlier and that the cat had been treated, although irregularly, with methimazole since that time. The referring veterinarian was previously suspicious of a thoracic mass on the basis of findings on thoracic radiographs that were not available for review at the time of referral. The cat was otherwise active with a good appetite and had a history of polydipsia, mild weight loss, and occasional vomiting. On physical examination, the cat had a grade 3/6 heart murmur and was of thin body condition. The rest of the findings on physical examination were unremarkable. Two-view thoracic radiographs were obtained to investigate the suspected mass (Figure 1).