Photodynamic therapy is used especially for superficial skin tumours.
The main indication to date has been squamous cell carcinoma in cats. Carcinoma in situ, i.e., early stages of the tumour, are particularly treatable.
One treatment usually suffices. However, if the tumour was not completely destroyed or if a further tumour develops, the treatment can be repeated without fear of side effects.
To make an assessment regarding optimal treatment (photodynamic therapy, radiotherapy or surgery) and as regards the prognosis of the tumour, we ideally need complete tumour staging (estimation of its extent). This includes histological diagnosis of the tumour, complete blood tests (haematology and blood chemistry), urine test, two lateral X-rays of the chest and aspiration of the regional lymph nodes. At very early stages, some of the staging can be omitted if the owner wishes.
In photodynamic therapy the animal is given a non-toxic substance (known as a photosensitiser). This can be superficial or intravenous. It is necessary to wait a few hours for this substance to accumulate optimally in the tumour. Radiation is then delivered, ideally with monochromatic light (laser light), which has no harmful effect on its own. However, a photochemical reaction takes place due to the interaction of the two components, which leads to destruction of the tumour cells. Since the animals have to lie completely still during the therapy and pain can occur during the radiation, they are given a short anaesthetic and pain relief.
When the substance is given intravenously (systemic injection of the photosensitiser) the animals are sensitive to light and must therefore be protected from direct sunlight for 10 - 14 days after the injection to avoid possible skin reactions. Cats should always be kept in the house or apartment after intravenous therapy with the photosensitiser and the rooms should be darkened as far as possible (blinds closed, curtains drawn). In dogs, longer walks should be kept to early morning or late evening; in the middle of the day, the animals should be taken out only very briefly to relieve themselves.
Acute reactions in the treated area can occur and these differ greatly individually. These are apparent in the form of oedema, hyperaemia/cyanosis and (more rarely) itching. Within the next few weeks, a thick crust develops at the treated site, which falls off on its own. Under this there is re-epithelialised skin, which can remain thinner and hairless for a few months.
© AniCura, Dr. Julia Buchholz