Evaluation of strontium 90 irradiation in treatment of cutaneous mast cell tumors in cats: 35 cases (1992–2002)

Jane M. Turrel Veterinary Oncology Specialties, 225 Carmel Ave, Pacifica, CA 94044.

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John Farrelly Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Rodney L. Page Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Margaret C. McEntee Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Abstract

Objective—To determine the efficacy of strontium 90 β irradiation in the management of cutaneous mast cell tumors (CMCTs) in cats.

Study Design—Retrospective case series.

Animals—35 client-owned cats with CMCTs.

Procedure—Medical records of cats with CMCTs in which tumors were radiated by use of a strontium 90 ophthalmic applicator from 1992 to 2002 were reviewed. Cats were included if CMCT was diagnosed, there were no other sites of MCT involvement at the time of treatment, and records contained adequate follow-up information to permit retrospective assessment of local tumor control.

Results—54 tumors in 35 cats were treated with a median dose of 135 Gy of strontium 90 β irradiation, resulting in local tumor control in 53 of 54 (98%) tumors with a median follow-up time of 783 days after treatment. Median survival time was 1,075 days. Adverse effects of treatment appeared to be infrequent and of mild severity.

Conclusions and Clinical Relevance—Results indicated that strontium 90 β irradiation resulted in long-term tumor control and should be considered an effective alternative to surgical resection in management of CMCTs in cats.

Abstract

Objective—To determine the efficacy of strontium 90 β irradiation in the management of cutaneous mast cell tumors (CMCTs) in cats.

Study Design—Retrospective case series.

Animals—35 client-owned cats with CMCTs.

Procedure—Medical records of cats with CMCTs in which tumors were radiated by use of a strontium 90 ophthalmic applicator from 1992 to 2002 were reviewed. Cats were included if CMCT was diagnosed, there were no other sites of MCT involvement at the time of treatment, and records contained adequate follow-up information to permit retrospective assessment of local tumor control.

Results—54 tumors in 35 cats were treated with a median dose of 135 Gy of strontium 90 β irradiation, resulting in local tumor control in 53 of 54 (98%) tumors with a median follow-up time of 783 days after treatment. Median survival time was 1,075 days. Adverse effects of treatment appeared to be infrequent and of mild severity.

Conclusions and Clinical Relevance—Results indicated that strontium 90 β irradiation resulted in long-term tumor control and should be considered an effective alternative to surgical resection in management of CMCTs in cats.

Mast cell tumors are common skin tumors in cats, accounting for 7% to 21% of reported cutaneous tumors.1-3 Mast cell tumors exist in the more common cutaneous-subcutaneous form and in the visceral form, in which abdominal organs (eg, spleen and intestine) are usually involved; a minority of affected cats develop both forms of the disease concurrently. Cutaneous MCTs in cats are frequently located in the dermis, although in some instances, the tumors extend into adjacent subcutis4; tumors typically appear as a small pimplelike mass or a well-defined dermal or subcutaneous mass. Metastasis to local lymph nodes or to visceral organs has been reported but is not common.5,6 Most CMCTs in cats have benign biological behavior and may be effectively treated with locally directed therapy alone.4,7,8 In histologic appearance and biological behavior, CMCTs in cats differ distinctly from those in dogs. In dogs, neoplastic mast cells are frequently distributed peripherally from and deep to the main tumor mass. Because of the small size and localized nature of CMCTs in cats, strontium 90 is a useful option for the treatment of this disease.

