Signalment

14-year-old neutered female domestic short-haired (DSH) cat.

Presenting signs

Clinical examination

Differential diagnoses

Diagnostic evaluation

In the light of her clinical history and signs it was decided to undertake thoracic radiographs first. These revealed the presence of a single solitary mass located in the cranial aspect of her right caudal lung lobe. No tracheobronchial lymph node enlargement was noted and there was no pleural effusion.

Treatment

In the light of her history and clinical findings, it was decided that the mass required removal, so after preanaesthetic blood screens had been undertaken, she was taken to surgery without further diagnostic evaluation and underwent an intercostal thoracotomy, during which a 2-cm diameter mass was identified within the right caudal lung lobe and excision of the affected lobe performed. Careful visual and digital examination of the draining lymph nodes was also undertaken but they were considered to be normal.

The cat was managed postoperatively with opioid and NSAID analgesia and made a slow but steady recovery, being discharged from the hospital 4 days postoperatively. Histopathology revealed the mass to be a moderately differentiated primary bronchial carcinoma. After careful discussion with the owner, no further treatment was undertaken at their request. The cat remained well for 16 months with no sign of recurrence before being lost to follow-up.

Theory refresher

Primary lung cancer in veterinary medicine is considered rare, accounting for approximately 1% of all canine cancer cases and less than 1% of all feline cancer cases. Primary lung cancer cases are usually epithelial in origin (i.e. carcinomas) with adenocarcinomas being the most common subtype reported. A precise diagnosis can be made by subcategorizing the tumour type depending on its location of origin (e.g. bronchial, bronchioalveolar or alveolar) and by the degree of histological differentiation. A clinical diagnosis is usually made by identifying a large, often solitary mass, or possibly multiple masses on thoracic radiographs. Primary lung cancer is more often seen in older animals and whilst there has been no gender or breed predilection shown in dogs, older female cats seem more likely to develop the disease than male cats.

It is also unusual for coughing to be a clinical sign of lung cancer in isolation; rather, many cancer cases which cough will also display signs such as tachypnoea/dyspnoea, lethargy, weight loss, reduced exercise tolerance and poor appetite. Careful questioning of the owner will also frequently establish that these signs have been gradually worsening over several weeks prior to presentation although sometimes the presentation can be acute in nature. Dyspnoea is usually noticed by the owner but any preceding tachypnoea may not be recognized, so again careful questioning to establish the rate of onset of the clinical signs is important. It is also very important to establish the nature of any cough, as many cases of lung cancer will have a non-productive cough and the suspicion for neoplasia will increase in older patients presenting with coughing that is not responsive to antibiotic treatment. However, it is important to remember that many lung cancers can initially be clinically silent, or cause more unexpected clinical signs, especially if the tumour metastasizes to an extrapulmonary site (e.g. to the brain, resulting in neurological signs or to the digits, causing lameness). An example of an unusual manifestation of lung cancer is hypertrophic osteopathy presenting as shifting lameness and warm and/or swollen limbs. The identification of hypertrophic osteopathy (HO) on a radiograph should certainly initiate a search for an underlying neoplastic cause, starting with thoracic radiographs (although HO has also been reported with a variety of other non-pulmonary neoplasms and some non-neoplastic diseases).

Metastatic lung cancer is considered to be much more common than primary lung cancer with almost any malignant tumour having the potential to spread to the pulmonary tissue by virtue of the anatomical nature of the vascular supply in the lungs. The identification of lesions on thoracic radiographs that could be consistent with pulmonary metastasis should therefore initiate a clinical investigation as to the location of a possible primary tumour. The tumours most often associated with pulmonary spread are malignant melanoma, osteosarcoma, mammary carcinoma and haemangiosarcoma (Fig. 7.1) but any malignant tumour could theoretically spread to the lung, so a thorough clinical examination and appropriate ancillary tests may be required in such cases.

Figure 7.1 Extensive pulmonary metastatic disease in a patient with primary splenic haemangiosarcoma

Diagnosis

Having obtained a careful history, the clinical investigation of a patient suspected to have cancer of the lower respiratory tract begins with a thorough physical examination, looking in particular for signs of abnormal breathing along with careful auscultation of the heart and percussion of all the lung fields, as well as a full systemic evaluation of the animal. However, for patients with pulmonary neoplasia the cornerstone of diagnosis in general practice remains careful thoracic radiography and for this, fully inflated (i.e. obtained under general anaesthesia) left and right lateral thoracic radiographs are still considered to be the minimum standard, whilst obtaining three views provides an optimum radiographic evaluation. Patterns commonly seen vary from discrete nodules or individual masses to poorly defined interstitial patterns. Consolidation of individual lung lobes can also be seen, as can enlargement of the tracheobronchial lymph nodes if there is nodal metastasis. However, it is important to remember three factors regarding thoracic radiography of neoplasia:

