Artículo Clínico

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Basal cell adenoma of the parotid: A revision based on four cases

Adenoma de células basales parotídeo: Revisión a propósito de cuatro casos

 

M.J. Pastor Fortea¹, J.I. Iriarte Ortabe², R. Ramos Asensi², M. Morey Mas², J. Caubet Biayna²,
A. Pozo Porta¹, I. Forteza-Rey Borralleras³

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Abstract: The basal cell adenoma is a specific type of adenoma, with a uniform, monomorphous histologic appearance that is dominated by basaloid cells and that does not have the myxochondroid tissue characteristic of mixed tumors. It may be divided on the basis of its morphologic pattern into four subtypes: solid, tubular, trabecular and membranous. We report four cases of basal cell adenoma subdivided as follows: one solid, one trabecular and two membranous subtypes. In all cases a conservative superficial parotidectomy was the treatment. Morphologic identification of the specific subtype is for descriptive purposes, except in the case of the membranous type, due to its tendency to be multifocal, its high recurrence rate, its occasional malignant transformation and its possible association in about onethird of the reported cases with dermal cylindromas. A close followup and screening of skin lesions is suggested for these tumors. Key words: Basal; Membranous; Multifocal; Relapse; Eccrine.

Resumen: El adenoma de células basales es un tipo específico de adenoma con una apariencia histológica uniforme y monomorfa, en el que predominan las células basaliodes sin el componente mixocondroide del tumor mixto. Atendiendo a su morfología pueden ser divididos en cuatro subtipos: sólido, tubular, trabecular y membranoso. Presentamos cuatro casos de adenoma de células basales localizados en glándula parótida: uno de tipo sólido, uno de tipo trabecular y dos de tipo membranoso, tratados mediante parotidectomía superficial conservadora en todos los casos. Esta división en distintos patrones morfológicos tiene una finalidad descriptiva, salvo en el subtipo membranoso por su mayor tendencia a la multifocalidad y a la recidiva, su ocasional transformación maligna, así como por su posible asociación en un tercio de los casos a tumores ecrinos dermales. Esto implica un seguimiento más estrecho y un despistaje de posibles lesiones cutáneas asociadas. Palabras clave: Basales; Membranoso; Multifocalidad; Recidiva; Ecrinos.

Recibido: 09-05-2004

Aceptado: 21-03-2005

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1 Médico Residente.
2 Médico Adjunto.
3 Jefe de Servicio
Servicio de Cirugía Oral y Maxilofacial
Hospital Son Dureta, Palma de Mallorca, España.

Correspondencia:
María Jose Pastor Fortea
C/ Illes Balears, 105 K
07015, Palma de Mallorca, España.
email: mjpastor@mundofree.com

 

Introduction

The basal cell adenoma (BCA) is a benign neoplasm with a uniform, monomorphous histologic appearance that is dominated by basaloid cells but that does not have the myxochondroid component of mixed tumors. They account for 2% of all primary salivary gland tumors and they are normally located in the parotid gland and in the minor salivary glands of the upper lip.¹

There has been much controversy about the terminology and the spectrum of tumors that belong to this group following their description by Kleinsasser and Klein in 1967.² In the monograph of the different histological types of salivary gland tumors by the World Health Organization (WHO) in 1972³ there were two categories of adenoma: Pleomorphic and monomorphic. Within monomorphic adenomas there were Warthin's tumors, Oncocytoma and Other Types, with no clear definition of the latter, meaning that various pathologists used the term monomorphic as a diagnostic entity. In a more recent revision of the histologic classifications of salivary gland tumors published by the WHO at the end of 1991, the term monomorphic is not used as a diagnostic category and variants are included of the specific types of adenoma, separated by their different morphologic features, and which included basal cell adenoma.^2,4

Clinical cases

We present four cases diagnosed as basal cell adenoma during 2000-2003, in a series of 92 parotidectomies performed at the University Hospital of Son Dureta during this time period.

The patients consisted of 2 men and 2 women between the ages of 17and 72 with a mean age of 43.7. The clinical was in all cases of a tumor-like mass of the parotid measuring between 1 and 3 cm in diameter that was not painful and which was hard and movable (Fig. 1).

Fine needle aspiration (FNA) puncture was systematically carried out and the results were suggestive of pleomorphic adenoma in two cases and monomorphic adenoma in the other two cases.

The radiologic tests using CT (computed tomography) scans and/or NMR (Nuclear Magnetic Resonance) showed in all cases a single tumorlike mass, that was welldefined, variable in size, with a homogenous consistency, except in one case in which images compatible with postpuncture bleeding could be seen (Fig. 2).

A conservative superficial parotidectomy was carried out in all cases, using a modified Appiani⁵ incision, with harvesting of a SMAS flap (Fig. 1.)

