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Introduction
Choristomas (Gr. chōristos, ‘separated’) are well-delimited formations consisting of histologically normal tissue located in an abnormal anatomical site.1 Their aetiology is related to failures in the normal course of embryonic development of the neural tube. While choristomas are frequently encountered in some parts of the body (eg, thyroid gland, pelvis), an intrapulmonary localisation is rarer. In particular, the presence of thymic tissue in the lung has never been documented in the English literature. Here, we illustrate a choristoma composed of mature thymic tissue in the lung of a fetus who died after 16 weeks of gestation.
Case presentation
A 21-year-old, gravida 1, para 0 woman experienced a spontaneous abortion at 16 weeks of gestation. Autoptic examination of the fetus revealed a female individual with mild clubfoot and a 0.4 cm nodule in the right lung. No other macroscopic birth defects were observed. At histological analysis, the lung nodule was composed of normal thymic tissue surrounded by a continuous fibrous capsule (figure 1). The detection of the normal thymus in its proper mediastinal location suggested a diagnosis of thymic choristoma of the lung.
Thymic choristoma in the lung of a female fetus at 16 weeks of gestation. A well-demarcated nodule measuring 0.4 cm in greatest dimensions was observed inside of the right lung (A, H&E, original magnification ×10). the nodule showed a lobulated appearance, with a dense outer cortex and an inner less dense medulla; it was surrounded by a capsule that was contiguous with normal pulmonary parenchyma in the canalicular phase of development and extended into the interior of the nodule forming septa (B, H&E, original magnification ×20). In the medulla, concentric, nest-like bodies consistent with Hassall's corpuscles were visible at higher magnification (C, H&E, original magnification ×100).
Discussion
The concept of choristoma, and its differentiation from hamartoma (ie, disorganised overgrowth of normal, variably mature tissue in its normal location) and heterotopia (ie, mass of normal mature tissue in an abnormal location but in the proper anatomical site), was first developed by Dr Eugen Albrecht in 1904 in the attempt to classify this heterogeneous group of conditions.2 However, unlike hamartoma and heterotopia, the term choristoma has fallen by the wayside of everyday pathology’s vocabulary. Hence, most of the available reports on thymic choristomas describe heterotopic masses. The presence of mature thymic tissue has been observed, although seldom, in the thyroid gland, in the neck region or in the posterior mediastinum.3 It is generally diagnosed in children, either as an incidental finding or as an indolent, slow-growing mass. Here, we provide evidence on the possible occurrence of thymic choristomas in the lung and that these tumour-like masses can arise at an early phase of the fetal maturation. Choristoma is a useful term to precisely define this type of displacement, and it might well be revived, especially in dealing with such vanishingly rare conditions.
Footnotes
Contributors GL and NF conceived the project. TR performed the initial diagnosis. SF and NF supervised the study. Image processing by GL. GL and RC wrote the first draft of the manuscript which was initially reviewed by NF. Subsequently, all authors contributed to and approved the final version of the manuscript. NF is the guarantor of this manuscript, taking full responsibility for its contents.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.