Where available, computed tomography (CT scanning) certainly provides a more sensitive technique for the detection of small lesions, so patients may be considered for referral for this evaluation if possible, especially if a thoracotomy and excisional surgery are being planned and there is uncertainty regarding the clinical stage of the disease. A recent study showed that CT was able to identify enlargement of the tracheobronchial lymph nodes due to the presence of metastatic disease in five cases in which the lymphadenopathy had not been identified on thoracic radiography. However, a different study revealed that although CT provides superior resolution compared to radiography (Figs 7.2–7.4), it is still associated with a significant number of both false-negative and false-positive findings. We have to therefore conclude that we do not have perfect imaging modalities available. So caution must always accompany presurgical evaluation and staging by imaging of a patient with possible lung cancer. However, as the identification of metastatic disease has a significant impact on treatment and prognosis, it is vital to attempt to fully stage these patients and perform the evaluations to the highest possible standards.

Figures 7.2–7.4 Lateral and dorso-ventral (DV) radiographs and a transverse CT scan from a dog with a prostatic carcinoma. The radiographs do not clearly show any metastatic lesions but CT clearly shows the presence of metastatic disease, as highlighted by the two black arrows (Fig. 7.4).

Images courtesy of Dr Ani Avner, Knowledge Farm Veterinary Specialist Referral Center, Israel

Identification of a malignant pleural effusion is generally considered a poor prognostic sign as it indicates diffuse intrapleural disease, although there are reports of successful treatments with intrapleural chemotherapy. The identification of neoplastic cells requires an experienced cytology specialist and samples should be obtained in both plain and EDTA tubes if the samples are being sent to an external laboratory.

Treatment

The first-line treatment for localized primary lung cancer is surgical excision of the lesion by complete lung lobectomy, as illustrated by the cat in this case example. In the light of this, and of the fact that radiographs do not provide a cytological or histological diagnosis, it may be desirable in some circumstances to try to obtain a definitive diagnosis before sending the patient to surgery. Fine needle aspiration of the mass, either by ultrasound guidance or by blind aspiration based on the radiographical position of the mass can be simple and useful. However, there is a risk of iatrogenic pneumothorax and some studies have shown fine needle aspiration of pulmonary masses to have a low diagnostic yield. Bronchoscopy and bronchoalveolar lavage (BAL) can also produce a diagnostic sample and one study has suggested that these are more sensitive techniques than radiography in the diagnosis of pulmonary lymphoma. However, for carcinomas, studies in human medicine have shown that the sensitivity of BAL in reaching a definitive diagnosis varies depending on whether or not the tumour can be visualized, with the diagnostic sensitivity falling when the tumour cannot be seen. It may therefore be best to recommend firstly that a definitive diagnosis before surgery is only sought, if for some reason, it will make a difference to the treatment undertaken, and secondly, that thoracotomy to undertake a complete surgical excision and obtain a histopathological diagnosis is still the optimum treatment if possible. Owners, however, need to be counselled that more extensive disease may be found at surgery even with the most advanced diagnostic imaging investigations having been performed.

An intercostal thoracotomy and lung lobectomy is the procedure of choice if a diagnosis of a solitary lung lobe mass has been made. Other options for removal include excision via thoracoscopy or a median sternotomy. Thoracoscopy removal necessitates specialized equipment with specialist surgeons and anaesthetists and is thus generally available only in certain referral practices. Additionally, due to poor visualization, conversion to a thoracotomy is sometimes required. The use of median sternotomy as an approach to lung lobectomy is reserved for those cases where the tumour is extremely large or where the exact location cannot be identified on radiographs. This approach is not recommended routinely due to the difficulty associated in accessing the hilar vessels and bronchi which are situated dorsally. Moreover, lymph node biopsy is more difficult with this approach. The use of CT to accurately identify the exact location of the tumour and the presence of enlarged local lymph nodes and/or metastatic disease is a useful presurgical imaging tool, however this may not be easily available.

To undertake surgery the patient is positioned in lateral recumbency with the affected lung lobe uppermost. A standard or modified intercostal thoracotomy from the fourth to the sixth intercostal space is usually sufficient to allow access to the lung lobe. Lung lobectomy can be performed either by using standard ligation techniques or by using surgical stapling equipment (TA-55 or TA-90; see suppliers, p. 201). The use of stapling equipment significantly reduces surgical time and allows for a secure lobectomy closure. Before the thoracotomy is closed the area is carefully checked for haemorrhage or leakage of air from the collapsed bronchi. This is achieved by flooding the thoracic cavity with warm sterile saline and submerging the surgical site. Any areas of haemorrhage or air leakages should be meticulously sutured. Local lymph nodes should also be inspected and biopsied if enlarged. A thoracic drain is placed after copious lavage and the thoracotomy site closed routinely.

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