Secondary lung tumours are neoplastic lesions originating at a site distinct from the primary lesion. Metastatic malignant neoplasms are the most common form of secondary lung tumours, when the primary tumour can be breast and colorectal cancer, melanoma and sarcoma. The most common mechanism of metastatic spread is haematogenous, via the pulmonary arteries.
The differential diagnosis of a patient with a known primary tumour and a lung nodule or mass should include secondary lung tumours, a synchronous second primary tumour and benign lesions. Spread to the lungs may also represent advanced neoplastic disease or an isolated early event.
Lung metastases are usually asymptomatic but may cause cough, shortness of breath or haemoptysis. Symptoms occur in up to 20 per cent of patients and depend on the proximity of the metastatic lesions to the central airways.
Secondary lung tumours may be identified following CT scanning as part of staging for malignancy. These are typically well-circumscribed, non-calcified nodules of less than 3cm seen in the peripheral lung parenchyma. Endotracheal and endobronchial metastases are less common.
Calcified pulmonary metastases may be seen with osteogenic sarcoma, ovarian cancer, colorectal malignancy and lymphoma. Cavitation may be a feature in pulmonary metastasis of sarcomas and squamous cell carcinoma.
Improvement in the resolution of scanning allows small lesions to be identified and treatment of metastatic disease to be initiated earlier. However, the risk of false positive lesions is increased. Positron emission tomography (PET) scanning may be useful since malignant cells demonstrate increased metabolic activity. This activity is less likely to be apparent if lesions are smaller than 7mm in size.
In patients with a history of sarcoma or melanoma with a solitary pulmonary nodule, the nodule is highly likely to represent a metastasis. Whether it is worth obtaining tissue diagnosis depends upon further management. Depending on the location of the lesion, percutaneous needle aspiration or bronchoscopy may avoid diagnostic surgery.
Most solid tumours that spread to the lung are relatively insensitive to currently available chemotherapy, so surgery may be the best option.
The natural history of a particular malignancy to metastasise predominantly to the lungs or to disseminate widely should also be considered when deciding on treatment.
When considering surgical treatment, the main considerations are that the primary site must be controlled, there is no systemic disease with the exception of resectable colorectal cancer metastases in the liver, and that all the pulmonary lesions can be removed leaving adequate lung tissue.
The patient should be a good surgical candidate with respect to co-morbidities.
Better outcomes may be achieved in patients with one or two pulmonary nodules, long tumour doubling times and long disease-free intervals. There is some evidence that the disease-free interval from the time of diagnosis of the primary cancer to the appearance of pulmonary metastasis is significant.
More than half of patients who undergo therapeutic pulmonary metastasectomy will experience recurrence, generally within the same lobe.
This may be because CT scans under-predict the number of lesions.
However, it is accepted that pulmonary metastasectomy may improve survival in patients with favourable tumour histology compared with those who undergo incomplete resection of the lung. Pulmonary metastasectomy may also confer a survival advantage even in the case of pulmonary recurrence.
With no controlled trials, much of the evidence is based on large case series. The populations are heterogeneous with pulmonary metastasis arising from different primary sites. In addition, different regimens of oncological treatment have been received.
The International Registry of Lung Metastases was established in 1991 to assess the long-term results of performing therapeutic resections. It included 5,206 cases of lung metastasectomy from 18 thoracic centres in Europe, the US and Canada. Single metastases accounted for 2,383 cases. The tumours included epithelial and germ cell tumours, sarcoma and melanoma.
Survival after complete metastasectomy was 36 per cent at five years and 26 per cent at 10 years. The corresponding values for incomplete resection were 13 per cent and 7 per cent, respectively.
Where metastasectomy is indicated, there are a number of approaches that may be used. Lung metastases may be removed by metastasectomy through a thoracotomy or video-assisted thoracoscopic surgery (VATS).
Regardless of the technique, resections should always be carried out conservatively as subsequent lung resections may be necessary.
VATS is possible if the pulmonary metastases are subpleural and are likely to be visible or palpable, and if there are only three or fewer. If, however, there are multiple lesions, thoracotomy will allow the surgeon to identify more lesions than seen on the CT scan.
For bilateral lung metastases the options include bilateral simultaneous or staged VATS or thoracotomy, or alternatively a median sternotomy.
Some surgeons confine the use of VATS to diagnosis and staging of pulmonary metastase.
The consensus opinion is that therapeutic metastasectomy should incorporate open approaches that allow bimanual palpation of the entire lung. Therefore the role of therapeutic VATS metastasectomy is still debated.
The follow-up of patients after pulmonary metastasectomy is controversial because there are no evidence-based guidelines. However, patients with recurrent pulmonary metastasis may benefit from further resection.
Dr Kochhar is an SHO in thoracic surgery and Mrs Harrison-Phipps is a consultant thoracic surgeon at Guy’s Hospital, London
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