Taken from an American website
Q. What is a Radiation Oncologist?

A. Radiation oncologists are physicians who are responsible for the
medically appropriate and technically correct radiation treatment of
patients with cancer. All Roswell Park Cancer Institute Radiation
Oncologists are board certified in therapeutic radiology by the American
Board of Radiology, licensed by the New York State Department of Health,
and comply with the American College of Radiology standards of care.

Q: What is radiotherapy and what is a radiation oncologist?
A: Radiotherapy or radiation oncology is the branch of medicine concerned
with treatment of tumors with high energy X-rays. This treatment can be
given in multiple small doses or a few large doses depending on the kind
of tumor and the goal of treatment. It can be given externally from a
machine called a linear accelerator or internally via radioactive sources
which are implanted into the body, brachytherapy. The radiation therapy
team usually consists of the physician who is called the radiation
oncologist and who prescribes the treatment and oversees the results and
side effects; the physicist and dosimetrist who help determine the best
treatment plan and who maintain the accuracy of the treatment; and the
therapist or technician who positions the patient on a daily basis and
administers the treatment

http://www.oncologychannel.com/oncologist.shtml
Oncologists are physicians who study, diagnose, and treat cancerous
tumors. They practice in hospitals and medical centers, university
hospitals, and research organizations.
To become certified as an oncologist, a candidate must first graduate from
an accredited medical school before entering into training as a specialist
(e.g., internist, pediatrician) and subspecialist (e.g., medical
oncologist, pediatric oncologist-hematologist). There are several oncology
specialties and subspecialties:

Gynecological oncology: The American Board of Obstetrics and Gynecology
(ABOG) examines and certifies obstetricians and gynecologists who choose
to acquire additional education and training to subspecialize in the
diagnosis and treatment of cancers of the female reproductive organs
(e.g., cervical cancer, breast cancer).

Medical oncology: The American Board of Internal Medicine (ABIM) examines
and certifies internists who choose to acquire additional education and
training to subspecialize in medical oncology, the use of medical and
chemotherapeutic treatments of cancer.

Medical oncology and hematology: The American Board of Internal Medicine
(ABIM) examines and certifies internists who choose to acquire additional
education and training in the dual subspecialty of medical oncology and
hematology (the treatment of malignancies of the blood and blood-forming
tissues).

Pediatric oncology and hematology: The American Board of Pediatrics (ABP)
examines and certifies pediatricians who choose to acquire additional
education and training to subspecialize in the diagnosis and treatment of
cancers in children (e.g., leukemia).

Radiation oncology: The American Board of Radiology (ABR) examines and
certifies radiation oncologists, who specialize in radiation treatment of
cancers.

Surgical oncology: The American Board of Surgery (ABS) examines and
certifies surgeons who are trained in several types of surgical
treatments, including biopsy, tumor staging, and tumor resection
(removal).
What is board certification?
The board certification process includes the following components:
Education
Candidates must have graduated from an approved medical school and must
have completed an ACGME accredited residency program. Graduate education
in a specialty and, if the physician chooses, a subspecialty follows.

Examinations
After satisfactory completion of graduate education in a specialty,
physicians may apply for certification. Applicants must pass the
certifying examination administered by the medical specialty board. If
they go on to receive additional education and training in a subspecialty
(e.g., pediatric oncology and hematology), they must complete the
education and training requirements and then apply for certification and
pass the examination administered by the medical specialty board.

Resources
To obtain detailed information regarding the educational requirements and
certification processes for the various oncology specialties and
subspecialties, visit the websites of the medical specialty boards listed
below:
American Board of Internal Medicine
American Board of Obstetrics and Gynecology (ABOG)
American Board of Pediatrics
American Board of Radiology (ABR)
American Board of Surgery (ABS)

Canadian website
WHAT IS A RADIATION ONCOLOGIST?
Radiation oncologists are specialists with training and experience in the
treatment of cancer.
They coordinate the care of many cancer patients and, when needed,
prescribe radiation and
other anti-cancer drugs (hormones, biological modifiers).

Oncology is a major medical discipline, especially when one considers
that one in three Canadians will develop cancer in their lifetime. In
contrast with physicians in
other subspecialties, radiation oncologists encounter and treat a variety
of problems which arise
regularly in the management of patients with cancer.. Daily interactions
with surgeons, medical oncologists,
pathologists, radiologists and others provide stimulation and
opportunities for learning.