Page 516 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

494 PART IV Specific Malignancies in the Small Animal Patient

Photodynamic Therapy

VetBooks.ir Photodynamic therapy is the use of applied light after the admin-
istration of a photosensitizer. The photosensitizer is activated by
the light to form free radicals, which results in the induction of
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cell death. Due to the limited penetration of light into the tissue,
the treatment depth is considered 5 mm or less. 21
Photodynamic therapy has been described for the treatment of
nasal planum SCC in cats. 21,22 In a large prospective study of 55
cats treated with photodynamic therapy using 5-aminolevulinic
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acid (5-ALA) as a photosensitizer, the overall response rate was
96% with 85% of cats having a CR. Fifty-one percent of cats with
a CR developed local recurrence with a median time to recurrence
of 157 days. At a median follow-up time of 38 months, 45% of cats
were alive without recurrence and 33% of cats were euthanatized
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• Fig. 24.3 Strontium plesiotherapy treatment of feline squamous cell car- because of local recurrence. In a study of 61 cats assessing response
cinoma. (Image courtesy Dr. Michael Kent.) to photodynamic therapy based on tumor staging, 49%, 12%, and
39% of cats demonstrated complete, partial, and no responses,
respectively. The rate of CR was significantly associated with stage;
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included clinical stage and proliferative fraction. The median CR was noted in 100% of noninvasive tumors measuring <1.5 cm
DFIs for 55 cats treated with megavoltage in two studies was in diameter, 56% of invasive tumors measuring <1.5 cm in diam-
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361 to 916 days, 3,12 with a MST of 902 days. Cats with larger eter, and 18% of invasive tumors measuring >1.5 cm in diameter.
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tumors had 5.4 and 6.3 times greater risk of recurrence and dying,
respectively, than cats with smaller tumors. For 15 cats treated Other Treatments
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with an accelerated RT protocol using protons, the PFS rate was
64% and the MST was 946 days. Side effects were considered Cryosurgery, using liquid nitrogen to freeze SCC lesions, has been
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minimal. described in two studies totaling 101 cats, 3,24 and is primarily indi-
cated for the treatment of small (<5 mm), superficial, and nonin-
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Strontium Plesiotherapy vasive SCC lesions. The response rate for cats with nasal planum
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SCC was 81% after two to three treatments. The local tumor
Strontium plesiotherapy is the application of a radioactive sub- recurrence rates are between 17% and 73% with a median DFI
3
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90
stance (strontium Sr90 or Sr) directly to the nasal planum tumor of 184 days to 26.7 months.
3
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(Fig. 24.3). When using this technique, less than 10% of the radi-
ation dose penetrates to a depth of 3 mm, and hence this treat-
ment is primarily indicated for cats with superficial and minimally SECTION B: NASAL CAVITY AND SINUS
invasive SCCs. Strontium plesiotherapy allows for a large dose TUMORS
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of radiation to be delivered to superficial lesions while avoiding
radiation dosing of deeper nonneoplastic tissues. In two stud-
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ies totaling 64 cats treated with Sr, the complete response (CR) SUSAN E. LANA AND MICHELLE M. TUREK
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rate was 88%; local recurrence was reported in 20% of cats, and
33% of cats developed new lesions in different locations. 15,16 Canine Sinonasal Tumors
The median progression-free interval was 1710 days, with 1- and
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3-year disease-free rates of 89% and 82%, respectively. The over- Incidence and Risk Factors
all MST was 3076 days, and cats with a CR had significantly lon-
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ger MSTs. Tumors of the nasal cavity and paranasal sinuses account for approxi-
mately 1% of all neoplasms in dogs. The average age of dogs with this
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Chemotherapy disease is approximately 10 years, although canine patients as young as
9 months have been reported. 26,27,28 Medium-to-large breeds may be
The most common reported use for chemotherapy in cats with more commonly affected. 26,29 It has been speculated, but is unproved,
nasal planum SCC is intralesional, either alone or with electro- that dolichocephalic breeds (long-nosed) or dogs living in urban envi-
chemotherapy. In one study of 23 cats treated with intratumoral ronments, with resultant increased nasal filtering of pollutants, may
administration of carboplatin to treat SCC of the nasal planum, be at higher risk for developing nasal cancer. 29–31 Exposure to envi-
there was no systemic toxicity and local side effects were minimal ronmental tobacco smoke has been associated with an increased risk
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and self-limiting. A CR was noted in 73% of cats, with a 30% of nasal cancer in a group of dogs in one study, but the same was not
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local recurrence rate and a mean PFS time of 16 months. Intra- true in another. Evaluation of DNA damage in biopsy samples of
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lesional carboplatin was combined with superficial orthovoltage the oropharyngeal region of dogs exposed to cigarette smoke showed
RT in six cats; all cats achieved a CR and five of six cats had local higher levels compared with those not exposed, suggesting that dogs
tumor control at 6 months posttreatment. 18 could be used as a sentinel for early DNA damage caused by expo-
Electrochemotherapy utilizing bleomycin has been described sure to environmental agents, although the numbers evaluated were
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in two case series for a total of 15 cats with nasal planum SCC. 19,20 small. Indoor exposure to fossil fuel combustion products, such as
A CR was noted in 75% to 86% of cats with a DFI between 7 to those produced by coal or kerosene heaters, may contribute to the sug-
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36 months. 19,20 No local or systemic side effects were noted. gested environmental component of this cancer.
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