Non-cellular morphologic markers in pleomorphic adenoma: A rare observation.

A 43-year-old male presented with complaint of swelling in the left parotid region for the past 7–8 years. The swelling was painless, insidious in onset, and progressively increased in size. Local examination revealed a single, well-defined, non-tender, and firm swelling measuring 3.5 × 3 cm in the left preauricular region. Fine-needle aspiration cytology (FNAC) stained smears show the following findings [Figure 1a-d].

Figure 1:

(a) Abundant fibrillary chondromyxoid material, numerous basophilic refractile structures (Giemsa, ×400), (b) Myoepithelial cell admixed with clusters of orangeophilic, refractile structures (Papanicolaou stain, ×1000), (c) Radially arranged, glossy, petal-like structures (Thin red Arrow) with blunt ends of refractile structures. (Papanicolaou stain, ×1000), (d) Few epithelial cells have intranuclear cytoplasmic inclusion (Thick Red Arrow) (Papanicolaou stain, ×1000).

Q1. What is your interpretation?

Tyrosine-rich crystalloid

Collagenous crystalloid

Amylase crystalloid

Calcium oxalate crystalloid

Answer: Q1-A. Tyrosine-rich crystalloid

Tyrosine-rich crystalloids (TRCs) are round to oval, refractile, floret-shaped orangeophilic (Papanicolaou stain) structures with symmetrical lobulated contour, central rounded core, and peripheral “rosette like” arrangement.[ TRCs appear reddish-brown with Millon’s stain and turn deep purplishred with the diazotisation coupling method, confirming the presence of tyrosine.[ Other histochemical reactions such as tryptophan, arginine, and sulfhydryl groups are also positive in TRC.[ Collagenous crystalloids (CCs) are non-refractile, eosinophilic, rounded structures composed of radially arranged needle-shaped fibers with central clear space, occupied by a fibroblast or a small blood vessel.[ Histochemically, CC stains bright blue with trichrome, black with methenamine silver, and gray to black on reticulin staining; while it is negative for Millon’s stain or diazotization coupling reaction.[ Alpha-amylase crystalloids (ACs) are non-birefringent having various shape such as rod like, rhomboid, cuboidal, square, polyhedral, or needle like, appear bright orange with Papanicolaou stains, deep blue by Giemsa stains, and pink by hematoxylin-eosin stains.[ Millon’s reagent or diazotization coupling reactions are negative.[ Calcium oxalate crystals (COCs) are birefringent, irregular or polygonal, transparent to pinkish structure.[ Millon’s reagent or diazotization coupling reactions are negative.[

Q2. What is your most probable diagnosis?

Mucoepidermoid carcinoma

Basal cell adenoma

Warthin tumor

Pleomorphic adenoma (PA)

Answer: Q2-D

PA is characterized by a variable admixture of ductal epithelial cells, myoepithelial cells, and mesenchymal matrix. The mesenchymal matrix may be either chondromyxoid matrix as in most of the cases or less commonly reveal amyloid, collagenous substance, and/or elastic fibers.[ Rare cytomorphological variations include presence of crystalloids such as TRC, CC, calcium oxalate crystalloid, and amylase crystalloids along with the presence of nuclear changes in epithelial component such as inclusions/grooves are also identified in PA.[

Q3. Which of the following statement is incorrect?

TRCs are refractile structures seen in both neoplastic and non-neoplastic lesions of parotid glands

Histochemical reactions (Millon’s stain and diazotization coupling reaction) help to confirm the presence of tyrosine.

Intranuclear cytoplasmic inclusions are the most commonly observed features in pleomorphic adenoma.

All of the above

Answer: Q3-C

TRCs are found in both neoplastic and non-neoplastic salivary gland lesions such as PA, myoepithelioma, carcinoma ex pleomorphic adenoma, adenoid cystic carcinoma, polymorphous low-grade adenocarcinoma, terminal duct adenocarcinoma, and parotid cysts.[ Intranuclear cytoplasmic inclusions in the epithelial component of PA are rare findings described in a few case reports.[

BRIEF REVIEW OF THE TOPIC

In 1953, Bullock first described the “innumerable crystalline aggregate” present in histologic section of PA as tyrosine crystals because its structure resembles that of tyrosine.[ Nochomovitz and Kahn (1974) described the physical and histochemical properties as refractile, rosette-like structure exhibiting a positive reaction with Millon’s reagent.[ On electron microscopy, TRCs are non-birefringent, electron dense, granular without internal structure thus designated as crystalloid.[ Based on the histochemical reaction and ultrastructural findings, the correct nomenclature is tyrosine-rich crystalloids rather than tyrosine crystal.[ The incidence of TRC in PA varies from 1.8 to 5%; however, Lemos et al. reported an incidence of up to 33% in African patients.[ The pathogenesis of TRC remains ambiguous. Thomas et al. proposed that as tyrosine plays an important role in pigment metabolism, therefore, there is high prevalence of TRC in parotid tumors in Black races.[ This hypothesis was refuted as tyrosine crystals do not get deposited in the tissues of patients suffering from tyrosinosis.[ Many authors hypothesized that TRC deposition is either a manifestation of disturbed protein synthesis or by precipitation onto stromal collagen secreted by myoepithelial cells.[ Morphology and histochemical stains may help to differentiate TRC from other crystalloids. Nuclear changes such as nuclear inclusion/grooves are rare cytomorphological variations seen in PA.[ In the present case, we emphasize on the presence of two rare cytomorphological features seen in PA.

SUMMARY

Since TRCs are the most common crystalloids found in PA, some authors suggest that TRC can serve as a non-cellular morphologic marker of salivary gland neoplasia which might be helpful in paucicellular smears. This case report also aims to emphasize the importance of this novel observation to the novice budding cytopathologists.