By Alice Villalobos, DVM
For Veterinary Practice News
Radiation therapy is used to manage 50 percent of human cancers. Approximately 7,500 radiation therapy machines are in use worldwide.
|Thin skin and hair loss are effects of radia-
tion therapy for mast cell cancer.
Courtesy of Dr. Villalobos
The industry’s technology went from orthovoltage in the 1950s to cobalt in the 60s and now to linear accelerators, or linacs. Four hundred older units will be replaced this year and an additional 300 new units will be installed. The cost of a new linac is $2.3 million.
The new Hi-Art TomoTherapy unit was developed by researchers in Wisconsin and is considered the biggest innovation in RTx since the 1950s. It combines a linac and a CT scanner.
Its price tag is $3.2 million.
In the past, a lack of facilities limited radiation therapy as an option for pets.
The need for heavy sedation or anesthesia for each treatment was another deterrent. Cost and logistics are additional factors for the pet owner to deal with.
|Short- and Long-term Risks of Radiation Therapy|
|Radioation therapy Field (Tumor Bed)||Short-term side effects||Long-term effects
Late onset Risks
|Brain, Cranium||Dermatitis, conjunctivitis, mucositis, acute otitis||Cataracts, hearing loss, mental dullness, necrosis|
|Nasal||Mucositis, dermatitis, oral sensitivity||Dental problems, cataracts, skin necrosis over bone|
|Throat/oral cavity/mandible||Mucositis, pain, (most severe if pet has not had dental prophy)||Dental problems, xerostomia, bone necrosis, sequestrum, hypothyroidism|
|Chest/thorax||Dermatitis, esophagitis||Fibrosis of lungs and heart|
Mast cell cancer
|Dermatitis, hair loss, local desquamation, degranulation reaction dermatits, self injury||Hair loss, scar formation, thin skin, later malignancy, scarring, permanent hair loss
Need for skin graft
|Legs/extremities, bone||Dermatitis, lameness, anemia, lymphcytopenia||Demineralization, necrosis, ligament failure, sequestrum|
|Abdomen and plevic area||Colitis, indigestion||Liver, gut, kidney problems|
|Anus||Mucositis, colitis||Stricture, loss of function|
These reasons kept RTx in the background as a seldom-used modality.
Times have changed and safer anesthetics are available.
The law of supply and demand has made RTx more accessible and a realistic option for pet owners who are near a facility and can afford the cost.
The good news is that our profession is inheriting used equipment at reasonable prices.
The specialty board of veterinary radiation oncology was established in 1994 and offers residency programs. There are approximately 50 RTx facilities devoted to the treatment of companion animal tumors in the United States and Europe, and we can expect more.
RTx is used for local tumor control and palliation.
RTx is most often used postoperatively to kill residual disease in surgical fields.
It may be used preoperatively to shrink large tumors, making them more amenable to surgical excision.
RTx may be used as a single modality to save structures in cases where the cancer is staged as local and believed to have low metastatic potential.
RTx is used with chemotherapy, hyperthermia and targeted therapy to enhance local control. Because RTx is strictly a localized treatment, other modalities are needed to address metastatic potential.
Basic Radiation Biology
Tumor tissue is more sensitive to RTx than normal tissue but adjacent tissue still suffers and may exhibit acute and late toxicity.
The dose-response curve for RTx is sigmoidal with curves at bottom and top. The steep part of the curve rises rapidly between the dose for tumor control and the dose for patient tissue toxicity.
If the patient stops at a threshold dose of 40 Gy for its tumor and does not finish the total prescribed dose (50-55 Gy), the probability of tumor control remains at the low end of the curve.
If the patient receives over 60 Gy, the side effects at the top end of the curve create too much patient tissue morbidity.
RTx doses range from 30 to 60 Gy, delivered in four to 30 treatment fractions over a period of three to six weeks depending on whether the treatment is palliative or curative.
The radiation therapist’s goal is to deliver the safest dose over the least amount of time so that the patient has the most benefit and the least risk of immediate and delayed side effects.
RTx is delivered via external beam (teletherapy), via implanted radioactive beads inside and around the mass (brachytherapy), or via radiochemicals that localize in specific sites (systemic therapy, such as iodine 131 in hyperthyroid cats).
Curative Radiation Therapy
– Post-surgical follow up to “sterilize” dirty margins in a sarcoma field.
– Post-operatively in anal sac carcinomas.
– Primary treatment for nasal tumors and facial squamous cell carcinoma.
– Primary treatment for some brain tumors.
