Polymorphous adenocarcinoma

Polymorphous adenocarcinoma (PAC) is a low-grade malignant neoplasm arising almost exclusively from minor salivary glands, with a strong predilection for the hard palate.[1][2][3] It is characterized by cytologic uniformity, architectural diversity, infiltrative growth, and neurotropism.[2][3] The tumor is considered the second or third most common malignancy of the minor salivary glands.[1][4] On imaging, PAC typically presents as a submucosal soft-tissue mass with variable bone involvement ranging from pressure remodeling to frank erosion; the radiographic appearance is nonspecific and overlaps with other minor salivary gland neoplasms.[5][6][7] Management centers on wide surgical excision, and the overall prognosis is favorable, with disease-specific survival exceeding 94% at 10 years.[8]

Current Classification

The 2017 WHO Classification of Head and Neck Tumours adopted the name polymorphous adenocarcinoma, removing "low-grade" from the prior designation to accommodate the recognition that a subset of tumors may exhibit higher-grade behavior.[1][9] Under this classification, cribriform adenocarcinoma of minor salivary glands (CAMSG) is included as a variant of PAC, although this grouping remains debated because of differences in nodal metastatic potential.[1][10]

Historical and Obsolete Names

• Polymorphous low-grade adenocarcinoma (PLGA) — the most widely used prior name, coined by Evans and Batsakis in 1984[2]
• Terminal duct carcinoma and lobular carcinoma — earlier designations reflecting putative histogenesis and morphologic resemblance to lobular carcinoma of the breast[4][11]
• Low-grade papillary adenocarcinoma — occasionally used before unification under the PLGA heading[11]

• Age: broad range (21–94 years); mean approximately 57–59 years[3][11]
• Sex: female predominance, with women comprising roughly two-thirds to over 70% of cases[3][11]
• Incidence: estimated annual incidence of 0.051 per 100,000 persons[9]
• Relative frequency: accounts for approximately 7% of all minor salivary gland tumors and roughly 20–41% of minor salivary gland carcinomas, making it the second or third most common malignancy at this site after mucoepidermoid carcinoma[4][9]
• Anatomic distribution:
  • Hard palate — most common site (approximately 60–73% of cases)[3][4][12]
  • Buccal mucosa — up to 20%[4]
  • Upper lip — approximately 12%[4]
  • Retromolar area, tongue base, and other intraoral sites — occasional[2][13]
  • Major salivary glands — rare; isolated parotid cases are reported, typically in the context of carcinoma ex pleomorphic adenoma[4]

Presentation

PAC characteristically presents as a painless, slow-growing, firm submucosal mass on the hard palate or other intraoral site.[3][14] The overlying mucosa is usually intact, though ulceration may develop with larger or long-standing lesions.[15] Tumor size at presentation averages approximately 2 cm (range 0.4–6 cm).[3] The clinical tempo is indolent; some tumors are present for years before diagnosis.[14][16]

Symptoms and Examination Findings

• Most patients are asymptomatic; the mass is often discovered incidentally during routine dental examination or prosthesis adjustment[16][17]
• Pain, when present, is uncommon and may suggest perineural involvement or secondary ulceration[18]
• Cranial nerve deficits are rare at presentation despite the tumor's histologic neurotropism[14]

Red Flags

• Rapid growth, pain, or cranial neuropathy should raise concern for high-grade transformation or an alternative diagnosis such as adenoid cystic carcinoma[14][19]
• Cervical lymphadenopathy at presentation is uncommon in classical PAC but is more frequent in the cribriform adenocarcinoma variant[1][10]

Because PAC arises predominantly in the soft tissues of the palate, many tumors produce no abnormality on conventional dental radiographs. Radiographic findings become apparent only when the tumor is large enough to involve the underlying palatal bone or, rarely, when it arises as a primary intraosseous lesion.[5][6]

Location and Distribution

• The posterior hard palate is the characteristic location, corresponding to the highest density of minor salivary glands[3][20]
• Lesions are typically unilateral and paramedian, reflecting the lateral distribution of palatal glands; midline origin is uncommon[20]
• Rare intraosseous examples have been reported in the maxilla and mandible[5][6]

