Year : 2009 | Volume
: 52 | Issue : 1 | Page : 103--105
Polymorphous low-grade adenocarcinoma of parotid gland: A rare occurrence
N Arathi1, Atul M Bage2,
1 Department of Pathology, SMV Medical College, Madagadipet, Pondicherry, India
2 Department of E.N.T., SMV Medical College, Madagadipet, Pondicherry, India
173, 12 B Main, 6th Block, Rajaji Nagar, Bangalore - 560 010
Polymorphous low-grade adenocarcinoma (PLGA) is a rare salivary gland malignant tumor of low aggressiveness, commonly occurring in minor salivary glands. Its occurrence in major salivary gland has been documented albeit rarely. The striking histological feature is architectural diversity combined with benign cytologic features. We report a case of PLGA arising from left parotid in a 25-year-old male patient. On light microscopy, varied patterns were seen .The cells were uniform with bland nuclei. Neural invasion was noted.
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Arathi N, Bage AM. Polymorphous low-grade adenocarcinoma of parotid gland: A rare occurrence.Indian J Pathol Microbiol 2009;52:103-105
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Arathi N, Bage AM. Polymorphous low-grade adenocarcinoma of parotid gland: A rare occurrence. Indian J Pathol Microbiol [serial online] 2009 [cited 2021 Jul 24 ];52:103-105
Available from: https://www.ijpmonline.org/text.asp?2009/52/1/103/44985
Polymorphous low-grade adenocarcinoma (PLGA) is a low-grade malignant tumor of salivary glands, most often arising from minor salivary glands with palate being a common site. Much less frequently cases have been described in major glands, usually but not always against a background of pleomorphic adenoma. Histologically various architectural patterns (like tubular, cribriform, papillary and solid) are seen with bland cytological features. Infiltration into the surrounding tissues is seen. Perineural invasion is noted in many cases.
We describe the clinicopathologic features of a case of PLGA arising from parotid gland.
A 25-year-old male patient presented with a painful left parotid gland swelling with a duration of 6 months. The nodule was retroauricular, firm to hard on palpation and measured 3 x 3 cm. A diagnosis of chronic parotitis was considered and superficial parotidectomy was done.
Per-operatively, the lesion was retroauricular, hard and compressed a branch of facial nerve.
Grossly, the tumor was well circumscribed but unencapsulated, lobulated and measured 3 x 3 x 1.5 cm; cut section was solid, pale white, homogeneous and firm. Adjacent normal looking salivary gland measured 4 x 3 x 0.5 cm.
Microscopically architectural variability was striking, with the following patterns - glandular, tubular, trabecular and solid [Figure 1a],[Figure 1b]. An occasional papillary area was seen. The glands and tubules were lined by a single layer of cells. The cells were round to ovoid to polyhedral, with the cytoplasm varying from eosinophilic to clear. Nuclear features were bland with vesicular nucleus, some showing inconspicuous nucleolus [Figure 2]. Hyperchromasia or mitoses were not seen. Lumen showed eosinophilic material at places, which was variably PAS positive. There were no areas of necrosis. Stroma showed extensive hyalinization. An area of neural invasion was noted. Periglandular adipose tissue showed invasion. There were no areas reminiscent of pleomorphic adenoma.
The term polymorphous low-grade adenocarcinoma (PLGA) was first introduced by Evans and Batsakis  to describe a specific category of salivary gland tumor amongst a host of heterogeneous salivary gland tumor labeled as adenocarcinoma "not otherwise specified." The 14 intraoral tumors in this group showed histologic diversity with cytologic uniformity. The behavior of these tumors was one of local aggressiveness with a potential for metastasis to regional lymph nodes.
In the second edition of WHO classification of salivary gland tumors published in 1991, it was classified under a separate category. PLGA commonly involves minor salivary glands; however, it has been described in major glands. , In the largest study of 164 cases from AFIP files, all were in minor salivary glands with palate being the commonest site.  Rare intraosseous cases have been described.  Clinically, wide age range has been described (23 to 94 years) with duration of symptoms ranging from few days to 40 years.  In the series of Castle et al. ,  the typical presentation was that of an asymptomatic mass lesion. A small number of patients (13 out of 164 cases) presented with mass accompanied by pain, bleeding or ulceration. However, these patients did not have more aggressive disease nor were they more prone to develop recurrences. Grossly, the tumor is usually unencapsulated, well circumscribed, lobular and firm.
Microscopically various architectural patterns could be seen in different areas (glandular, trabecular, tubular, cribriform, "Indian file" and solid). Luminal eosinophilic material seen in our case has not been commonly described. Myxoid change in the background can be seen. In contrast to architectural polymorphism, the nuclei are uniform and bland with absent or negligible mitoses. Castle et al .  detected neurotropism in majority of cases with a targetoid appearance due to concentrically arranged tumor cells around central nerve twig.
The differential diagnoses of PLGA include adenoid cystic carcinoma (ACC) and pleomorphic adenoma. Both ACC and PLGA can have similar architectural patterns; however, the cells in ACC tend to be smaller with hyperchromatic nuclei and coarser chromatin. Mitoses are numerous. It is important to differentiate these two entities, as the prognosis and treatment are significantly different. A study by Penner et al .  explores the role of c-kit in differentiating PLGA from adenoid cystic carcinoma (ACC). The study supports a role for c-kit IHC in the differential diagnosis of ACC and PLGA, but the authors opine that more number of cases needs to be evaluated to draw firm conclusions.
The infiltrative growth pattern is an important diagnostic clue favoring PLGA when the differential includes pleomorphic adenoma. Neurotropism, if present, also favors PLGA. Various immunohistochemical markers have been found to be positive like vimentin, cytokeratin, S-100, CEA, SMA and GFAP. ,,, Proliferative markers like Ki-67 and p53 show variability in intensity and percentage of cells positive but generally the intensity is weak. 
The study conducted by Castle et al .  on 164 cases of PLGA arising from minor salivary gland showed excellent long-term prognosis. Local recurrences can occur. Rarely regional lymph node metastasis and distant (lung) metastasis have been documented. , There is no large series study of PLGA cases arising from major gland. Nagao et al .  opine that PLGAs of minor and major salivary glands have similar clinicopathologic characteristics. Rare cases undergoing transformation to a higher grade have been described, with a possibility of radiation therapy playing a role in the transformation. 
In conclusion, this relatively new entity of salivary gland tumor though common in minor glands can arise from the major salivary gland. Variability in architectural pattern with bland cytological features is a diagnostic clue for PLGA, along with peripheral infiltration and perineural invasion.
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