Radiotherapy in animals
More and more veterinary cancer patients are treated with multimodal therapy approaches. In human oncology, radiotherapy is an important treatment modality for malignancies, most commonly as a part of a multimodal approach to cancer treatment. About 50% (or even more in developed countries) of people with cancer will be treated with radiotherapy. In veterinary medicine, the number of animals with cancer treated with radiotherapy is lower, in part due to limited access to radiotherapy units.
To understand how radiotherapy works, one needs to be familiar with the basics of tumour occurrence and growth. Cancer cells are, in part, similar to normal cells with the cell cycle being basically the same, but lacking control mechanisms. The biological behaviour of tumours, their aggressiveness, depends mostly on the degree of loss of these control mechanisms. When radiotherapy is considered as a treatment option, this behaviour of the tumour is very important. When it is small (i.e. microscopic), it is usually metabolically very active. This metabolic and cellular activity is later reduced. Radiotherapy is most effective when tumours are very active. Radiotherapy can be used as a sole treatment strategy, but it is more commonly used as a (neo)adjuvant treatment to the surgical removal of the tumour, which can be either partial or complete with residual microscopic disease. Such reduction of tumour mass makes the tumour more responsive to radiotherapy.
The basics of radiation-induced damage to (tumour) cells are related to radiation-energy-induced DNA damage. For the cell to die, DNA should be irreversibly damaged. However, at each radiotherapy session, only a limited number of cells receive a lethal dose of radiation. In addition, radiation-induced damage or even death can occur in healthy surrounding cells; therefore, the goal of radiotherapy is to induce death in as many cancer cells while preserving as many healthy cells as possible. Detailed planning of radiotherapy is, therefore, required for the radiation to reach mostly tumour cells. The total dosage of radiation is determined based on the dose needed to eliminate tumour cells. Furthermore, biological behaviour of the tumour and surrounding healthy tissues determine the time between radiation sessions. Each radiation dose (fraction) is determined based on the survival capability of the most sensitive healthy tissue in the irradiation field.
Irradiation protocols can be described as curative (when the intent of radiotherapy is to destroy the tumour) or palliative (when the intent of radiotherapy is to reduce the size of the tumour and improve quality of life). A standard curative protocol consists of 18-20 fractions of 3 Gy (18-20 × 3Gy) (Gy means ‘Gray’, a derived unit of ionising radiation dose), five to six times weekly. Some tumours, which have a tendency of early metastasis (e.g., canine oral malignant melanoma) and usually receive a multimodal treatment approach, can be treated with radiotherapy using a palliative protocol, using fewer fractions and higher doses per fraction to achieve good results. A typical palliative protocol is 6 × 5 Gy or 6 × 6 Gy or even 4 × 8-9 Gy. Palliative treatment protocols are also used to relieve pain in some tumours, such as osteosarcoma. Some other tumours (e.g., squamous cell carcinoma), in which cells divide rapidly (high doubling time), should be irradiated twice daily to minimise recovery of the tumour cells between fractions. Such accelerated protocols are shorter and last 9 days (14 × 3,2-3,5 Gy).
Radiotherapy comes with side effects due to damage induced in healthy cells. Side effects can be acute or chronic (late). Acute side effects are expected and can be seen as dermatitis, hair loss or mucositis, while late side effects can be seen as serious damage to nerve tissues, cicatrisation of tissues and osteonecrosis. Acceptable late side effects are alopecia and cataracts. Acute side effects appear within weeks of the start of radiotherapy and usually spontaneously regress within a few weeks post radiotherapy. Late side effects may develop years after irradiation, and the risk of developing late side effects remains for life. Many of the side effects of radiotherapy can be avoided by the proper planning of treatment, which is based on advanced imaging of the tumour, most commonly CT. Special software is required for radiotherapy planning, and the goal is to irradiate the tumour while avoiding surrounding healthy tissues as much as possible
Some of the tumours that are more commonly treated with radiotherapy (either alone or as a part of multimodal therapy) in Animal Hospital Postojna include mast cell tumours, soft tissue sarcomas, nasal tumours, brain tumours, oral tumours such as malignant melanoma and squamous cell carcinoma, squamous cell carcinoma of the head and neck region, anal sac adenocarcinoma, localised nasal and cutaneous lymphomas, solitary plasmacytomas.
It must be noted that successful radiotherapy can only be performed with an animal under general anaesthesia, so they can lie still for the session. For the purpose of radiotherapy, adapted anaesthesia protocols are used, which enable recovery of the animal within minutes of the completion of the radiotherapy session. Each patient is thoroughly evaluated before anaesthesia to ensure that the animal is a good anaesthesia candidate and that radiotherapy can be safely performed.
If you have noted any problems with your animal, please consult your veterinarian.
Text by Janoš Butinar