Ectopic hamartomatous thymoma is distinct from lipomatous pleomorphic adenoma in lackingPLAG1aberration

Branchioma: immunohistochemical and molecular genetic study of 23 cases highlighting frequent loss of retinoblastoma 1 immunoexpression

Branchioma is an uncommon benign neoplasm with an adult male predominance, typically occurring in the lower neck region. Different names have been used for this entity in the past (ectopic hamartomatous thymoma, branchial anlage mixed tumor, thymic anlage tumor, biphenotypic branchioma), but currently, the term branchioma has been widely accepted. Branchioma is composed of endodermal and mesodermal lineage derivatives, in particular epithelial islands, spindle cells, and mature adipose tissue without preexistent thymic tissue or evidence of thymic differentiation. Twenty-three branchiomas were evaluated morphologically. Eighteen cases with sufficient tissue were assessed by immunohistochemistry, next-generation sequencing (NGS) using the Illumina Oncology TS500 panel, and fluorescence in situ hybridization (FISH) using an RB1 dual-color probe. All cases showed a biphasic morphology of epithelial and spindle cells with intermingled fatty tissue. Carcinoma arising in branchioma was detected in three cases. The neoplastic cells showed strong AE1/3 immunolabeling (100%), while the spindle cells expressed CD34, p63, and SMA (100%); AR was detected in 40-100% of nuclei (mean, 47%) in 14 cases. Rb1 showed nuclear loss in ≥ 95% of neoplastic cells in 16 cases (89%), while two cases revealed retained expression in 10-20% of tumor cell nuclei. NGS revealed a variable spectrum of likely pathogenic variants (n = 5) or variants of unknown clinical significance (n = 6). Loss of Rb1 was detected by FISH in two cases. Recent developments support branchioma as a true neoplasm, most likely derived from the rudimental embryological structures of endoderm and mesoderm. Frequent Rb1 loss by immunohistochemistry and heterozygous deletion by FISH is a real pitfall and potential confusion with other Rb1-deficient head and neck neoplasms (i.e., spindle cell lipoma), especially in small biopsy specimens.

Branchioma with a nested/organoid morphology: molecular profiling of a distinctive potentially misleading variant and reappraisal of potential relationship to CD34-positive/Rb1-deficient tumors of the neck

Branchioma (previously called ectopic hamartomatous thymoma, branchial anlage mixed tumor, or thymic anlage tumor) is a rare lower neck lesion with an adult male predominance and an uncertain histogenesis. Except for 4 cases, all branchiomas described in the literature were benign. Recently, HRAS mutation was detected in one case, but still little is known about the molecular genetic background of this rare entity. We herein report the histological, immunohistochemical, and molecular genetic analysis of a branchioma with a nested/organoid (neuroendocrine-like) morphology in a 78-year-old man. Histology revealed classical branchioma areas merging with nested/organoid cellular component lacking conventional features of malignancy. Immunohistochemistry was positive for high-molecular-weight cytokeratins. CD34 was expressed in the spindle cell component. Moreover, the tumor cells showed near-complete loss of retinoblastoma (RB1) expression (<1% of cells positive). All neuroendocrine markers (synaptophysin, chromogranin, and INSM1) were negative. Next-generation sequencing (TSO500 Panel) revealed 5 pathogenic/likely pathogenic mutations including 1 mutation in KRAS and 2 different mutations in each of MSH6 and PTEN. FISH and DNA sequencing were negative for RB1 gene alterations. To our knowledge, this is the first report of a branchioma showing misleading nested/organoid morphology and the first report on Rb1 immunodeficiency in this entity, in addition to multiple gene mutations revealed by NGS.

Adenocarcinoma arising in branchioma with a KRAS and TP53 mutation

Branchioma is a rare benign neoplasm occurring in the lower neck. Occurrence of malignant neoplasms arising in branchioma is extremely rare. Here, we report a case of adenocarcinoma arising in branchioma. A 62-year-old man had a right supraclavicular mass measuring 7.5 cm in diameter. The tumor contained an adenocarcinoma component encapsulated in a benign branchioma component. The adenocarcinoma component consisted of high- and low-grade components, with the former accounting for 80% of the adenocarcinoma. The high-grade component was immunohistochemically characterized by diffuse strong p53 expression, while the low-grade component and branchioma component were negative for p53. Targeted sequencing analysis for the branchioma and adenocarcinoma components revealed that the adenocarcinoma component harbored pathogenic mutations in KRAS and TP53. No definitive oncogenic drivers were detected in the branchioma component. Based on these immunohistochemical and molecular findings, we suggest that the KRAS mutation contributed to the pathogenesis of the adenocarcinoma, and the TP53 mutation played a key role in the transition from low-grade to high-grade adenocarcinoma.

