Is There A Missing Rib?

A 9 year old Jack Russell Terrier presented with a history of occasional cough over the previous 2 weeks, swelling of the caudodorsal left chest wall and dyspnoea. X-ray and CT images are presented. Answers at the end!

X-Rays:

These show a diffuse increased soft tissue/fluid opacity throughout the chest cavity on left lateral view. Note that the cardiac and diaphragmatic silhouettes remain visible. The VD shows a homogenous increased opacity on the left side of the chest with a mediastinal shift to the right. No air is seen in the left chest while there is air in the right lung. There is lumbarisation of T13 which has a partial rib on the left side. Only one rib is seen coming from T10 on the lateral film. There is poorly marginated destruction of the left 10th rib and a soft tissue mass extending into the caudal thoracic wall. The diaphragm is straight and displaced caudally.

Chest and Abdominal CT:

There is a 9.0 x 7.4 x 6.5 cm irregular apparently cavitated mass arising from the left 10th rib which is almost totally destroyed with some heterogeneous amorphous new bone formation also. There is an associated very large ipsilateral pleural effusion (mean 19HU) with total atelectasis of the entire left cranial and caudal lobes and the accessory lobe.

There is a marked mediastinal shift with the heart and most mediastinal structures virtually completely located in the right hemithorax. There is partial atelectasis of the right cranial lung lobe but the right middle and caudal lobes are all aerated. No pulmonary metastasis is seen. No thoracic lymphadenopathy is seen. There is also marked caudal displacement of the ipsilateral hemidiaphragm and caudomedial displacement of the left liver lobes and the gastric fundus.
The mass has a markedly contrast enhancing rim consistent with a cartilage cap and multiple irregular striated contrast enhancing tendrils extending throughout its volume. The enhancing rim is thin in places but irregularly thickened elsewhere. There is a clear demarcation between the rim and surrounding soft tissues. No suggestion of invasion of these tissues is seen. The 9th and 11th ribs are displaced but otherwise are not involved. No evidence of regional or distant metastasis is seen.

This is highly likely to be an aggressive, highly destructive neoplasm such as a high grade chondrosarcoma for example. Differentials would include osteosarcoma, fibrosarcoma, haemangiosacroma for example. The mass was surgically excised and histologic diagnosis was osteosarcoma.

Below are chest films taken 6 months post-operatively at which time the patient was clinically back to normal.

Author

Keely Breen