However, long-term observational studies are desperately needed to guide the development of evidence-based surveillance algorithms designed to detect late tumor progression." Also (see Medical and Gene Therapies, above): "Basic science and identification of genes, molecular pathways, and networks related to tumor growth are likely to change our approach to treatment including conservative management." Incidence of Sporadic VS
The U.S. National Institutes of Health (NIH) consensus statement for Acoustic Neuroma in 1991 estimated that 2–3,000 cases of VS were diagnosed in the U.S. annually—an incidence rate of about 10 per million of population per year. In 2015, researchers at the Cleveland Clinic in Ohio used population-based data of the Central Brain Tumor Registry of the U.S. to calculate an incidence of 10.9 per million of population, or about 3,300 cases of VS per year. A higher incidence up to 29.3 per million of population was found for the 65-74 year-old age group. There was no significant difference in incidence by gender. Incidence was higher in Asian Pacific Islanders, and lower in African Americans and Hispanics. The annual number of diagnosed VS increased significantly worldwide by the early 1990s with the introduction of magnetic resonance imaging (MRI). Notably, epidemiologists in Denmark (population of 5.7 million in 2015) reported 193 cases of VS for 2015—an incidence of 34 per million of population per year. The first MRI scanner in Denmark was functional in 1989, and by 2015 the number increased to approximately 100.
Complications
Infection
Repeated trauma
Bursting and reformation
Dysphagia (in the case of a large ranula)

Causes
Minor trauma to the floor of the mouth is thought to damage the delicate ducts that drain saliva from the sublingual gland into the oral cavity. The lesion is a mucous extravasation cyst (mucocele) of the floor of mouth, although a ranula is often larger than other mucoceles (mainly because the overlying mucosa is thicker). They can grow so large that they fill the mouth. The most usual source of the mucin spillage is the sublingual salivary gland, but ranulae may also arise from the submandibular duct or the minor salivary glands in the floor of the mouth. A cervical ranula occurs when the spilled mucin dissects its way through the mylohyoid muscle, which separates the sublingual space from the submandibular space, and creates a swelling in the neck. It may occur following rupture of a simple ranula. Rarely, ranulae may extend backwards into the parapharyngeal space. Mechanism
The fluid within a ranula has the viscous, jellylike consistency of egg white. Diagnosis
The histologic appearance is similar to mucoceles from other locations. The spilled mucin causes a granulation tissue to form, which usually contains foamy histiocytes. Ultrasound and magnetic resonance imaging may be useful to image the lesion. A small squamous cell carcinoma obstructing the Wharton duct may require clinical examination to be distinguished from a ranula. Criteria
Mostly seen in young children and adolescents, both sexes are equally affected. Swelling in floor of mouth, which may be painful. Mostly unilateral, on one side of frenulum.