A strontium 90 ophthalmic applicator is a sealed-source β emitter that decays with a half-life of 28 years. Treatment with strontium 90 irradiation entails placement of a sealed radiation source in contact with the surface of the lesion to be irradiated and is also referred to as plesiotherapy (the prefix plesiois Greek for close or near). Strontium 90, in equilibrium with its daughter product yttrium 90, emits β particles with a maximum energy of 2.27 MeV (megaelectron volts), with lower-energy β particles being filtered out by the steel encapsulation.9 The maximum dose of radiation is deposited at the skin surface; irradiation drops off rapidly with increased depth, with only approximately 5% of the delivered dose reaching deeper than 4 mm into the tissue.10 A single large dose of radiation may be delivered by direct application of the strontium 90 applicator to a superficially located tumor with negligible risk of delivery to underlying healthy tissues. The active diameter of a strontium 90 ophthalmic applicator is 8.5 mm. One application field is used for small lesions, and multiple overlapping fields are required for lesions that approximate or exceed the diameter of the radiation source. The dose of radiation delivered depends on the length of time during which the source is in contact with the site and the radioactivity of the source at the time of use. Strontium 90 irradiation has been used successfully to treat small superficial lesions (primarily squamous cell carcinoma in situ or early stages of invasive squamous cell carcinomaa) in cats, but the effectiveness of this type of treatment for CMCTs in cats has not been reported. The objectives of the present study were to evaluate the efficacy of strontium 90 irradiation as a locally acting treatment modality for CMCTs in cats and to determine the clinical outcome.

Criteria for Selection of Cases

Medical records of cats with CMCTs treated with strontium 90 β irradiation at Veterinary Oncology Specialties in Pacifica, Calif, from 1992 to 2002 were reviewed. Cats with single or multiple CMCTs that were treated with strontium 90, had records that contained sufficient clinical and follow-up information, and that did not have evidence of visceral mast cell disease at the time of initial evaluation at the hospital were eligible for inclusion in the study.

Procedures

Information was collected from medical records or by telephone contact with referring veterinarians or owners and included signalment, location, number and size of CMCTs, treatment details (eg, number of applications and radiation dose), acute and late irradiation adverse effects, tumor recurrence at a treated site, development of other CMCTs or evidence of metastatic disease, and additional treatments. Survival time was calculated from the date of strontium 90 treatment to the date of death or last known follow-up contact. For the purpose of evaluating efficacy of strontium 90 for local tumor control, all tumors treated with strontium 90 and that had sufficient follow-up data for assessment were included in analysis.

Confirmation of the diagnosis of CMCT was performed via cytologic evaluation of an aspirate of the mass, with or without histologic assessment of a biopsy specimen. Treatment consisted of 1 application field or multiple overlapping fields for lesions that approximated or were larger than the active diameter of the radiation source, respectively. Irradiation was applied with a strontium 90 ophthalmic applicator (Figures 1 and 2).b The dose of radiation delivered to each site was 120 to 150 Gy, and time required to deliver the dose varied according to the activity of the strontium 90 source at the time of use.

Figure 1
Figure 1

Photograph of an MCT on the inner aspect of the pinna of a cat that was treated with 1 field of β irradiation by use of a strontium 90 ophthalmic applicator. The applicator was placed in contact with the surface of the tumor. Notice the circular impression that was visible immediately after completion of the treatment, indicating that the tumor was positioned centrally in the radiation field.

Citation: Journal of the American Veterinary Medical Association 228, 6; 10.2460/javma.228.6.898

Figure 2
Figure 2

Photograph of an anesthetized cat undergoing treatment for an MCT. The strontium 90 ophthalmic applicator is applied to the surface of the tumor on the inner aspect of the pinna. The sealed strontium 90 radioactive source is mounted on a rod that has a flexible hinged end, allowing the active head to be rotated 180°. The strontium applicator is held by the rod, and a plexiglass shield protects the operator from radioactive β particles emitted by the strontium 90 source.

