Author : Bhadresh Bundela
Lung cancer
There are several types of cancer of the lung, distinguished by the appearance down the microscope. The commonest type of lung cancer is changing; whereas squamous carcinoma accouned for up to half of all cases in the 1970's, adenocarcinoma is increasing in incidence and has overtaken squamous histology as being the most common histological sub-type in some countries at present.Large cell carcinoma accounts for 10-15% of the total.
There is a particular subtype called small cell cancer, which is important to recognise separately from the other types as management varies from the general principles that apply for all the other types. Also called ‘oat’ cell carcinoma (because the cells look oat seed shaped down the microscope and under the high power lens) small cell cancers account for 20% of the total numbers of lung cancer. Rare types are giant cell carcinoma and the truly lung (as distinct from bronchial) alveolar cell carcinoma.
Incidence of lung cancer
Lung cancer is the leading cause of cancer deaths in the Western world, accounting for 30% of male cancer related deaths, which translates into a total death toll in excess of 30,000 persons per year in the UK.
Causes of lung cancer
The rise in lung cancer mortality during the twentieth century was statistically linked with the increase in smoking and that risk is higher in heavier smokers than it is in light smokers. If the nation reduces the consumption of tobacco, the incidence of this highly lethal disease will fall. On cessation of smoking, the risk falls but remains above that of the background population even after 15 years. Atmospheric pollution, ionising radiation and various occupational exposures to carcinogens such as the benzpyrenes predispose in a fashion similar to smoking; asbestos is an important carcinogen in its own right. What is of great interest to epidemiologists is the change in histological types of lung cancer encountered in Western society, even over the last twenty years. There has been a shift in incidence away from squamous lung cancer (the most common type in the 1940's to 1970's) to adenocarcinoma currently; this is not adequately explained by changes in tobacco or cigarette composition and remains enigmatic. There is renewed interest in a familial tendency to the disease, and many studies give credence to this concept. Some smokers are more at risk than others if they have a familial tendency. Genetic differences in the means of detoxifying carcinogens entering the body may be the link here.
Screening for lung cancer
The natural history of lung cancer is such that there is not a long pre-invasive phase. This does not therefore make it a good cancer candidate for a screening programme and most health programmes are concentrating on trying to stop the population from smoking. Nevertheless, for patients who are heavy smokers, an annual chest x-ray from 45-50 years of age seems to be intuitively a good idea. The new generation of spiral CT scanners now available have brought the possibility of CT screening of those at higher risk to the fore; several publications suggest that such routine CT screening of higher risk individuals will bring to light a higher number of potentially curable lung cancers (i.e at an operable stage). This is a developing subject and computer software is being developed to over-read the scans.
Symptoms of lung cancer
The presenting symptoms are easily ranked as: cough (often a change in character, e.g. more persistent and sometimes varying in intensity with the patient’s position) sometimes with blood (a sinister symptom), breathlessness, and chest pain. In addition, it is not unusual for the patient to complain of weight loss at presentation. Of course, this is a cancer that spreads (metastasises) early in its natural history and it is not that unusual for the disease to present with complaints due to metastases (secondary spread) e.g. bone pain from metastatic spread or headaches from brain spread etc.
Diagnosis of lung cancer
The doctor first performs a chest x-ray and if this shows anything suspicious he will run a CT scan of the chest. The analysis of the sputum for abnormal cells is the next test (sputum cytology) and is a very accurate diagnostic tool if positive, and this is usually the case if the patient is coughing up blood; it is less often positive otherwise.
If the tumour is not diagnosed by the sputum cytology, the doctor orders a bronchoscopy (a test where a telescope is manipulated down the throat into the wind-pipe (the trachea) and into the bronchial tubes to directly see any tumour arising from the walls of these tubes.
It is worth noting here that almost all lung cancers are bronchial cancers and arise from the walls of these bronchi. When the tumour is seen or an abnormal area discovered at bronchoscopy, the doctor will take a piece of tissue (biopsy) from this area for subsequent analysis down the microscope for a certain/pathological diagnosis.
If the tumour is not easily seen down the bronchoscope and a single lung shadow on the x-ray could be a birthmark (hamartoma etc.) then a PET scan is useful. This is a functional scan which is ‘hot’ when there is tumour there in the lung but ‘cold’ when the chest x-ray abnormality is one of any of the lung benign shadows which include congenital hamartomas or scars from old injury. If the result is equivocal or PET scan positive and the lesion not able to be biopsied on bronchoscopy, then a transthoracic fine needle biopsy directed by CT is diagnostic in most cases.
The message from all the above is that the diagnosis of lung cancer has to be made from sputum cytology or biopsy – that is: down the microscope. Sometimes the patient presents with spread of the cancer outside the chest and then the diagnosis can be made from biopsy of a metastasis, e.g. an abnormal lymph node in the neck.
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