Squamous cell carcinoma of the lung
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Squamous cell carcinoma accounts for ~30-35% of all lung cancers and in most instances is due to heavy smoking 3. Historically it was the most common type of lung cancer but in many countries has gradually declined over the past four decades with a rise in adenocarcinoma of the lung, which is now the most common in many series 4.
In general, squamous carcinomas are encountered more frequently in male smokers, and adenocarcinoma in female smokers, although these results vary from series to series 4.
In addition to smoking, exposure to toxic agents (e.g. nickel) may increase the risk of developing squamous cell carcinoma 9.
Clinical presentation depends on the location of the tumor and is largely independent of histology.
More peripheral tumors, if not found incidentally on imaging, usually present when larger, invading into chest wall (e.g. Pancoast tumor) 3.
Although squamous cell carcinoma of the lung is traditionally known to arise centrally (66-90%), the incidence of peripherally located squamous cell carcinoma is increasing 1-4.
Macroscopically these tumors tend to be off-white in color, arising from, and extending into a bronchus. They invade the surrounding lung parenchyma and can extend into the chest wall. Larger tumors have a tendency to undergo central necrosis 4.
Squamous cell carcinoma of the lung is characterized by intercellular bridging and/or keratinization of the individual cells or squamous pearls. These characteristics vary depending on the degree of differentiation with the poorly differentiated form exhibiting the least remarkable features and greater mitotic activity.
Four subtypes are recognized 4:
small cell (not to be confused with small cell lung cancer)
Squamous cell carcinoma consistently expresses P63 and is negative for TTF1 6. Other squamous immunomarkers include CK5/6 or 34BE12. Differentiation of squamous cell carcinoma from adenocarcinoma is vital as a response to cytotoxic and biological agents will differ.
While it is not possible to differentiate squamous cell lung cancer from other types of lung cancer on plain film, there are a few generic features that would raise suspicion of a lung malignancy.
Lung cancer is relatively infrequently found on chest radiographs due to the combination of difficulty in visualizing small lesions and the fact that even when the lesion is objectively visible, it is not seen by the reporting radiologist. The diagnostic confidence is the greatest when the lesion is at least 8-10 mm 5.
The appearance depends on the location of the lesion. The more central lesions may merely appear as a bulky hilum, representing the tumor and local nodal involvement. Lobar collapse may be seen due to obstruction of a bronchus. When the right upper lobe is collapsed and a hilar mass is present, this is known as the Golden S sign.
A more peripherally located mass may appear as a rounded or spiculated mass. Cavitation may be seen as an air-fluid level. Chest wall invasion is difficult to identify on plain films unless there is destruction of an adjacent rib or evidence of soft tissue growing into the chest wall.
Pleural effusions may also be seen, and although it is associated with a poor prognosis, not all effusions are due to malignant involvement of the pleural space. Some are due to venous obstruction or represent a parapneumonic effusion 4.
CT is the modality of choice for the evaluation of possible lung cancer.
Certain morphological features can be suggestive of squamous cell carcinoma, but these are by no means definitive, with significant overlap with other histological types. More importantly, cross-sectional imaging enables staging of the disease and, together with the histological grading and clinical performance status, will dictate the prognosis and treatment.
Central tumors often result in intraluminal obstruction and cause lung collapse and/or obstructive pneumonitis. Peripheral tumors may be seen as a solid nodule/mass with or without an irregular border 1,2. The irregular margin can be attributed to a desmoplastic reaction or infiltrative growth 1,2. Similar to central tumors, peripheral tumors can also result in obstructive changes such as a mucocele.
Cavitation is a frequent finding in primary lung squamous cell carcinoma (some report as high as 82% 10), but can also be encountered in metastatic squamous cell carcinoma. Cavitation is secondary to tumoral necrosis. In other instances, squamous cell carcinoma can have a central scar with the peripheral growth of the tumor.
When squamous cell carcinoma presents as a peripheral solid nodule, follow-up is as per the Fleischner Society guidelines.
Treatment and prognosis
Classification of lung cancer into histological and immunohistochemical subtypes has a bearing on oncologic therapy. Survival is dependent on performance status at diagnosis and the stage of disease (see non-small cell lung cancer staging). Overall, stage-for-stage comparison of the survival rate for squamous cell carcinoma is better than for adenocarcinoma 6. Differentiation between other types of non-small cell lung cancer (NSCLC), namely. adenocarcinoma/large cell lung cancer, and squamous cell carcinoma is also crucial, as the response to cytotoxic therapy differs. One such difference is the decreased efficacy of pemetrexed in squamous cell carcinoma patients 7.
Different types of NSCLC respond differently to biological agents. High expression of EGFR in some squamous cell carcinoma patients may benefit from cetuximab (monoclonal antibody against EGFR). However, lung squamous cell carcinoma patients receiving bevacizumab (monoclonal antibody against VEGF) have a high incidence of pulmonary hemorrhage, thus it is only used for non-squamous cell carcinoma NSCLC 7.
Patients with cavitating squamous lung carcinoma (cSLC) are believed to harbor an aggressive, chemoresistant disease with distinct features and fare poorly 11.
The differential diagnosis depends on the location and appearance of the mass.
Generic differentials for individuals features are as follows:
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