ABSTRACT. The object of this study was to evaluate hypofractionated multiportal field and two-portion (rostral and caudal portions divided by the eyelid) radiation therapy for canine nasal tumors. Sixty-three dogs underwent multiportal hypofractionated radiation therapy. The radiation field was divided into rostral and caudal portions by the eyelid. Treatments were performed four times for 57 dogs. The median irradiation dose/fraction was 8 Gy (range, 5-10 Gy); the median total dose was 32 Gy (10-40 Gy). Improvement of clinical symptoms was achieved in 53 (84.1%) of 63 cases. Median survival time was 197 days (range, 2-1,080 days). Median survival times with and without destruction of the cribriform plate before radiotherapy were 163 and 219 days, respectively. There was no significant difference between them. No other factors were related to survival according to a univariate analysis. All radiation side effects, except one, were grade I according to the VRTOG classification. It was not necessary to treat any dogs for skin side effects. One dog (1.6%) developed an oronasal fistula 1 year after completion of radiation therapy. This radiation protocol may be useful in reducing radiation side effects in dogs with cribriform plate destruction.KEY WORDS: canine, hypofractionated radiotherapy, multiportal fields, nasal tumor, palliative.J. Vet. Med. Sci. 73(2): 193-197, 2011 Tumors involving the nasal cavity and nearby sinuses are uncommon in dogs, and they account for approximately 1% of all reported canine tumors [29]. Because nasal tumors metastasize rarely, late in the course of disease, therapy is directed at controlling localized disease [19]. Without treatment, the median survival time of dogs has been reported to be 1.5-4.1 months [22,24,30]. Surgery alone never prolongs survival time [16,22], but it may also deteriorate quality of life. In regard to chemotherapy, Langova et al. reported the effectiveness of alternating doses of doxorubicin and carboplatin in conjunction with oral piroxicam [14], though their sample size was small [14]. Radiation therapy is the treatment of choice [29], and reported survival times have ranged from 7. 4 -47.7 months [1-3, 8, 11-13, 17, 20, 21, 23, 27, 28, 30].Various radiation treatment protocols in veterinary medicine have been reported. For total dose increasing, 19 [11] or 21 [5] fractions at 3 Gy/fraction over 1 month have been used. Because this method requires frequent anesthesia and is expensive, it is a heavy burden on the dog and owner. Hypofractionated radiotherapy has been used to address these issues [7,18].Hypofractionated radiation therapy results in late side effects that are more severe than with fractionated radiation therapy; blindness was reported following radiation treatment of dogs with nasal tumors [7,18]. To address this, multiportal field and two-portions (rostral and caudal portions divided by the eyelid) radiotherapy (Fig. 1) have been used to deliver the radiation dose to the tumor and achieve a decreased dose to surrounding tissue and, thus, pr...