DOI:10.2214/AJR.07.2292
AJR 2007; 189:W254-W261
© American Roentgen Ray Society
Basal Cell Adenoma Versus Pleomorphic Adenoma of the Parotid Gland: CT Findings
Nai-Chi Chiu1,
Hsiu-Mei Wu1,
Yi-Hong Chou1,
Wing-Yin Li2,
Yi-You Chiou1,
Wan-Yuo Guo1 and
Cheng-Yen Chang1
1 Department of Radiology, Taipei Veterans General Hospital and School of
Medicine, National Yang-Ming University, 201 Shih-Pai Rd., Section 2, Taipei,
Taiwan.
2 Department of Pathology, Taipei Veterans General Hospital and School of
Medicine, National Yang-Ming University, Taipei, Taiwan.
Received March 21, 2007;
accepted after revision May 18, 2007.
WEB
This is a Web exclusive article.
Address correspondence to H. M. Wu
(hmwu{at}vghtpe.gov.tw).
Abstract
OBJECTIVE. Basal cell adenoma is a rare benign epithelial tumor of
the salivary gland. The objective of this study is to present the CT findings
of parotid basal cell adenoma. We also compare CT findings of basal cell
adenoma with those of pleomorphic adenoma, the most common parotid tumor, to
determine whether any features on CT can help differentiate these two
entities.
CONCLUSION. Basal cell adenomas of the parotid gland are located
chiefly in the superficial lobe. They are generally round, well-circumscribed
tumors that show heterogeneous enhancement on CT. The age of the patient and
the attenuation on unenhanced and contrast-enhanced CT may help in
differentiating basal cell adenoma from pleomorphic adenoma of the parotid
gland.
Keywords: adenoma • basal cell adenoma • CT • head and neck imaging • parotid gland • pleomorphic adenoma
Introduction
Basal cell adenoma is a rare benign epithelial tumor of the salivary gland,
accounting for 1–2% of all salivary gland epithelial tumors
[1]. More than 80% of basal
cell adenomas arise in the major salivary glands, mostly the parotid gland
[2]. Clinically, they usually
present as a painless palpable mass, indistinguishable from other salivary
gland tumors, including pleomorphic adenoma, the most common benign tumor of
the salivary gland. Histologically, basal cell adenomas are classified as
monomorphic tumors composed chiefly of basaloid cells organized with a
prominent basal cell layer and distinct basement membrane–like material.
They lack the myxochondroid stromal component of pleomorphic adenomas
[2]. The basaloid cells are
sometimes indistinguishable from adenoid cystic carcinoma, especially when
diagnosed by cytology alone
[3].
The purpose of this study is to present the unenhanced and
contrast-enhanced CT features of the rare benign parotid gland tumor, basal
cell adenoma, and compare them with those of the most common benign parotid
gland tumor, pleomorphic adenoma.
Materials and Methods
A retrospective search through the histopathology records and PACS
(SmartIRIS, Taiwan) records in our institution from January 2003 to December
2005 revealed 11 patients with basal cell adenoma and 17 patients with
pleomorphic adenomas who underwent preoperative CT. All these patients had
undergone superficial or total parotidectomy according to the location of the
tumor. None of the patients underwent biopsy before scanning.
CT was performed with Genesis, HiSpeed, or RP scanners (GE Healthcare) or
Somatom Plus 4 or Sensation 16 scanners (Siemens Medical Solutions) scanners
in a single-slice mode with contiguous 5-mm slices from the skull base to the
thoracic inlet using a soft-tissue algorithm. For contrast-enhanced images, a
bolus IV dose of 75 mL of nonionic contrast material (350 mg I/mL) was given.
Scanning was initiated 30 seconds after the contrast injection was completed
[4].
All images were reviewed on the PACS. For visual assessment, the images
were reviewed at the same window width and level in all patients. Tumor
location was divided into superficial and deep lobes of the parotid by the
location of the retromandibular vein. Size was expressed in terms of maximal
perpendicular dimension on the transverse plane in millimeters. A tumor was
considered to have well-circumscribed margins if it was well demarcated from
the rest of the parotid gland throughout all scans. A "lobulated"
tumor was one showing an undulated margin with indentations or sulci. For
assessment of the unenhanced and contrast-enhanced attenuations of the tumors,
a circular region of interest (ROI) as large as possible (8–35 mm) was
drawn on the soft-tissue components of the tumor, excluding obvious cystic or
necrotic areas. The enhancement patterns were divided into homogeneous
(uniform) and heterogeneous (nonuniform). The "cystic area" was
defined as having a CT attenuation of 20 H or less and having a round or ovoid
shape.
