Thoracic CT for pulmonary metastases in dogs

Why lung staging changes decisions

Whether a tumour has already spread to the lungs is usually central to the treatment plan. How likely spread is depends heavily on tumour type. In 536 dogs with non-pulmonary malignant neoplasia that had CT at a single hospital (Lamb et al., Veterinary Radiology & Ultrasound, 2019), multiple pulmonary nodules were present at the first CT in:

Tumour type	Dogs with multiple nodules at first CT
Haemangiosarcoma	24 of 58 (41%)
Osteosarcoma	14 of 55 (26%)
Carcinoma	20 of 80 (25%)
Histiocytic sarcoma	5 of 21 (24%)
Soft tissue sarcoma	13 of 57 (23%)
Adenocarcinoma	11 of 60 (18%)
Melanoma	5 of 37 (14%)
Lymphoma	10 of 76 (13%)
Mast cell tumour	2 of 47 (4%)
Squamous cell carcinoma	0 of 17 (0%)

Grade matters too. In 137 dogs with cutaneous or subcutaneous soft tissue sarcoma (Villedieu et al., Journal of the American Veterinary Medical Association, 2021), nodules suggestive of metastasis were found in 6% with grade 1, 6% with grade 2 and 38% with grade 3 tumours. The authors considered lung staging low-yield for grade 1 or 2 tumours, especially those present for 3 months or less.

CT compared with thoracic radiographs

• Size threshold. In 18 dogs with confirmed pulmonary metastases (Nemanic et al., Journal of Veterinary Internal Medicine, 2006), only 9% of the nodules found on CT were visible on radiographs. CT detected nodules down to about 1 mm, while radiographs needed about 7–9 mm to show them reliably. CT also found nodules in more lung lobes.
• Patient level. In 33 dogs with neoplasia (Armbrust et al., Journal of the American Veterinary Medical Association, 2012), CT found nodules or masses in 21. At least one of three radiograph readers saw them in 17 of those 21 dogs. Radiographic sensitivity ranged from 71% to 95% between readers. The 4 dogs missed on radiographs were all large or giant breeds with osteosarcoma, and the authors recommend thoracic CT in these dogs if finding nodules would change treatment.
• Osteosarcoma staging. In a pilot study of 15 dogs with appendicular bone tumours (Oblak et al., Veterinary and Comparative Oncology, 2015), thoracic CT showed lesions that were not visible on radiographs. In 39 dogs staged with whole-body CT (Talbott et al., Veterinary Surgery, 2017), pulmonary metastasis was considered definitive in 2 (5%), and 2 more had soft tissue masses consistent with concurrent, non-metastatic malignancies.

Lung CT protocol at a glance

Metastasis screening needs well-aerated lungs, because collapsed or poorly aerated lung can hide or mimic nodules. In anaesthetised dogs, a fixed positive end-expiratory pressure valve fitted immediately after induction reduced atelectasis (Allison et al., Journal of Small Animal Practice, 2017), and in normal dogs scanned under injectable sedation, no atelectasis was identified (Hunt and Wallack, Veterinary Radiology & Ultrasound, 2021). It is worth agreeing positioning, ventilation and the order of scanning with the anaesthetist before the thorax is imaged.

What the radiologist looks at

CT finds more than radiographs, and not every nodule is a metastasis.

• One nodule or many. A single nodule was seen at first CT in 33 dogs (6%). Of 9 with follow-up CT, the nodule disappeared in 2, did not change in 3, and enlarged with new nodules in 4. The authors call a solitary nodule an indeterminate finding that may be unrelated to the tumour.
• Ground-glass lesions in bone tumours. Equivocal ground-glass lesions progressed in 3 of 4 cases, and the authors advise treating them as suspicious for metastasis.
• Age changes. In a comparison of young and old dogs (Hornby and Lamb, Veterinary Radiology & Ultrasound, 2017), visible foci of heterotopic bone were more common in old dogs (62% vs 14%). Lung collapse was associated with old age, greater body weight and anaesthesia. The authors judged that neither difference should lead to a misdiagnosis of pulmonary disease.
• Atelectasis. Collapsed or poorly aerated lung, which can hide or mimic nodules.

What your report will include

Every Veheri case is read by a radiologist or neurologist, and when you order you can choose Diplomate-only reporting or the full specialist panel. You can chat directly with the specialist assigned to your case, and a simplified pet owner report helps you explain the findings to the owner.

When you take the report forward:

• A solitary nodule that would change the plan: a follow-up CT after an interval is often worth discussing before deciding on one scan, because its behaviour on follow-up varied.
• CT does not give a histological diagnosis. A nodule that does not behave as expected on follow-up still needs sampling or clinical judgement.
• Older radiographs are not a like-for-like baseline, because CT finds smaller nodules than radiographs.

What to include when you request a thoracic CT report

• signalment: age, breed and sex
• tumour type, grade and site, and how long the mass has been present
• whether the result will change treatment
• previous thoracic radiographs or CT, for comparison
• sedation or general anaesthesia, recumbency and ventilation used during the scan
• any known lung disease or previous thoracic surgery

How Veheri can help

Veheri’s radiologists report veterinary CT studies remotely for clinics, and offer expert support with CT protocols, positioning and acquisition. For cats, see thoracic CT in cats, and for wider staging, our oncology imaging guide.