Citation: Journal of the American Veterinary Medical Association 228, 6; 10.2460/javma.228.6.898

Results

The search of records yielded 49 cats with CMCT that were treated with strontium 90 β irradiation from March 1992 to February 2002. Of those, 4 cats were excluded because of concurrent visceral MCT at initial evaluation and 10 were excluded because of incomplete clinical or follow-up information. Among the 35 cats that met the inclusion criteria were domestic shorthairs (n = 21); domestic longhairs (7); Siamese or Siamese cross (2); Persian (2); and Maine Coon, Himalayan, and Balinese (1 each). The value for both mean and median age of cats at the time of diagnosis was 10 years, and the range was 1.8 to 22 years. Twenty cats were spayed females, and 15 were neutered males.

In 18 of the 35 (51%) cats, CMCT was confirmed histopathologically from a biopsy specimen of one or more of the tumors, and in 17 cats, CMCT was confirmed via cytologic assessment of a needle aspirate specimen. Of the 17 cats with a cytologic diagnosis, cytologic evaluation was performed by a commercial laboratory in 11 cats and by one of the authors (JT) in 6 cats. Strontium 90 was used to treat multiple tumors in 6 cats: 3 cats had 2 tumors, and 1 cat each had 4, 6, and 9 tumors. In cats with multiple tumors, a presumptive diagnosis of CMCT was made for the masses that were not evaluated on the basis of similarity in appearance to the mass or masses on the same cat that were aspirated or biopsied. Three of the additional tumors in cats with CMCTs were confirmed as CMCT via histopathologic assessment.

The 35 cats were treated for 54 tumors. Information regarding the number of applications or fields used for individual tumors was available for 43 (80%) tumors. Number of application fields ranged from 1 to 9 fields, with a median of 2 fields. Total radiation dose ranged from 120 to 150 Gy, with a median of 135 Gy.

Regarding tumor location, 40 (74%) tumors were located on the head at sites that included the eyelid (14), pinna or base of the ear (14), chin (5), lip (2), and other locations on the head (5). Ten tumors were located on the trunk, and 4 were located on limbs, including 3 on a forelimb and 1 on a hind limb. Information on tumor size at the time of treatment was available for 18 (33%) tumors: sizes ranged from 1 to 25 mm, with a median of 4.5 mm. In 2 cats, there was no evidence of disease at the time of strontium 90 treatment, but treatment was performed because of incomplete surgical margins in 1 cat and previous local recurrence following complete surgical removal in the second cat. The other 52 strontium 90 treatments were performed because of macroscopically evident disease.

Thirteen of the 35 (37%) cats received glucocorticoids as treatment for CMCT prior to referral; most of those cats had multiple tumors or other concurrent diseases. Although treatment with glucocorticoids may have contributed to the efficacy of strontium 90 in that group, the effect could not be ascertained because local control of tumors in cats that were or were not treated with glucocorticoids was similar (median, 770 vs 1,028 days in cats that were or were not treated with glucocorticoids, respectively, and local recurrence of tumor at only 1 site).

One cat had recurrence of tumor at the site of previous strontium 90 application approximately 2 years after treatment. The recurrent mass was removed surgically, and the cat died approximately 1 year later of unrelated disease. The remaining 53 (98%) tumor sites were free of disease at the time of data compilation, with a median follow-up time of 783 days (range, 41 to 3,256 days).

One cat that received irradiation treatment for a CMCT on the metatarsus developed metastasis to the popliteal lymph node 237 days after strontium 90 treatment. Metastasis was histopathologically confirmed on an excisional biopsy specimen of the lymph node. Another cat was euthanized because of clinical signs related to an abdominal mass 1,021 days after strontium-90 irradiation, and a third cat developed an abdominal mass that was detected via abdominal palpation 123 days after strontium 90 irradiation but was lost to follow-up. A definitive diagnosis was not made in either of the cats with abdominal masses.

Mean and median survival times were 1,223 and 1,075 days, respectively, from the first strontium 90 treatment, with a range of 41 to 3,256 days. Further analysis of survival was not performed because none of the cats died as a result of local tumor-related disease and regional tumor metastasis was reported in only 1 cat. Adverse effects of radiation therapy were mild and included alopecia, crusting, pruritus, leukotrichia, and thinning and depigmentation of the skin. Although some of the cats were returned to Veterinary Oncology Specialties for recheck examinations, follow-up information was derived primarily from referring veterinarians' records and telephone conversations with owners.