– Local control of oral tumors (acanthomatous epulis, squamous cell carcinoma, malignant melanoma, fibrosarcoma).
– Pre-operatively in soft tissue sarcoma to decrease size and viability of mass.
– Intra-operative therapy (using a megadose) to sterilize an open tumor bed.
– May be used in some induction protocols for lymphoma.
– Half-body RTx in some induction protocols for canine lymphoma.
Generally, skin and mucous membranes of the oral cavity, eyelids, genitourinary and GI tracts become the dose-limiting factor in external beam RTx. Hair may return a different color after initial alopecia.
At higher doses to the skin, the hair follicles are completely damaged.
Immediate or acute reactions occur in tissues that normally undergo mitosis. Late reactions occur in tissues that have low regeneration rates such as bone, nerves, brain and the ocular lens. The skin undergoes desquamation, both dry and moist, with pruitis, erythema and rash.
Mucositis with odoriferous salivation and secondary anorexia and weight loss often develop in patients treated for oral and nasal tumors.
The symptoms start when the patient is three-quarters of the way to completion of the RTx schedule.
Some pet owners want to quit at this stage. Proper education and supportive care for each one of their concerns is essential.
Support from the referring veterinarian is important to help pets and their families through the rough times that RTx may impose.
Conjunctivitis, kerato conjunctivitis sicca and cataract formation are common when the eyes are included in the RTx field or when treating nasal and brain tumors.
Pneumonitis and pericarditis with effusion may develop following radiation to the lung field. This is especially worrisome in cats treated for feline vaccine-associated sarcoma.
Cats develop cardiac and respiratory problems four to six months following the completion of RTx and may have to face pericardectomy with fibrotic lungs. Colitis, tenesmus and bleeding may result if the colon and rectum are in the field.
Fibrosis and loss of elasticity of the bladder may be a side effect with radiation to the sub-lumbar nodes.
Anemia and lymphocytopenia or general bone-marrow suppression may occur if enough bone is in the field.
Palliative Radiation Therapy
– To reduce cancer pain and tumor growth with osteosarcoma.
– To shrink bleeding facial and nasal squamous cell carcinoma.
– To shrink oral and pharyngeal masses (SCC, MM, FSA, tonsilar SCC).
– To treat inoperable brain tumors.
– To shrink mediastinal masses and large lymph nodes in resistant lymphoma.
– To reduce the size of large primary or metastatic lesions.
Note: If your client would consider RTx, early referral for consultation will save steps. CT scans are programmed to interface and reconstruct images for computerized treatment planning instead of MRI.
The closer the CT scan is to the time of treatment, the more accurate it is, because tumors change shape and size over time. This has been a major problem in human RTx and is solved with the new Hi-Art TomoTherapy unit.
Bones also undergo the delayed effect of necrotic sequestrum, which may occur years after treatment. Delayed effects such as xerostomia (no saliva) may appear after therapy of the oral cavity.
Side effects from the treatment of brain tumors with RTx are rare in dogs and cats but may include nausea, vomiting, pain, papilledema and unresponsive neurologic problems related to late effect necrosis of brain tissue.
Treatment of radiation side effects requires good nursing care and pain control. The RTx site should be kept clean and treated with water-soluble aloe vera gel topicals used for burn patients.
Oral mucositis is best treated by frequent oral lavage with saline and tea. The pet must be fed a bland, soft-food diet.
If anorexia develops, a feeding tube to keep up caloric intake becomes essential, especially in cats. Water-soluble ophthalmic steroid preparations help deal with conjunctivitis provided there is no corneal ulceration. Artificial tears are indicated for KCS.
For colitis, use the recommend low residue diets, steroid enemas and stool softeners. Bone sequestra need to be removed.
Steroids, anti-emetics and steroid-containing otic solutions help alleviate the side effects from CNS radiation; however there are no effective treatments for late effects such as myelomalacia.
Alice Villalobos, DVM, owns Animal Oncology Consultation Service in Woodland Hills, Calif. She received the 1999 Bustad Companion Animal Veterinarian Award and is associated with VCA Clarmar and Coast animal hospitals in Torrance and Hermosa Beach, Calif.
This article first appeared in the October 2004 issue of Veterinary Practice News.
10-1-2004Radiation Therapy in Veterinary Cancer ManagementRadiation Therapy in Veterinary Cancer ManagementRadiation Therapy in Veterinary Cancer ManagementRadiation Therapy in Veterinary Cancer ManagementRadiation Therapy in Cancer ManagementRadiation therapy is used to manage 50 percent of human cancers. Approximately 7,500 radiation therapy machines are in use worldwide.