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basaloid carcinoma of the lung

Wednesday 8 October 2014

Basaloid carcinoma of the lung was first included as a distinct entity in the 1999 WHO schema of lung tumors based on its poorer prognosis when compared to conventional non-small cell lung carcinoma.

In its initial description in 1992, only 50% of the tumors were pure basaloid carcinomas while the rest were associated with areas of squamous cell carcinoma, adenocarcinoma, adenosquamous carcinoma, or classic large cell carcinoma.

The same authors used an arbitrary cutoff of 60% of a basaloid pattern in order to qualify for this type of tumor.

Subsequently, basaloid carcinomas were included in the WHO schemata of 1999 and 2004 either as a subtype of squamous cell carcinoma or large cell carcinoma depending on the presence or absence of squamous differentiation.

In 2003, Kim et al.40 presented a series of 35 cases, 18 of which were pure basaloid carcinomas and 17 of which were basaloid squamous cell carcinomas. Interestingly, in this series, the survival was similar to that of conventional non-small cell carcinoma questioning the hypothesis that basaloid carcinomas of the lung carry a worse prognosis.

In 2014, Rossi et al.10 demonstrated that all of their 24 basaloid carcinomas in a series of 121 large cell carcinomas showed squamous differentiation by immunohistochemical techniques, thereby allowing reclassification of all these tumors as variants of poorly differentiated squamous cell carcinoma.

From the data presented in the literature, it appears that basaloid carcinoma may represent a form of poorly differentiated squamous cell carcinoma or very rarely adenocarcinoma.

Basaloid carcinoma of the lung is listed as another variant of pulmonary large cell carcinoma.

These tumors are characterized by a solid nodular growth or formation of anastomosing trabecular structures. A hallmark of this tumor is its prominent peripheral palisading.

Contrary to large cell neuroendocrine carcinomas, the tumor cells are small and uniform and contain hyperchromatic nuclei with indistinct nucleoli. Although cytoplasm is scant, nuclear molding as typically seen in small cell carcinomas is absent. Comedo-type necrosis is common and rosette-like structures may be occasionally seen.

Squamous differentiation should be absent, otherwise the tumor would have to be classified as the basaloid variant of squamous cell carcinoma.

Although focal positivity of tumor cells may be seen with neuroendocrine markers, high molecular-weight cytokeratin is diffusely expressed in these tumors contrary to neuroendocrine carcinomas.