Discussion

Results indicated that strontium 90 β irradiation is an effective local treatment modality for cats with CMCTs. The low rates of local tumor recurrence and metastasis (1/35 [3%] cats) in this group of cats indicated that the treatment is effective for local control of CMCT and may be considered an alternative treatment for CMCT in situations in which surgery is not feasible or practical because of tumor location. A higher local recurrence rate (15.6%) was reported7 in a group of 32 cats that underwent surgical excision of CMCT. Strontium 90 treatment may also be useful in cats with multiple CMCTs, in which multiple local excisions would otherwise be necessary. Given the short treatment time needed for each field, multiple CMCTs could be treated with strontium 90 during a single anesthetic episode.

Because the present study was retrospective in nature, it was impossible to control for certain factors related to treatment, such as administration of glucocorticoids and dose of radiation delivered. The study design also resulted in a limited ability to determine or assess adverse effects associated with the irradiation treatment because cats received the strontium 90 treatment at the referral center but returned to the primary-care veterinarian for follow-up examinations and routine care.

A subset of cats received glucocorticoids prior to or concurrent with strontium 90 irradiation. Although the differences were not significant, median survival times were shorter in cats that were treated with glucocorticoids. The cats that received glucocorticoids often had multiple MCTs and may have had more aggressive forms of disease. In cats with multiple MCTs or that develop additional MCTs, it is recommended that a splenic aspirate be performed. In the present study, cats with evidence of splenic involvement were excluded from analysis because additional treatment measures (ie, splenectomy) were recommended.

Adverse effects of strontium 90 treatment are usually considered to be mild despite the delivery of high single doses (120 to 150 Gy) of radiation to the skin. One limitation of this study was the lack of a standardized recheck schedule to monitor cats for adverse effects, but it is unlikely that adverse effects were common on the basis of information from previous reportsa and given the absence of cats that were examined because of complications of treatment over an extended follow-up period. Adverse effects observed in the small number of cats that had follow-up examinations at Veterinary Oncology Specialties after receiving irradiation treatment were mild.

Cats with single CMCTs that were treated with strontium 90 had excellent outcomes. Tumor biopsy was not performed in all cats, but it is known that histologic grades of CMCTs in cats are not useful for determining prognosis.4 Therefore, cytologic evaluation of tumor aspirates may be used for prompt diagnosis of CMCT and initiating treatment with strontium 90 irradiation. The low rate of distant metastasis of MCT observed in cats in the present study was similar to rates in earlier reports4,7,11 on surgical treatment of CMCT, confirming the effectiveness of adequate locally directed treatment for these tumors in cats. In some cats with multiple CMCTs, the diagnosis of CMCT was not confirmed in each mass but was assumed on the basis of similarity of gross appearance to other tumors on the same cat that were cytologically or histologically evaluated. It is possible that some of the masses in cats with multiple cutaneous masses were not CMCTs, which would have falsely increased the response rate.

Results suggest that strontium 90 irradiation is a safe and effective alternative to surgical resection for treatment of CMCT in cats, and adverse effects appeared to be mild and self-limiting. These results are in agreement with those from earlier reports, in which the outcome was excellent for cats that had CMCT without visceral involvement and that received effective locally directed treatment for the tumors.

MCT

Mast cell tumor

CMCT

Cutaneous MCT

a.

Van Vechten MK, Théon AP. Strontium-90 plesiotherapy for treatment of early squamous cell carcinomas of the nasal planum in 25 cats (abstr), in Proceedings. Annu Conf Vet Cancer Soc 1993;13:107–108.

b.

Isotope Products Laboratories, Valencia, Calif.

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