Pathology recheck was performed in four tumors and resulted in revision to
the final diagnosis in two (i.e., atypical basal cell adenoma or
adenocarcinoma rather than typical basal cell adenoma). In one tumor, the
recheck was due to marked deviation of CT findings from those of most other
tumors, with an extremely cystic appearance and internal
fluid–sedimentation level. In the second tumor, it was due to an
exceptionally young patient age and indistinct tumor margins, and in the last
two tumors, to the original pathologic diagnosis of atypical basal cell
adenoma (Table 1).
The Mann-Whitney test was used for the statistical analysis of differences
in tumor size, location, margin, unenhanced and contrast-enhanced attenuation,
and contrast enhancement patterns between basal cell adenomas and pleomorphic
adenomas.
Results
Of the 11 patients with basal cell adenoma, typical or not, a palpable mass
without tenderness was the initial presentation in nine patients and an
incidental finding during a health examination in the other two. The initial
presentation in all 17 patients with pleomorphic adenoma was palpable mass
without tenderness or painful sensation.
Of the seven patients with typical basal cell adenoma, five underwent both
unenhanced and contrast-enhanced CT and two underwent only contrast-enhanced
CT. The ages of these patients ranged from 43 to 86 years (mean, 63.9 years).
Two patients were women and five were men
(Table 2). The masses involved
the superficial lobe of the parotid gland in all seven patients. The mean
diameter was 1.7 x 1.9 cm (range, 0.8 x 1.0 to 2.4 x 2.5
cm). No significant difference was seen in the largest diameter of lesions
between basal cell adenomas and pleomorphic adenomas (mean, 1.5 x 2.0
cm; range, 0.7 x 0.8 to 3.1 x 3.5 cm).
Two of the seven typical basal adenomas showed homogeneous enhancement
(Fig. 1A,
1B), and another three showed a
large eccentric cystic component (Fig.
2A,
2B,
2C,
2D). The remaining two cases
showed heterogeneous enhancement with small low-attenuation components (Fig.
3A,
3B,
3C). All seven tumors were
round or ovoid and were marginally well circumscribed. The mean CT attenuation
(in Hounsfield units) of the soft-tissue part of the tumors on unenhanced CT
scans was 46.80 ± 10.62 (SD) H. The soft-tissue components showed
intense enhancement after contrast injection (mean attenuation, 91.43 ±
26.70 H).
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Fig. 3A —66-year-old man with typical basal cell adenoma. CT shows
small cystic components (arrows, B) in tumor on unenhanced
(A) and contrast-enhanced (B) scans, which is compatible with
pathologic appearance.
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Fig. 3B —66-year-old man with typical basal cell adenoma. CT shows
small cystic components (arrows, B) in tumor on unenhanced
(A) and contrast-enhanced (B) scans, which is compatible with
pathologic appearance.
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All 17 patients with pleomorphic adenoma underwent both unenhanced and
contrast-enhanced CT. The ages of these patients ranged from 18 to 77 years
(mean, 47.5 years). Ten were women and seven were men. All 17 tumors were well
circumscribed. Tumors were either round (n = 14) or lobulated
(n =3). The mean diameter was 1.5 x 2.0 cm (range, 0.7 x
0.8 to 3.1 x 3.5 cm). Twelve tumors (71%) were enhanced homogeneously.
The remaining five cases showed heterogeneous enhancement with some
low-attenuation components. The mean CT attenuation of the soft-tissue portion
of the tumors on unenhanced CT was 34.35 ± 12.86 H. The soft-tissue
components showed intense enhancement after contrast injection (mean
attenuation, 66.06 ± 26.35 H).
No significant difference was seen in the largest dimension between basal
cell adenomas and pleomorphic adenomas. A statistically significant difference
existed in the mean CT attenuation of basal cell adenoma and pleomorphic
adenoma on either unenhanced or contrast-enhanced images (p = 0.037
and 0.042, respectively, analysis of variance)
(Table 3).
One of the basal cell adenomas had an atypical CT appearance: a lobulated
mass lesion with an exclusive cystic component and internal
fluid–sedimentation levels. Pathology review revealed a typical basal
cell adenoma with chronic and fresh hemorrhage, chronic inflammation, and
fibrosis of the capsule (Fig.
4A,
4B,
4C,
4D,
4E).
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Fig. 4B —76-year-old man with typical basal cell adenoma and
intratumoral hemorrhage. Contrast-enhanced CT scans show lobulated contour,
distinct margin, fluid–sediment level (arrows, B) and
ring enhancement (arrowheads, B).
