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What is Chronic sclerosing sialadenitis and explain in detail?
Chronic sclerosing sialadenitis is a chronic (long-lasting) inflammatory condition affecting the salivary gland. Relatively rare in occurrence, this condition is benign, but presents as hard, indurated and enlarged masses that are clinically indistinguishable from salivary gland neoplasms or tumors. It is now regarded as a manifestation of IgG4-related disease.Involvement of the submandibular glands is also known as Küttners tumor, named after Hermann Küttner (1870–1932), a German Oral and Maxillofacial Surgeon, who reported four cases of submandibular gland lesions for the first time in 1896.

Presentation
The inflammatory lesions in Küttners tumor may occur on one side (unilateral) or both sides (bilateral), predominantly involving the submandibular gland, but is also known to occur in other major and minor salivary glands, including the parotid gland.
Overall, salivary gland tumors are relatively rare, with approximately 2.5–3 cases per 100,000 people per year seen in the Western world; however, salivary gland malignancies account for 3–5% of all head and neck cancers. However, salivary tumors show a great deal of morphological diversity, as well as variations in the nature of the lesion (malignant vs. benign):  approximately 20% to 25% of parotid tumors, 35% to 40% of submandibular tumors, and more than 90% of sublingual gland tumors are malignant. This situation underscores the diagnostic challenges in respect of Küttners tumor; despite being benign, this condition mimics the clinical appearance of malignancy in the salivary gland.
The swollen masses of Küttners tumor are generally painful, and patients are advised surgical resection (known as sialadenectomy) of a part or whole of the glandular tissue upon suspicion of possible malignancy. It is only upon post-surgical histopathology of the excised mass that the diagnosis of Küttners tumor is definitively made.

Histological features
The histopathological features that characterize Küttners tumor include:

Heavy infiltration of the glandular tissue by lymphocytes (predominantly activated B-cells and helper T-cells) as well as plasma cells (collectively known as Lymphoplasmacytic Infiltrate).
Presence of reactive lymphoid follicles in the infiltrate, marked by a lack of atypical lymphoid cells (this is in sharp contrast to the presentations in lymphoma).
Atrophy and loss of acini (groups of secretory cells found in the salivary glands).
Encasement of the glandular ducts in thick fibrous tissues, as a result of chronic presence of inflammatory infiltrate in that area - a condition known as periductal fibrosis.
Eventual periductal and interlobular (inside the gland) sclerosis (replacement of regular tissue with hard connective tissue).

Pathogenesis
The cause and pathogenesis of this chronic condition are not very well understood. Several factors have been postulated:

Formation of a hard salivary calculus or sialolith by accumulation of calcium salts in the duct of the salivary gland (a process known as Sialolithiasis)