Update from the 5th Edition of the World Health Organization Classification of Head and Neck Tumors: The Neck and Lymph Nodes, Metastasis, and Melanocytic Tumors

The changes made in the fifth edition of the WHO Classification of Head and Neck Tumors demonstrate the recent diagnostic, histopathological, and molecular advances in the field, and this updated information will hopefully lead to improved and standardized tumor subtyping. This review summarizes the changes related tumors and tumor-like lesions of the neck and lymph nodes (Chapter 11), metastasis to the head and neck region (Chapter 15), and melanocytic tumors (Chapter 10).

Biphenotypic Branchioma: A Better Name Than Ectopic Hamartomatous Thymoma for a Neoplasm with HRAS Mutation

Ectopic hamartomatous thymoma is a rare neck lesion originally thought to represent a non-neoplastic hamartoma, even though thymic origin has been questioned, and there is uncertainty about whether the lesion is a neoplasm. We investigated the genetics by performing targeted next generation sequencing (NGS). Three cases were identified from the authors' consultation files. A custom, targeted NGS panel including 1385 pan-cancer-related genes was performed on all cases. Three patients included 2 males and 1 female, aged 50, 58 and 70 years, respectively (mean 59.3 years), with tumors arising in the low anterior neck. All cases showed classical histologic features of EHT, with one case showing intraductal carcinoma in association with the EHT. By targeted NGS, one case harbored a hotspot HRAS mutation (p.Gln61Lys), while the other two cases only showed non oncogenic variants. Dual mesoderm and endoderm derivation/differentiation (biphenotypic) has been previously recognized, with epithelial and myoepithelial components, and arising from the apparatus contributing to neck development (branchial apparatus). Thus, EHT has been shown to have genetic alterations in HRAS. These findings, without evidence of thymic derivation or an ectopic tissue location, strongly support that EHT is a true neoplasm. The name biphenotyic branchioma more correctly reflects the true nature of this dual mesoderm and endoderm derived tumor occurring in the lower neck.

Ectopic Hamartomatous Thymoma: A Review Of The Literature With Report Of New Cases And Proposal Of A New Name: Biphenotypic Branchioma

Ectopic hamartomatous thymoma (EHT) is a rare benign neoplasm of the lower neck suggesting branchial origin. Despite use of the term thymoma in the nomenclature, there is no evidence of thymic origin or differentiation. It affects middle-aged adults with a remarkable male predominance. To date less than 80 cases have been reported in the English literature. We present here two additional cases of EHT. The first is a benign case in a 31-year-old man, showing typical histological features. The second is a malignant case in a 70-year-old woman, showing intraductal carcinoma arising in intimate association with an EHT. These cases are presented in the context of a review of cases reported in the English literature. The exact origin has not been identified, but is considered to be of branchial apparatus, creating a quandary about the best terminology. Recently, the designation "branchial anlage mixed tumor" or "thymic anlage tumor" were proposed, but do not quite reflect the true nature of the neoplasm. To avoid taxonomic confusion, international consensus on terminology is desired. As this entity is a neoplasm that shows dual mesoderm and endoderm derivation/differentiation, we propose a new name "biphenotypic branchioma."

Loss of expression of Plag1 in malignant transformation from pleomorphic adenoma to carcinoma ex pleomorphic adenoma

PLAG1 (pleomorphic adenoma gene 1) is frequently activated in pleomorphic adenoma (PA). Carcinoma ex pleomorphic adenoma (CXPA) arises in PA, and PLAG1 expression is believed to be maintained from PA to CXPA, as it can contribute to the carcinogenesis process. To evaluate if PLAG1 is a good marker of malignant transformation from PA to CXPA as well as to evaluate if PLAG1 expression is associated with progression and histopathologic subtype of CXPA. Forty PAs, 21 residual PAs (without malignant transformation), and 40 CXPAs were analyzed by immunohistochemistry with PLAG1 antibody. The proportion of positive neoplastic cells was assessed according to a 2-tiered scale: >10% to 50%, and >50% positive cells. The CXPA group was classified according to histopathologic subtype and invasiveness degree. Thirty-seven PAs (92.5%), 15 residual PAs (71%), and 14 CXPAs (35%) were positive for PLAG1. In relation to the CXPA group, among the intracapsular cases, myoepithelial carcinoma and epithelial-myoepithelial carcinoma showed the highest level of PLAG1 expression. PLAG1 expression is lost when PA undergoes malignant transformation, possibly due to other pathway activation and different clone cells. In addition, PLAG1 expression seems to be present mainly in low-grade carcinomas and in cases with early phase of invasion, due to its regulation of oncogene-induced cell senescence. In CXPA, PLAG1 expression was most associated with myoepithelial differentiation. This way, loss of PLAG1 expression can be considered a hallmark of CXPA carcinogenesis, mainly when there is only epithelial differentiation.