Definition

 Pleomorphic adenoma is a tumour of variable capsulation characterized microscopically by architectural rather than cellular pleomorphism. Epithelial and modified myoepithelial elements intermingle most commonly with tissue of mucoid, myxoid or chondroid appearance.

Synonym Mixed tumour

ICD-O code 8940/0

 Epidemiology

 Pleomorphic adenoma is the most common salivary gland tumour and accounts for about 60% of all salivary neoplasms {2439}. The reported annual incidence is 2.4-3.05 per 100,000 population {244, 2053}. The mean age at presentation is 46 years but the age ranges from the first to the tenth decades {703}. There is a slight female predominance {703,2711}.

Localization

About 80% of pleomorphic adenomas arise in the parotid, 10% in the submandibular gland and 10% in the minor salivary glands of the oral cavity, nasal cavity and paranasal sinuses and the upper respiratory and alimentary tracts {703}. The lower pole of the parotid gland is the most common location but deep lobe tumours can present as a parapharyngeal mass. The accessory parotid is occasionally involved.

Clinical features

Pleomorphic adenomas usually are slow growing painless masses. Small tumours typically form smooth, mobile, firm lumps but larger tumours tend to become bossellated隆起 and may attenuate the overlying skin or mucosa. Multifocal, recurrent tumours may form a fixed mass. Pleomorphic adenomas are usually solitary but they may show synchronous or metachronous association with other tumours, particularly Warthin tumour, in the same or other glands {2298}. Pain or facial palsy are uncommon but are occasionally seen, usually in relation to infarcted tumours. The size of most tumours varies from about 2-5 cm but some reported cases have been massive {388}. In the palate, tumours are usually seen at the junction of the hard and soft palate unilaterally. In the hard palate they feel fixed due to the proximity of the underlying mucoperiosteum.

 Macroscopy

 Pleomorphic adenomas tend to form well-defined, ovoid or round tumours. They are often encapsulated but the capsule varies in thickness and may be partially or completely absent, particularly in predominantly mucoid tumours. Those developing in the minor glands usually have a poorly developed or absent capsule. In major gland pleomorphic adenomas there is a distinct tendency for the tumour to separate from the capsule when handling the specimen. The outer surface of larger tumours is frequently bossellated. The cut surface is typically homogeneous and white or tan. It may have a glistening appearance where there are cartilaginous or myxochondroid areas. There may be areas of haemorrhage or necrosis. Recurrent tumours are usually multifocal and may be widely dispersed.