For the conventional histologic examination formaldehyde fixation was carried out at 4% with paraffin embedding and hematoxylin-eosin staining.

Two basal cell monomorphic adenomas were diagnosed of the membranous subtype. One was of the solid subtype and the other trabecular. Within the membranous type one was multinodular, and the remainder were uninodular (Figs. 3-7).

All the patients were discharged two days after surgery. One patient, who experienced a complication during the immediate post-operative period, developed a local hematoma that was drained and the evolution was favorable.

There were no cases of facial paralysis. One of the cases developed slight paresis of the marginal branch of the facial nerve that disappeared three months later.

To date there have been no cases of local relapse or of malignant transformation.

In the membranous basal cell adenoma cases, screening was carried out of possible associated cutaneous lesions, that proved negative.

Discussion

Basal cell adenomas account for approximately 2% of all primary tumors of the salivary glands, according to the AFIP register of salivary gland tumors.¹ On the basis of their morphologic pattern they can be divided into four subtypes: solid, trabecular, tubular and membranous.⁴

The mean age of the patients is 58, with a small peak in incidence in the 70s decade. It is more predominant in females with a 2:1 ratio, although the membranous subtype has a similar distribution in both sexes.

The basal cell adenoma is most commonly located in the parotid gland, in 74% of cases approximately, followed by the salivary glands of the upper lip, with this intraoral location being the most common in 7.5%. The remainder is distributed amongst the accessory salivary glands of the buccal mucosa, the submaxillary gland, the salivary glands of the palate, and other oral locations and the neck.

There is a remarkable histological similitude between eccrine dermal tumors (cylindromas) and basal cell adenomas, and there is certain diathesis with these tumors and basal cell adenoma of the parotid gland. Many of the adenomas associated with cutaneous tumors are dermal cylindromas. As a result of this and the histologic resemblance between both tumor-like masses, Batsakis and Brannon⁶ called parotid tumors, dermal analogue tumors, although Headington and cols.,⁷ who were the first to describe the association between both types of tumors, coined the term membranous-type basal cell adenoma.

The most common clinical finding in these adenomas is of a tumor-like mass that tends to be uninodular and well-defined, except in the membranous subtype where it can be multifocal. It has a firm consistency, although on occasions it can be a cystic tumor, softer on compression and moving to palpation except when located in the soft palate. With regard to their macroscopic features, with the exception of some membranous type tumors that are multinodular or multifocal, these are solitary tumors that have well-defined borders. In most cases they measure less that 3 cm in diameter on excision, they can range between 1.2 and 8 cm. The most common location within the parotid gland is in the superficial portion of the lateral lobe, and they usually have a well-defined capsule. Intraoral monomorphic adenomas, although being welldefined tumors, do not tend to be encapsulated.

The surface color is usually uniform and it tends to have a homogenous texture. Many years ago, following the initial description of these tumors by Kleinsasser and Klein,² the term monomorphic was interpreted as isomorphic with the absence of myoepithelial differentiation. More recent studies, including electron microscopy and immunohistochemistry have shown that basal, ductal or myoepithelial differentiation is produced to different degrees in basal cell adenomas, as occurs in other types of salivary gland tumors. The monomorphic character of these tumors is the result of a growth pattern that is to a greater or lesser degree uniform, and that does not have the myxochodroid tissue that is characteristic of mixed tumors. In contrast with other types of adenoma also called monomorphic, the basal cell adenoma is dominated by basaloid cells. The term basaloid should be used for optic microscopic terms as it avoids the necessity of determining whether the cells are basal, myoepithelial or ductal, as this is on occasions difficult with these tumors without further study with electron microscopy or immunohistochemistry.

As we have mentioned previously, basal cell adenomas can be divided into four subtypes based on their histological morphology: solid, trabecular, tubular and membranous. Despite these different morphologic patterns, they have basic histologic features that help in their diagnosis.

With regard to the different morphologic subtypes, a tumor can have one or more of these growth patterns, but most basal cell adenomas can be classified into one of these categories based on the predominating pattern. With the exception of the membranous type, the identification of these subtypes is for merely descriptive purposes and it does not imply different biological behavior.

The most common subtype is the solid variant, which is made up of neoplastic epithelial cells set out in islands or strands with a dominant lobular or rounded pattern, and it has a layer of hyperchromatic peripheral cells set out in a palisade pattern. The stroma that separates these additional epithelial cells is made up of dense connective tissue.

A net of tight interwoven bands of basaloid cells characterizes the trabecular subtype. The stroma in these tumors is formed by loose connective tissue.

The tubular subtype is less common and it is characterized by ductal differentiation. The tubular lumina appear to have a border of cuboidal ductal cells, and at the periphery several layers of basaloid cells can be observed. The epithelial aggregates are closely packed, and they have very little stroma around them.

