Chemotherapy and radiation therapy are confusing topics. When complicated terminology is combined with the anxiety associated with a diagnosis of cancer, it’s easy to understand how things become blurry. Further complicating things are those veterinarians who cross specialties. How can an owner be expected to keep it all straight?


Chemotherapy is defined as the use of chemical substances to treat disease. Conventionally, we think of chemotherapy in relation to treating cancer. Chemotherapy can be administered orally, intravenously (through a vein), topically (on the skin), subcutaneously (under the skin), intramuscularly (into a muscle), intratumorally (injected directly into a tumor), or intracavitary (given directly into a body cavity).


Adjuvant chemotherapy is prescribed after a tumor is removed and we are hoping to treat any microscopic residual cancer cells that may have spread from the tumor prior to surgery. An example of adjuvant chemotherapy is treating a dog with osteosarcoma with a drug such as carboplatin following amputation of the affected limb.


Neoadjuvant chemotherapy is used prior to surgical removal of a tumor or treatment with radiation therapy. The goal is to reduce the size of the tumor, affording the patient a less complicated “next step.” Neoadjuvant chemotherapy plays a big role for many human cancers, but unfortunately has a fairly limited role in veterinary medicine. Neoadjuvant chemotherapy can be helpful in treating and reducing the size of cutaneous mast cell tumors, thereby making them more “amenable” to surgery.


Induction chemotherapy is used to cause remission of disease. This would be the treatment of choice for blood borne cancers such as lymphoma or leukemia. Induction chemotherapy is often combined with consolidation and/or maintenance chemotherapy to maintain a long-term remission.


Regardless of how it’s used, chemotherapy is considered to be first line when the efficacy of the drug(s) has been proven during previous clinical trials and is the most effective treatment known for the particular disease in question. 


Second line chemotherapy (otherwise known as “rescue” or “salvage" chemotherapy) is prescribed when first line treatment is ineffective, or recurrence of disease is detected following initial treatment.


Radiation therapy involves the use of ionizing radiation to treat tumors. Radiation therapy is most commonly delivered by a machine outside of the body (external beam radiation), but also can be administered from a handheld source very close to the body (Strontium-90), via implantable radiation sources (brachytherapy), or even systemically, where radioactive substances travel in the bloodstream (e.g., 131I [Iodine-131] for treating feline hyperthyroidism).


Radiation therapy can also be used in the adjuvant or neoadjuvant setting. Prior to starting radiation treatment, patients typically undergo a CT scan of the affected area. The images obtained by the scan are used to plan the number and specific site of administration of the radiation treatments, as well as to delineate any anticipated side effects.


Patients must be positioned exactly the same way for each treatment, which means pets must be anesthetized every time they receive radiation. Various molds, “bite blocks,” or other devices may be constructed to facilitate accurate patient positioning. Markings are made along the skin and regions of fur may be clipped as well.


Chemotherapy can be administered simultaneously with radiation therapy in what are known as radiosensitizing protocols. The goal of this form of therapy is to increase the efficacy of the individual radiation treatment. Patients are monitored carefully, as side effects can be more pronounced.   


A board certified medical oncologist is trained in the safe handling, use, and administration of chemotherapy drugs, as well as the treatment of patients with chemotherapy. Medical oncologists spend time learning the principles of radiation oncology and are capable of managing radiation cases, but they are not considered board-certified radiation oncologists. In the U.S., veterinarians achieve board certification via meeting requirements put forth by the American College of Veterinary Internal Medicine


Radiation oncologists are specifically trained in the physics and biology of ionizing radiation and the treatment of cancer patients with radiation therapy. They are specialized in the art and science of radiation treatment planning. Radiation oncologists spend time learning medical oncology during their training, but are not considered board certified in medical oncology. To achieve board certification in radiation oncology in the U.S., veterinarians must complete requirements put forth by the American College of Veterinary Radiology.


It’s common for medical oncologists to offer radiation therapy to patients even while not having a radiation oncologist on site at the facility where the treatments are being administered. Those facilities most often use remote treatment planning, where either a veterinary radiation oncologist or human dosimetrist (who is not a veterinarian) receives the images generated by the pre-treatment CT scan and devises the treatment plants. The plans are sent to the medical oncologist, who oversees the treatments.

Likewise, some radiation oncologists elect to administer chemotherapy or immunotherapy treatments, either with or without having concurrent medical oncologists on staff.


In a perfect world, pets would always be treated by the veterinary specialist possessing the most specialized training for their disease. This isn’t always possible based on geography, finances, or other unforeseen circumstances. However, far too many times pets are not offered ideal treatment because a lack of communication and education. This can occur when an owner or primary care veterinarian is unsure or unaware of the qualifications of the attending veterinary specialist or even when there’s a misrepresentation of what a facility has to offer (e.g., specialty or primary care hospitals with no medical or radiation oncologist on staff that offer “oncology” as a service).


Owners should not be afraid to ask about the credentials of the doctor taking care of their pet, and specialists should do a better job of educating the public about the pros and cons of when they are acting outside of their “board certified” role. And primary veterinarians must be honest with owners about their limitations when it comes to practicing specialty medicine.


We are responsible for making sure owners know exactly what we can and can’t do, and to let them know when someone could do it better.


Dr. Joanne Intile


Image: Tonhom1009 / Shutterstock