Baylor University Medical Center Proceedings July 2017 - 323

Most patients present with symptoms of large airway irritation or
obstruction including cough, hemoptysis, wheezing, chest pain,
and obstructive pneumonia (2-4,
11). PMEC may be missed on
chest radiographs; however, CT
imaging typically demonstrates a
well-deﬁned endobronchial mass
(2, 3). PMEC is frequently positron emission tomography avid
(3).
d
c
PMEC is composed of varying proportions of mucus-secreting cells, intermediate cells, and
epidermoid (or squamoid) cells,
lacking keratinization and/or in
situ carcinoma of the overlying
epithelium (2, 3, 5). The tumor
cells display various patterns of
growth, including mucin-filled
cysts, nests, islands, and tubules
or glands (2-4). Occasionally
Figure 2. Microscopic examination of the submucosal mass. (a) A well-circumscribed mass composed of a predomi- stromal calcifications with or
nant population of clear cells (hematoxylin and eosin [H&E], 40×). (b) The cytologically bland neoplastic cells with without a granulomatous reacample clear cytoplasm and small round nuclei (H&E, 200×). (c) Foci of epidermoid/squamoid cells and rare mucin tion are also observed in areas
(H&E, 200×). (d) An increased amount of mucus-secreting cells intermixed with intermediate cells in other areas of of mucous extravasation (3, 11).
the neoplasm (H&E, 200×).
Immunophenotypically, PMEC is
positive for pan cytokeratin, CK7, CK5/6, p63, and p40. S100,
following lobectomy, the patient exhibited no recurrence or
CEA, and EMA show variable staining. PMEC is negative for
metastasis. The patient continued with surveillance bronchosCK20, TTF1, SOX 10, napsin A, androgen receptor, calponin,
copy and chest CT scans every 6 months for 5 years.
SMA, and smooth muscle myosin heavy chain (3-6, 12).
PMEC is associated with a translocation involving CRTC1
DISCUSSION
(also known as MECT1) on 19p13 and MAML2 on 11q21 in
PMEC is a rare malignant neoplasm typically aﬀecting paat least a subset of cases. Identiﬁcation of the CRTC1-MAML2
tients <30 years of age, although the age range is wide (2-4).
fusion gene is now considered to be pathognomonic for this
entity, and the fusion gene can be detected by real-time PCR
or FISH in both low-grade and high-grade MEC (3, 5, 7, 8,
13, 14). Recent studies of PMEC demonstrate MAML2 rearrangement in 60% to 100% of tested cases (5, 15, 16) detected
by FISH or real-time PCR. Additionally, there are documented
reports of EGFR mutations in PMEC with involvement of exon
18 (L861Q) (3, 17). Other studies demonstrated EGFR expression without EGFR ampliﬁcation or mutation (18).
Diﬀerentiating between squamous cell carcinoma (SCC)
and PMEC is challenging, considering the overlap in the immunophenotype (both are keratin, p63, and p40 positive).
Although not speciﬁc, features more frequently observed in
high-grade PMEC than SCC are 1) exophytic endobronchial
growth, 2) absence of carcinoma in situ, and 3) absent keratinization and squamous pearl formation (3). Pulmonary adenocarcinoma is typically positive for TTF1, CK7, and napsin A
and negative for CK20 and p63. Adenosquamous carcinoma
Figure 3. Break-apart FISH probe demonstrating one fusion signal and separaof the lung exhibits a mixture of adenocarcinoma (TTF1 position of the red and green signals (arrows), indicating rearrangement of MAML2
tive) and SCC (p63 positive). Metastatic carcinoma, such as
(1000× oil immersion). A normal or intact MAML2 locus would show the red and
clear cell renal cell carcinoma, is excluded using immunostains
green signals in close proximity, producing a yellow color.
a

July 2017

b

Primary mucoepidermoid carcinoma of the lung with prominent clear cells

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Table of Contents for the Digital Edition of Baylor University Medical Center Proceedings July 2017