Histopathology

Pleomorphic adenoma shows a remarkable degree of morphological diversity. The essential components are the capsule, epithelial and myoepithelial cells, and mesenchymal or stromal elements. The capsule varies in thickness and presence. A quantitative study showed the thickness ranged from 15-1750 mm {2732}. When tumours were serially sectioned areas of capsular deficiency were seen in all cases {1418}. In predominantly mucoid pleomorphic adenomas, the capsule may be virtually absent and the tumour abuts onto the adjacent salivary gland. Most tumours show areas where finger-like processes extend into the capsule. In addition, the tumour sometimes bulges through the capsule and forms what appear to be separate satellite nodules. These satellites are invariably attached to the main tumour by an isthmus {1418,1986}. There is a tendency for clefts to form close to and parallel with the capsule. These clefts are within the tumour itself and leave tumour cells attached to the capsular wall. The epithelial component shows a wide variety of cell types including cuboidal, basaloid, squamous, spindle cell, plasmacytoid and clear cells. Rarely, mucous, sebaceous and serous acinar cells are seen. These cells are cytologically bland and typically have vacuolated nuclei, without prominent nucleoli, and a low mitotic frequency. The epithelium usually forms sheets or duct-like structures. There is a wide range of epithelial cellularity; sometimes, the epithelial component forms the bulk of the tumour (cellular pleomorphic adenoma). This phenomenon has no prognostic significance. The ducts show cuboidal luminal cells and there may be an abluminal layer of myoepithelial cells. These may be morphologically similar to the luminal cells or have clear cytoplasm and hyperchromatic and somewhat angulated nuclei. In limited material tumours showing these features could easily be confused with adenoid cystic carcinoma and epithelial-myoepithelial carcinoma. The ducts often contain eosinophilic secretory material and are usually small but may be distended to form microcysts. Squamous metaplasia, sometimes with the formation of keratin pearls, can be seen in both ducts and sheets and occasionally there is mucous metaplasia or conspicuous clear cell change. These appearances can be confused with mucoepidermoid carcinoma. More rarely, sebaceous cells or serous cells with zymogen granules are seen. Another rare feature is the presence of multinucleated epithelial cells. Myoepithelial cells may form a fine reticular pattern or sheets of spindle-shaped cells. These may be palisaded forming a Schwannoma–like appearance. A very distinctive appearance is seen when the myoepithelial cells are plasmacytoid or hyaline {1552}. Focal oncocytic change is not uncommon but occasionally the entire tumour is affected and may be misdiagnosed as an oncocytoma {1973}. Crystalloid material in the form of collagenous crystalloids, tyrosine and oxalate crystals are occasionally present {324}. The mesenchymal-like component is mucoid/myxoid, cartilaginous or hyalinised and sometimes this tissue forms the bulk of the tumour. Cells within the mucoid material are myoepithelial in origin and their cellular periphery tends to blend into the surrounding stroma. The cartilage-like material appears to be true cartilage and is positive for type II collagen and keratan sulphate. Occasionally it is the major component of the tumour. Bone may form within this cartilage or form directly by osseous metaplasia of the stroma. Deposition of homogeneous, eosinophilic, hyaline material between tumour cells and within the stroma can be a striking feature of some tumours. It forms globular masses or sheets and typically is positive with stains for elastin.  This material can push apart epithelial  elements to give a cylindromatous or cribriform  appearance that is readily mistaken  for adenoid cystic carcinoma. Some longstanding  tumours show increasing hyalinisation  and the epithelial component is  progressively effaced. It is important,  however, to scrutinise the residual epithelial  elements of such old, scarred pleomorphic  adenomas as there is a significant  risk of malignant progression in such  tumours {85}. Tumours that have a lipomatous  stromal component of 90% or  more have been called lipomatous pleomorphic  adenomas {1881, 2299}.  More extensive inflammation and necrosis  can be seen following spontaneous  infarction or fine needle aspiration. In  such tumours there may be an increase  in mitotic figures and some cellular atypia  {361,1495}. In addition, squamous  metaplasia may be present and these  changes can be mistaken for malignancy.  Some tumours show cystic degeneration  with the neoplastic elements forming  a rim around a central cavity.  Occasionally tumour cells can be seen  within vascular spaces {475}. These are  usually within the body of the tumour or  at the periphery and this is assumed to  be a peroperative phenomenon.  Sometimes this is seen in vessels distant  from the main tumour mass. However,  this finding does not appear to have any  significance in terms of tumour behaviour  and, in particular, the risk of metastasis.

 Immunoprofile

 The inner ductal cells in the tubulo-glandular  structures are positive for cytokeratin  3, 6, 10, 11, 13, and 16, whereas the  neoplastic myoepithelial cells are irregularly  positive for cytokeratin 13, 16, and  14 {311}. The neoplastic myoepithelial  cells co-express vimentin and pan-cytokeratin  and are variably positive for S-100  protein, a-smooth muscle actin, GFAP,  calponin, CD10 and muscle-specific  actin (HHF-35) {545}. Modified myoepithelial  cells in these tumours are also  reactive for p63 {214}. The non-lacunar  cells in the chondroid areas are positive  for both vimentin and pan- cytokeratin,  whereas the lacunar cells are positive  only for vimentin {1776}.  The spindle-shaped neoplastic myoepithelial  cells around the chondroid areas  express bone morphogenetic protein  (BMP) {1083} whereas the inner ductal  cells in the tubulo-glandular structures  and the lacuna cell in the chondroid  areas express BMP-6 {1397}. Type II collagen  and chondromodulin-I is present in  the chondroid matrix {1396}. Aggrecan is  present not only in the chondroid matrix  but also in the myxoid stroma and in the  inter-cellular spaces of the tubulo-glandular  structures {2898}. 