Periphery and Shape

• When bone involvement is present, the margins are variable:
  • Smooth, well-corticated pressure remodeling ("cupping" or saucerization of the palatal bone) is seen with slow-growing tumors[5][15]
  • Irregular, poorly defined borders with cortical disruption suggest infiltrative bone invasion and are more characteristic of malignancy[6][21]
• In the rare intraosseous variant, the lesion may appear as a radiolucency with irregular scalloped borders; the only feature suggesting malignancy may be an irregular cortical margin[6]

Internal Architecture

• The soft-tissue mass itself is radiolucent (soft-tissue density) on conventional radiographs and does not produce a mineralized matrix
• Intratumoral calcifications are occasionally observed on cross-sectional imaging and, when present in a palatal minor salivary gland tumor, are significantly associated with malignancy[21]
• The original histopathologic description noted intratubular calcification as an occasional microscopic finding[2]

Effects on Teeth and Adjacent Structures

• Palatal tumors may displace or remodel the alveolar process; frank root resorption of adjacent teeth is uncommon but has been described[5][16]
• Tooth displacement may occur with larger lesions extending into the alveolar ridge[16]
• The nasal floor and maxillary sinus walls may be thinned or eroded by superiorly extending tumors[7]

Extent and Cortical Involvement

• Bone involvement ranges from superficial cortical erosion to full-thickness palatal bone destruction with extension into the nasal cavity or maxillary sinus[2][7][21]
• Deep submucosal extension is characteristic of minor salivary gland carcinomas generally and may be underestimated on conventional radiographs[22]
• Extension into the pterygopalatine fossa, when present, is a significant finding associated with malignancy and potential perineural spread along the greater palatine nerve and V2 division of the trigeminal nerve[21][23]
• CBCT provides superior detection of cortical thinning, cortical destruction, and expansion compared with panoramic radiography, particularly in the maxilla where superimposition limits panoramic assessment[24][25]

Growth Pattern

• The tumor grows by infiltration rather than expansion, consistent with its unencapsulated histologic nature[2][3]
• Bone marrow infiltration may occur without obvious cortical destruction, a pattern shared with other minor salivary gland carcinomas[22]

Recognized Variants

• Intraosseous PAC: exceedingly rare; presents as a radiolucent lesion within the jaw mimicking an odontogenic cyst or other intraosseous pathology, with intact overlying mucosa. The cyst-like radiographic appearance with an irregular cortical border may be the only clue to malignancy.[5][6]

Soft-Tissue Characterization

On contrast-enhanced CT, PAC appears as a well-defined to ill-defined enhancing submucosal mass.[7][20] MRI demonstrates intermediate T1 and intermediate-to-high T2 signal intensity; ill-defined margins, absence of a capsule, and irregular shape on MRI are features that help distinguish low-grade minor salivary gland malignancies from benign tumors such as pleomorphic adenoma.[26] Bone marrow infiltration without cortical destruction is better depicted on MRI than CT.[22][27]

Perineural Spread

Histologic neurotropism is common in PAC, but macroscopic (imaging-detectable) perineural spread along named nerves is far less frequent than in adenoid cystic carcinoma.[1][28] When suspected, MRI with fat-suppressed post-contrast sequences is the modality of choice, demonstrating nerve enlargement, abnormal enhancement, and foraminal widening along the greater palatine nerve, V2 division, or pterygopalatine fossa.[27][29][30] On bone-window CT, erosion or enlargement of the palatine canal, pterygopalatine fossa, or foramen rotundum may serve as an indirect sign of perineural spread.[31]

Staging Considerations

NCCN and ASCO guidelines recommend contrast-enhanced CT and/or MRI from skull base to clavicle for salivary gland malignancies, with chest CT as clinically indicated.[27][32] FDG-PET/CT may be considered for advanced or recurrent disease.[27]

The radiographic differential of a submucosal palatal mass with variable bone involvement centers on other minor salivary gland neoplasms and selected non-salivary entities. Imaging alone cannot reliably distinguish among these; biopsy is required.[21][23]

The highest-risk diagnostic pitfall is misclassification of PAC as adenoid cystic carcinoma (or vice versa), because the two tumors share palatal location, infiltrative growth, cribriform architecture, and neurotropism but differ markedly in prognosis and treatment intensity.[36][37] When histologic distinction is uncertain on small biopsy, molecular testing for PRKD1 E710D hotspot mutation (PAC) versus MYB-NFIB fusion (adenoid cystic carcinoma) can resolve the diagnosis.[33][38]