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Fig. 4C —76-year-old man with typical basal cell adenoma and
intratumoral hemorrhage. Contrast-enhanced CT scans show lobulated contour,
distinct margin, fluid–sediment level (arrows, B) and
ring enhancement (arrowheads, B).
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Fig. 4D —76-year-old man with typical basal cell adenoma and
intratumoral hemorrhage. Photomicrographs of tumor specimen show well-defined
tumor capsule with fibrotic change (arrows, D) and
hemosiderin-laden macrophages (arrows, E), which correspond to
intratumoral hemorrhage.
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Fig. 4E —76-year-old man with typical basal cell adenoma and
intratumoral hemorrhage. Photomicrographs of tumor specimen show well-defined
tumor capsule with fibrotic change (arrows, D) and
hemosiderin-laden macrophages (arrows, E), which correspond to
intratumoral hemorrhage.
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The final diagnosis of one of the four pathologically reviewed cases
changed from typical basal cell adenoma to atypical basal cell adenoma because
of the newly identified capsular invasion, nerve bundle encasement, and focal
cribriform pattern of tumor cells (Fig.
5A,
5B,
5C,
5D). This tumor showed
ill-defined margin on CT images. Also, the patient's age was much younger than
the mean age of those with typical basal cell adenomas (mean age, 60.6
± 14.9 years).
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Fig. 5A —Tumor originally diagnosed as typical basal cell adenoma in
33-year-old woman. CT shows indistinct margin (arrows, A) and
prominent contrast enhancement on unenhanced (A) and contrast-enhanced
(B) scans. Because of atypical image appearances and age deviation
compared with other cases, pathology review was suggested. Diagnosis was
verified as atypical basal cell adenoma.
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Fig. 5B —Tumor originally diagnosed as typical basal cell adenoma in
33-year-old woman. CT shows indistinct margin (arrows, A) and
prominent contrast enhancement on unenhanced (A) and contrast-enhanced
(B) scans. Because of atypical image appearances and age deviation
compared with other cases, pathology review was suggested. Diagnosis was
verified as atypical basal cell adenoma.
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Fig. 5C —Tumor originally diagnosed as typical basal cell adenoma in
33-year-old woman. Photomicrographs of tumor specimen show capsular invasion
(arrows, C) and focal cribriform pattern of tumor cells
(arrows, D).
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Fig. 5D —Tumor originally diagnosed as typical basal cell adenoma in
33-year-old woman. Photomicrographs of tumor specimen show capsular invasion
(arrows, C) and focal cribriform pattern of tumor cells
(arrows, D).
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One of the two atypical basal cell adenomas showed capsular extension and
adenoid cystic features (focal cribriform pattern) at the subcapsular region
(Fig. 6A,
6B,
6C,
6D). The other one had
capsular extension, mild nuclear atypia, and loss of the peripheral palisading
appearance of a tumor cell nest, which was suggestive of basal cell
adenocarcinoma (Fig. 7A,
7B,
7C,
7D,
7E). Although both of these
tumors showed capsular extension on pathologic examination, the CT features of
the former are indistinguishable from those of the typical basal cell adenoma.
From this point of view, the CT features cannot completely exclude malignancy,
and thus all lesions deemed to be basal cell adenoma on CT probably should be
resected and analyzed by a pathologist.
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Fig. 6A —56-year-old woman with atypical basal cell adenoma.
Unenhanced (A) and contrast-enhanced (B) CT scans show round
tumor with distinct margins and heterogeneous contrast enhancement with
low-attenuation areas (arrows, B). Image patterns are similar
to those of typical basal cell adenoma.
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Fig. 6B —56-year-old woman with atypical basal cell adenoma.
Unenhanced (A) and contrast-enhanced (B) CT scans show round
tumor with distinct margins and heterogeneous contrast enhancement with
low-attenuation areas (arrows, B). Image patterns are similar
to those of typical basal cell adenoma.
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Discussion
Salivary gland neoplasms represent less than 3% of all tumors in the
general population. Approximately 88% of salivary gland neoplasms are of
epithelial origin, and benign adenomas account for 65.5%. Basal cell adenoma
accounts for 1–2% of all salivary gland epithelial tumors
[1], and more than 80% of them
arise in the major salivary glands, mostly the parotid gland
[2], as in those cases
presented here. Although some articles have dealt with basal cell adenoma,
ours may be the first report to show the CT findings of this rare benign
epithelial tumor of the salivary gland.
