Definition Radiotherapy using a strontium-90 applicator is called contact therapy. The treatment consists of applying a radioactive source of Strontium that emits beta radiation (electrons) directly onto the surface of the lesion to be treated.
The radioactive source is 0.8 cm in diameter and is mounted on a 15-cm shaft, along which slides a 1-cm-thick Lucite shield. The source we are using is available with a mask to allow delivery to particular areas.
As opposed to x-rays, the beta radiation produced by the applicator source has a marked attenuation in biological tissues resulting in a sharp dose fall-off, making it particularly suitable for treatment of superficial skin and ocular surface lesions without exposure to the underlying normal tissues.
Treatment technique The maximal radiation dose provided by the source is delivered at the lesion surface with a rapid decrease in dose with depth; < 10% of the surface dose penetrates to a depth of 3 mm. For lesions requiring > 1 probe application, adjacent fields overlapped by 2 mm are used.
During treatment, the strontium-90 surface applicator is placed directly onto the surface of the lesion for irradiation. Treatments are done under brief general anesthesia to ensure accurate positioning of the probe and to minimize personnel exposure.
The applicator is carefully applied directly on the surface of the lesion. The Lucite shielding protects the operator’s hands from scattered electrons.
Because each application takes 3 to 4 minutes, the applicator is held with a stand during treatment to minimize radiation exposure to personnel.
Treatment requires only 1 anesthesia and treatment times are short (3 to 5 min/ application). Lesions in multiple areas can be treated during a single anesthetic episode. For large treatment areas, overlapping fields restricted to tumor tissues can be used. Usually one treatment is sufficient. Treatment doses of 120 to 150 Gy are given in a single session.
Indications
Treatment is curative for superficial lesions of the skin, including carcinoma in situ, Bowen’s disease, sun-induced pre-cancerous lesions and squamous cell carcinoma (SCC) less than 2 mm thick such as those on the nose, eyelids and face of light-colored cats. Surface radiotherapy may also be used as primary or ancillary treatment for superficial fibrovascular infiltrates of the cornea that are not responsive to medical treatment.
Northern California Veterinary Specialists, Inc.
7425 Greenhaven Drive
Sacramento, CA 95831
Phone (916)231-0696
Fax (916)231-0691
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The maximal radiation dose provided by the source is delivered at the lesion surface with a rapid decrease in dose with depth; < 10% of the surface dose penetrates to a depth of 3 mm. For lesions requiring > 1 probe application, adjacent fields overlapped by 2 mm are used. 