Diagnostic Microscopic Features

The hallmark of PAC is cytologic uniformity combined with architectural diversity:[2][3][4]

• Cytology: isomorphic cells with round-to-oval vesicular ("open") nuclei, inconspicuous nucleoli, and scant-to-moderate eosinophilic cytoplasm; mitotic figures are rare; nuclear grooves may be present[3][9][11]
• Architecture: multiple coexisting growth patterns within a single tumor, including tubular (most common), solid, cribriform, papillary, trabecular, fascicular, single-file ("Indian file"), and strand-like arrangements[2][3][9][39]
• Borders: unencapsulated and infiltrative, with peripheral single-file or small tubular growth[2][9]
• Neurotropism: targetoid (concentric) perineural growth around small nerves is characteristic and common[9][11]
• Stroma: variable fibromyxoid to hyalinized stroma; chondroid matrix (a feature of pleomorphic adenoma) is absent[11]
• Other findings: intratubular calcification, tyrosine-rich crystals, and mucoid material within microcysts are occasionally seen[2][9]

Immunohistochemistry

• Diffuse S-100 and EMA positivity; CK7 positive[40][41]
• p63 expression is variable and non-patterned (unlike the organized biphasic pattern in adenoid cystic carcinoma)[19]
• c-Kit (CD117) expression is weak or absent, in contrast to strong expression in adenoid cystic carcinoma[36][42]
• GFAP is negative, distinguishing PAC from pleomorphic adenoma[34]

Molecular Findings

• PRKD1 E710D hotspot mutation is pathognomonic for classical PAC and serves as a definitive diagnostic marker[9][19][38]
• The cribriform adenocarcinoma variant instead harbors PRKD gene family rearrangements (PRKD1–3 fusions, including ARID1A-PRKD1 and DDX3X-PRKD1), which may explain its more aggressive nodal behavior[9][38]

High-Grade Transformation

Rare cases undergo transformation to high-grade carcinoma, defined by loss of cytonuclear uniformity, necrosis, and elevated mitotic rate (>5 per 10 HPF).[11][19] High-grade transformation is associated with more aggressive clinical behavior and may occur de novo or after multiple recurrences.[19]

Workup and Referral

Incisional biopsy is the standard diagnostic approach for intraoral minor salivary gland masses.[1][14] NCCN guidelines recommend cross-sectional imaging (CT and/or MRI) from skull base to clavicle and chest CT as clinically indicated for salivary gland carcinomas.[27] Multidisciplinary consultation is advised.[27]

Definitive Treatment

• Wide surgical excision with clear margins is the primary treatment[1][3][12][14]
• Margin status is the most important modifiable prognostic factor; positive or unknown margins are significantly associated with local recurrence[12][39]
• Adjuvant radiotherapy is considered for positive margins, advanced-stage disease, or the cribriform adenocarcinoma variant, although definitive evidence of survival benefit is lacking[1][12][14]
• Elective neck dissection is not routinely performed for classical PAC given the low rate of nodal metastasis; however, it may be warranted for the cribriform variant, which has a substantially higher rate of cervical nodal involvement[1][10]

Recurrence and Outcomes

• Local recurrence occurs in approximately 13–19% of cases across large series, and recurrences may be delayed, occurring up to 15–24 years after initial surgery[12][17][43]
• Cervical lymph node metastasis occurs in roughly 9–14% of PAC overall but is substantially more frequent in the cribriform adenocarcinoma variant[10][16][39]
• Distant metastasis is rare in classical PAC[3][8]
• Disease-specific survival is excellent: 10-year rates of 94–99%[8][12]
• Recurrences generally respond well to salvage surgery[12][14]

Prognostic Factors

• Positive surgical margins — strongest predictor of local recurrence[12][39]
• Papillary growth pattern of more than focal extent — associated with increased risk of cervical nodal metastasis[39]
• Cribriform adenocarcinoma variant — higher nodal metastatic rate[1][10]
• High-grade transformation — rare but associated with aggressive behavior[11][19]

Long-term surveillance of at least 10 years is essential given the propensity for late recurrence.[12][14][43]