The tonsils are a collection of oval shaped, lymph tissue, or white blood cells, at the back of your mouth that gather there to help fight infection. Cancer arising from tissue of tonsils is called tonsillar cancer. It is usually included in a wider category of cancer called Oropharynx which is a group including the tongue, the tonsils, the soft palate, and the walls of the pharynx. This can even occur as a metastasis from small cell lung carcinoma. Even if you have had tonsillectomy, you can have tonsil cancer as some remnants of tissue remain even after surgery.
Mostly they are squamous cell carcinomas. 
Signs and symptoms of Tonsil Cancer
A persistent sore throat and difference in size of tonsils is a red sign to book an appointment with your doctor. Initially, might be considered as an inflammation because of an infection but if antibiotic regime doesn’t help then it should raise the suspicion of tonsil cancer and lead to further investigation.
In later stages one can notice enlarged lymph nodes in surrounding areas, weight loss and recurrent infection. 
Risk Factors of Tonsil Cancer
Traditionally most common cause is linked to alcohol and tobacco use, But may also occur in people who don’t drink and smoke and has noticed trend of increase in such patients. Recent researches have also suggested a link between cancer and HPV( Human Papilloma Virus).
Prognosis of Tonsil Cancer
It usually depends on age, stage and general health of the patient. But statistically, tonsil cancers related to HPV (Human Papilloma Virus) tend to have higher survival rates than other causes. Though, once eliminated it is suggested to have a regular follow up to make sure there is no re-occurrence or spread to nearby tissue.
So, Make sure to book follow up appointments as suggested by your doctor.
Tests and Diagnosis of Tonsil Cancer
If you have a persistent or chronic sore throat and a your tonsils differ in size then you must book an appointment with An otolaryngologist (ear, nose and throat doctor). He will examine the area and determines if a biopsy is needed. In biopsy a tissue sample is obtained and sent to a pathologist for microscopic investigation to help determine the characteristic and staging of the cancer. Biopsy is still the gold standard for diagnosing and confirming the cancer.
Staging of Tonsil Cancer
Stage 0 – Carcinoma in situ
Abnormal cells are found in the lining of which may become cancer and spread
Cancer has formed, delimited, hasn’t spread and is 20 millimetres or smaller
No spread noticed yet. Cancer has formed and is larger than 20 millimetres but not larger than 40 millimetres.
No spread yet but Cancer is larger than 40 millimetres or any size and has spread to only one lymph node
Also its less than 30mm in lymph node
Cancer has spread to nearby areas, bone, jaw, lymph nodes and may even surround the arteries.
Treatment of Tonsil Cancer
Surgery and radiation go hand in hand but radiation is preferred as it has less side effects than surgery.
In advances cases, chemotherapy is also suggested.
Among the newer treatments a combination of radiation and molecular targeting agents called EGFR (epidermal growth factor receptor) inhibitors are being used. 
 Tonsillar Metastasis of Small Cell Lung Carcinoma., Unsal M, Kutlar G, Sullu Y, Yurtlu S., Clin Respir J. 2015 Jan 26. doi: 10.1111/crj.12275., http://www.ncbi.nlm.nih.gov/pubmed/25620524