Basal cell adenoma is classified as a subtype of monomorphic adenomas,
which were first described and adequately documented as a distinct clinical
and pathologic entity by Kleinsasser and Klein in 1967
[5]. Currently, basal cell
adenomas are classified by the World Health Organization (WHO) as one of nine
subcategories of salivary gland epithelial tumors. Basal cell adenoma, as
defined by WHO, is a distinctive benign neoplasm composed chiefly of basaloid
cells organized with a prominent basal cell layer and distinct basement
membrane–like structure with no myxochondroid stromal component, as seen
in pleomorphic adenomas
[6].
The common clinical feature of basal cell adenoma is a slowly growing,
asymptomatic, freely movable parotid mass. With respect to sex distribution,
many previous reports have described a female predominance in basal cell
adenomas and in pleomorphic adenomas
[1,
7–9].
In our study, pleomorphic adenomas were also found more frequently in women.
However, our study found no significant difference in the distribution by sex
of basal cell adenomas (Table
3). This finding may be due to the predominance of men in the
older patient population at our institute, a tertiary veterans hospital.
The average age of patients with basal cell adenomas has been reported to
be approximately 57.7 years [2,
7], more than a decade older
than the average age of those with pleomorphic adenoma
[1]. In this study, patients
with basal cell adenomas were also significantly older than those with
pleomorphic adenomas (mean ages, 60.6 ± 14.9 years vs 47.5 ±
15.4 years; p = 0.034, analysis of variance).
In general, basal cell adenomas are assumed to be rather small tumors, less
than 3 cm in their greatest dimension, and smaller than pleomorphic adenomas
[1,
6,
7,
10]. The same trend was also
seen in our study. However, no significant difference was seen in the largest
diameter of lesions between these two entities.
The absence of chondroid tissue and myxoid stroma are the main
histopathologic features that help to differentiate basal cell adenomas from
pleomorphic adenomas. According to a previous study, the myxoid stromal
component shows low attenuation on unenhanced CT and bright intensity on
T2-weighted imaging, and the attenuation is not as low as that of fluid
[11]. This may explain why
pleomorphic adenomas showed lower CT attenuation than basal cell adenomas on
unenhanced and contrast-enhanced CT in our study (mean, 46.80 ± 10.62 H
vs 34.35 ± 12.86 H; p = 0.037, analysis of variance)
Histopathologically, basal cell adenomas have characteristic numerous
endothelium-lined vascular channels, in which small capillaries and venules
are prominent, in the microcystic areas of the adenoma
[12,
13]. The strong enhancement
after contrast injection and the presence of hemorrhagic components in basal
cell adenoma (Fig. 4A,
4B,
4C,
4D,
4E) may be related to the
vascular architecture [10,
14].
According to previous reports, cyst formation is a main histopathologic
feature of basal cell adenoma, presenting in more than one half of tumors
examined (26/40, 65%) [1,
10,
14]. The imaging appearances
of typical basal cell adenomas are compatible with the histopathologic
features. All seven typical basal cell adenomas in our study showed cystic
components microscopically. Three of the seven cases showed peripherally large
cystic components. Two showed some small intratumoral cystic components. As
with patient 1 in Table 1, the
CT appearances of extremely cystic change with only a thin layer of soft
tissue marginally correspond to the features of intratumoral hemorrhage and a
fluid–sediment level seen on pathology (Fig.
4A,
4B,
4C,
4D,
4E).
The final diagnosis of basal cell adenoma in a 33-year-old woman (Fig.
5A,
5B,
5C,
5D) was verified to be
atypical basal cell adenoma due to the findings of the capsule extension,
nerve bundle encasement, and a focal cribriform pattern during the pathology
recheck. Although to our knowledge no study has been performed of incidence
and prognosis of patients with basal cell adenoma in different age groups,
incidences of 75–80% of benign salivary gland neoplasms in adults but
only 40–65% in adolescents have been reported
[15–17].
It is particularly true that malignant parotid tumors are more frequent in
young persons [15]. This may
suggest that basal cell adenomas in young adults may be not as benign as those
in older patients. Also, in cases like this 33-year-old patient, it is also
important to rule out other monomorphic tumors with basaloid cells such as
adenoid cystic carcinomas, which frequently present as well-enhanced tumors
with ill-defined margins in the middle-aged population
[3].
In conclusion, basal cell adenoma usually appears as a round tumor in older
patients and has a smooth, distinct margin and a high incidence of internal
macro- or microcysts. The soft-tissue components show relative high
attenuation on unenhanced CT and enhance well on contrast-enhanced CT. Basal
cell adenoma should be included in the differential diagnosis of parotid
tumors in older patients. Careful pathologic review should be performed when a
basal cell adenoma is diagnosed in a young adult or in a tumor with an
indistinct margin on imaging.
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Abstract
Introduction