Lastly, the membranous subtype is similar to the solid variant of basal cell adenoma except that it is frequently multinodular and it is encapsulated in approximately half the cases. Its distinctive feature is a thick eosinophilic hyaline layer that forms a wide rim around the edge of the islands of basaloid cells. This hyaline material, which is PAS-positive, corresponds as demonstrated by electron microscopy, to a duplicated basal membrane. In addition, small accumulations of hyaline coalescent material are often present in the intracellular space of tumor aggregates. The stroma is usually composed of dense connective tissue.

In addition to the common clinical features of basal cell adenomas, such as their most common location being in the parotid and their being movable asymptomatic masses, this tumor-like subtype has two clinical features of its own. One is its association with the dermal cylindroma, the trichoepithelioma or the eccrine spiradenoma of the scalp. Approximately a third of the cases published describe synchronous dermal cylindromas. These are also located in the scalp, nipples, back, thighs, legs and scrotum. They can be located in any part except for the palms, the soles and the axillae. There are commonly many tumors - more than ten in number. In patients with membranous basal cell adenoma of the parotid the screening of any skin lesions in the scalp or in any other location is therefore advisable. Similarly, the parotid gland should be checked in those patients that present with dermal cylindroma of the scalp. Other clinical features of the membranous basal cell adenoma is its tendency to be hereditary. This tendency tends to more evident in patients with multiple tumors than those that present with a solitary form. Therefore, in patients with multiple synchronous tumors a family investigation is advisable. ⁸

The principal entities that should be distinguished from the basal cell adenoma are the mixed tumor, the adenoid cystic carcinoma and the basal cell adenocarcinoma. The most common location for the canalicular adenoma, which is different to the basal cell adenoma, is the upper lip and it should be differentiated histologically from the trabecular subtype in particular.

The chondromyxoide tissue of the mixed tumor distinguishes it from the basal cell adenoma, although some mixed tumors are very cellular and they possess minimal quantities of myxochondroid tissue. Although the use of immunohistochemistry for distinguishing one type of salivary gland from another is full of errors, positive immunoreactivity for the glial fibrillary acid protein (GFAP) speaks in favor of the mixed tumor.

With regard to the adenoid cystic carcinoma, the cytomorphologically common cribiform pattern in this type of tumor is rare in the basal cell adenoma, and when it does appear it is accompanied by solid growth patterns or by typical trabeculae. Infiltration and perineural invasion helps to distinguish the adenoid cystic carcinoma from the basal cell adenoma.

The basal cell adenocarcinoma is a malignant variant of the basal cell adenoma and it can be distinguished from the basal cell adenoma on the basis of its characteristic growth and its more aggressive behavior. These growth characteristics include invasion into the parotid gland parenchyma and the adjacent tissues such as fat, muscle and bone as well as vascular and perineural invasion. A mitosis count that is 3-10 higher per field is highly suggestive of malignancy although a smaller number does not guarantee that the tumor is benign. The prognosis is good.

With the exception of the membranous type, the recurrence rate is very low, and even inexistent when conservative treatment is given adequately. This signifies a superficial parotidectomy, a submaxillectomy, an excision of the sublingual gland or an excision with adequate margins for intraoral locations.

Unlike other basal cell adenoma subtypes, the membranous subtype has a recurrence rate of approximately 25%,⁶ probably as a result of it being more commonly multifocal and not encapsulated. For this reason some authors advise a total parotidectomy⁷ with this variant, due to the rate of recurrence being attributed to incomplete resection as a result of it being multifocal. Nevertheless the study method of these patients in most cases does not allow knowing the histological subtype before the surgery. For this reason we only carry out a total parotidectomy in those cases in which multiple nodes have been observed radiologically.

The malignant transformation of these tumors has been described, and Batsakis⁶ in fact refers to a rate of 28% of membranous adenomas and 4% for other types of basal cell adenomas. Due to these factors a close follow-up is advised of membranous basal cell adenomas.

Establishing within basal cell adenocarcinomas which ones arise from a preexisting benign lesion and which ones are new is extremely difficult to establish.

Conclusion

The basal cell adenoma is defined as a diagnostic category within the WHO's monograph of salivary glands published at the end of 1991. It is a benign epithelial neoplasm that histologically has a uniform and monomorphous appearance. It is predominated by basaloid cells and it does not have the myxochondroid component of the mixed tumor.

It can be divided into four subtypes for merely descriptive purposes based on its morphology: solid, tubular, trabecular and membranous. The exception is the membranous subtype as a result of its greater tendency to be multifocal and recur, and of its occasional malignant transformation as well as its association in approximately a third of the cases with dermal eccrine tumors. It requires a close follow-up, cutaneous lesions should be screened, and a total parotidectomy may even have to be performed in the event of it being multifocal.

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