Genetics

 Cytogenetics  Extensive cytogenetic studies of pleomorphic  adenomas have shown that  approximately 70% of the tumours are karyotypically abnormal {306,1639, 2239}. Four major cytogenetic subgroups may be discerned: > Tumours with rearrangements involving 8q12 (39%) > Tumours with rearrangements of 12q13-15 (8%) > Tumours with sporadic, clonal changes not involving 8q12 or 12q13-15 (23%) > Tumours with an apparently normal karyotype (30%). Whereas t(3;8)(p21;q12) and t(5;8)(p13;q12) are the most frequently observed translocations in the first subgroup, a t(9;12)(p24;q14-15) or an ins(9;12)(p24;q12q15) are the most frequent rearrangements seen in the second subgroup. In addition, many variant translocations have been identified in which a number of other chromosome segments are found as translocation partners of both 8q12 and 12q13-15. Secondary chromosome changes, including trisomies, dicentrics, rings and double minutes, are found in about onethird of the cases with abnormal karyotypes. Previous studies have also indicated that patients with karyotypically normal adenomas are significantly older than those with rearrangements of 8q12 (51.1 years versus 39.3 years, p < 0.001) and that adenomas with normal karyotypes are often more stroma rich than tumours with 8q12 abnormalities {306}. Molecular genetics The target gene in pleomorphic adenomas with 8q12 abnormalities is PLAG1, a developmentally regulated zinc finger gene {82,1279,2701}. Translocations involving 8q12 commonly result in promoter swapping/substitution between PLAG1 and a ubiquitously expressed translocation partner gene, leading to activation of PLAG1 expression. The breakpoints invariably occur in the 5´- noncoding regions of both the target gene and the promoter donor genes. The most commonly observed fusions are CTNNB1-PLAG1 and LIFR-PLAG1, resulting from t(3;8)(p21;q12) and t(5;8)(p13;q12) translocations, respectively {1279,2701}. Recently, cryptic gene fusions involving CTNNB1-PLAG1 and SII-PLAG1 were also found in karyotypically normal adenomas {82}. The PLAG1 protein is a nuclear oncoprotein that functions as a DNA-binding transcription factor. Deregulation of PLAG1 target genes, including IGF2, is likely to play a major role in the genesis of pleomorphic adenomas {2700}. The target gene in adenomas with rearrangements of 12q14-15 is the high mobility group protein gene, HMGA2 (a.k.a. HMGIC) {878,879,2269}. HMGA2 encodes an architectural transcription factor that promotes activation of gene expression by modulating the conformation of DNA. The protein contains three DNA-binding domains that bind to the minor groove of AT-rich DNA. The majority of breakpoints in HMGA2 occur within the third large intron, resulting in separation of the DNA-binding domains from the highly acidic, carboxy-terminal domain. Two fusion genes, HMGA2-NFIB and HMGA2-FHIT, have been identified in adenomas with ins(9;12) and t(3;12), respectively {878,879}. Since no common functional domain has been found among the translocation partners, the critical event seems to be the separation of the DNA-binding domains from potential mRNA destabilizing motifs in the 3´- UTR, leading to deregulation of HMGA2 oncoprotein expression. High-level expression of HMGA2 resulting from gene amplification was recently suggested to be of importance for malignant transformation of pleomorphic adenomas {2194}. The five PLAG1- and HMGA2-containing fusion genes so far identified are all tumour specific and may therefore be used as diagnostic markers for pleomorphic adenomas. The fusions may be detected either by RT-PCR or by interphase fluorescence in-situ hybridization {878,879,1279,2701}. Molecular studies of the RAS and ERBB2 oncogenes have shown that mutation and activation of RAS frequently occur in pleomorphic adenomas, particularly in tumours with PLAG1 activation {1727, 2198,2464,2465}, whereas amplification and/or overexpression of ERBB2 seem to be rare {2198,2465}. Similarly, TP53 alterations are infrequent in adenomas {1907, 2198,2734}. In contrast, mutation and overexpression of TP53 are found in a relatively high proportion of carcinoma ex pleomorphic adenomas {1491,1907, 2169}. In addition, recent studies have shown that the TP53-related genes TP63 and TP73, which are novel myoepithelial markers, are overexpressed in basal and myoepithelial cells in pleomorphic adenomas {214,2734}. The pathogenetic relevance of the latter observations is uncertain. Studies using the human androgen receptor gene assay have demonstrated that the stromal and epithelial cells in pleomorphic adenomas are clonal and derived from the same progenitor cell {1455}. Finally, it was recently demonstrated that pleomorphic adenomas contain Simian virus 40 (SV40) DNA sequences and express the SV40 large T antigen, suggesting that this oncogenic virus may be involved in the genesis and/or progression of this tumour {1643}.

Prognosis and predictive factors

Although pleomorphic adenoma is a benign tumour it can cause problems in clinical management due to its tendency to recur and the risk of malignant transformation. Recurrences are rare in the minor glands but in a meta-analysis of parotid tumours 3.4% of tumours recurred after 5 years and 6.8% after 10 years with a range of 1-50% {1083}. The variation of frequency of recurrence in this survey probably reflected the inclusion of cases reported before superficial parotidectomy became a widely used treatment and the variability of long-term follow-up. Some single centre, long-term surveys however, have shown recurrence rates as low as 1.6% {2169}. Recurrences appear to be much more likely in younger patients {1436,1681}. The possible reasons for recurrences or persistence in pleomorphic adenoma include:

- The diffluent nature of predominantly mucoid tumours {2157}.

- The variability of the thickness of the capsule, together with the tendency of the tumour to invade the capsule {1065}.

- Tumour nodules bulging through the capsule.

 - Intratumoural splitting beneath the capsule.

- It is probable that the tumour cells have low biological requirements and this enables them to survive when spilt into the operative site. Many recurrent pleomorphic adenomas are multifocal and some are so widely distributed that surgical control becomes impossible.