The most common causes of cancer are tobacco, obesity and lifestyle, exposure to radiation and several infections (viruses and bacteria). There are few patients who don’t have any of the known risk factors, but they can still develop cancer. It possibly occurs through some derangement in genetic composition of the cells called mutations.
 

No. Cancer is not contagious. It doesn’t spread by living in close contacts, sharing food, hugging or kissing. Rather, several cancer patients while they are on chemotherapy or immunosuppressive treatment may get secondary infections from other infected person coming in close contact. For their safety we should follow good hygiene practices.
 

There are few cancer types which have a tendency for familial aggregation. A small proportions of patients with breast cancer, ovarian cancer, prostate cancer, colon cancer, few brain tumors, muscle sarcomas etc may have genetic predisposition. There are few harmful genetic mutations which may run in families and may predispose the carriers of the mutations for development of cancer.
 

All cancers are not familial. If several first degree (parents or sibling) and/or second-degree relatives (maternal or paternal uncles and aunts) have had cancer, then you need to consult an oncologist or genetic counsellor. They will assess the familial risk based on the pattern in the family and accordingly advise the genetic test after genetic counselling. Carrying these genes doesn’t mean that one has cancer. It simply means that the carrier has higher risk of developing cancer in their lifetime as compared to general population. Although we can’t change our genes, but we can certainly suggest few recommended screening and preventive strategies. Carrying out these genetic tests without proper risk assessment and genetic counselling is not advisable as it would simply increase the anxiety.
 

That depends on the type of cancer and the stage at presentation. If detected early and treated appropriately, many cancers are associated with pretty high cure rates. Survival probabilities have improved even in advanced stages more recently with advancement in technology and treatment modalities. With targeted treatments and immunotherapy many patients are living long with reasonably good quality of life in many advanced cancers which were earlier considered untreatable.
 

Various  modalities commonly used in cancer treatment are-
Surgery
Radiotherapy
Chemotherapy
Targeted therapy
Immunotherapy
The choice of treatment depends on type of cancer, stage at presentation, age, general condition of patient, pathology and molecular reports. All these factors are taken into consideration before taking treatment decision. Many cancers require mult-modality treatments e.g. surgery followed by chemo or radiotherapy, concurrent chemo-radiotherapy, chemo-immunotherapy etc. Consultations with surgical oncologist, radiation oncologist and medical oncologist is required in such scenarios.
 

Almost 70 % of all cancers are considered related to tobacco and alcohol. Stopping consumption of tobacco and alcohol in any form reduces cancer risk with time. Similarly preventing obesity, healthy diet and regular exercise also reduce the risk of cancer. Prevention of several viral infections through vaccination like HPV have been shown to reduce the risk for development of cancer cervix  (mouth of uterus) in women.
Several screening modalities like mammography (breast cancer), pap smear (cervix cancer), low dose CT scan of chest for high-risk individuals (lung cancer), oral cavity examination (mouth cancer) can be useful to detect cancers at early stage.  
 

Radiotherapy is one of the cancer treatment modalities using ionisation radiation (mostly X-rays) to kill cancer cells. Radiation is normally delivered by linear accelerator.

IMRT (INTENSITY MODULATED RADIOTHERAPY) 
IGRT (IMAGE GUIDED RADIOTHERAPY)
VMAT (VOLUMATRIC MODULTED RADIOTHERAPY)
SRT (STERIOTACTIC RADIOTHERAPY)
SRS (STERIOTACTIC RADIOSURGERY)
Newer radiotherapy techniques are more precise and has fewer side effects so patient has better tolerance to the treatment.

Radiotherapy is local treatment so as the side effects are. All most all the side effects are irreversible. Skin hyper pigmentation, nausea , vomiting ( abdominal radiation) , local alopecia are the common side effects. 
 

Yes surgery can be avoided or unnecessary for early laryngeal cancer, nasopharyngeal cancer, base of tongue cancer, oesophagus cancer, cervical cancer (IIB onwards), urinary bladder cancer as organ preservation.