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). This has been proposed as the most common cause for Küttners tumor of the submandibular gland, with sialoliths observed in an appreciable proportion of cases. However, sialolith involvement may not be found in many cases.
Abnormalities of the salivary gland ducts leading to excessive accumulation or retention of ductal secretions, which can excite chronic inflammations. Immune, especially autoimmune, cause - which has gained steam, given the observation that the tissue of the glands is overrun with lymphoid immune cells  and fibrous connective tissue, as well as corroboration from markedly similar lesions (with histologic and immunohistochemical findings) seen elsewhere in the body. The presence of abundant Immunoglobulin G4 (IgG4) associated with Plasma cells infiltrating into the salivary glands, as well as increased serum IgG4 concentration, has been noted with patients with Küttners tumor.This chronic condition is primarily observed in adult (40–70 years) patients. However, Küttners tumor, with prominent immunopathological features, has been described in an 11-year-old boy in Brazil in 2012. Diagnosis
Given the difficulties of a definitive pre-operative diagnosis, the clinical entity of Küttners tumor has so far remained significantly under-reported and under-recognized. In recent times, armed with a better understanding of the occurrences and observable features of this condition, surgeons are increasingly depending upon pre-operative ultrasonography along with Fine-needle aspiration cytological (FNAC) examinations to make an accurate presumptive diagnosis, and according to one estimate, about 44% of patients undergoing submandibular resection are found to have this condition. In the ultrasonogram, Küttners tumor is characterized by a diffuse, heterogeneous zone of echo-shadows. The FNAC finds cells greatly reduced in number (called paucicellularity) along with scattered tubular ducts against a backdrop of lymphoplasmacytic infiltration and fibrous depositions.