Oncology is the branch of medicine that studies tumors (cancer) and seeks to understand their development, diagnosis, treatment, and prevention. A medical professional who practices oncology is an oncologist. The term originates from the Greek onkos (ογκος), meaning bulk, mass, or tumor and the suffix -ology, meaning "study of".

The oncologist often coordinates the multidisciplinary care of cancer patients, which may involve physiotherapy, counseling, clinical genetics, to name but a few. On the other hand, the oncologist often has to liaise with pathologists on the exact biological nature of the tumor that is being treated.

Oncology is concerned with:

• The diagnosis of cancer
• Therapy (e.g., surgery, chemotherapy, radiotherapy and other modalities)
• Follow-up of cancer patients after successful treatment
• Palliative care of patients with terminal malignancies
• Ethical questions surrounding cancer care
• Screening efforts:
• of populations, or
• of the relatives of patients (in types of cancer that are thought to have a hereditary basis, such as breast cancer).

Diagnosis

The most important diagnostic tool remains the medical history: the character of the complaints and any specific symptoms (fatigue, weight loss, unexplained anemia, fever of unknown origin, paraneoplastic phenomena and other signs). Often a physical examination will reveal the location of a malignancy.

Diagnostic methods include:

• Biopsy, either incisional or excisional;
• Endoscopy, either upper or lower gastrointestinal, bronchoscopy, or nasendoscopy;
• X-rays, CT scanning, MRI scanning, ultrasound and other radiological techniques;
• Scintigraphy, Single Photon Emission Computed Tomography, Positron emission tomography and other methods of nuclear medicine;
• Blood tests, including Tumor markers, which can increase the suspicion of certain types of tumors or even be pathognomonic of a particular disease.

Apart from in diagnosis, these modalities (especially imaging by CT scanning) are often used to determine operability, i.e. whether it is surgically possible to remove a tumor in its entirety.

Generally, a "tissue diagnosis" (from a biopsy) is considered essential for the proper identification of cancer. When this is not possible, empirical therapy (without an exact diagnosis) may be given, based on the available evidence (e.g. history, x-rays and scans.)

Occasionally, a metastatic lump or pathological lymph node is found (typically in the neck) for which a primary tumor cannot be found. This situation is referred to as " carcinoma of unknown primary", and again, treatment is empirical based on past experience of the most likely origin.