Page 521 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 24 Tumors of the Respiratory System 499

rapidly and slowly renewing tissues, respectively, and depend on or seizures, and optic nerve degeneration), bone (osteonecrosis),
daily dose to the tissue, total dose, overall treatment time, and and skin (fibrosis). 104,105,119,122,127–130 Late complications develop
126
The severity of side effects can there-
months to years after therapy and are generally irreversible. The
volume of tissue treated.
VetBooks.ir fore vary among protocols and also between individuals on the most commonly observed clinically relevant late effects in dogs
treated with definitive RT for sinonasal tumors are those affecting
same treatment schedule, depending on the tissues included in
the radiation field. Acute toxicities associated with irradiation ocular structures when it is not possible to spare the eyes using
of sinonasal tumors are dose- and schedule-dependent and can conformal radiation delivery. 104,119 Late ocular changes in the dog
involve the oral cavity (oral mucositis), eye (keratoconjunctivitis typically occur 6 to 9 months after RT and most often include
and blepharitis), nasal cavity (rhinitis), and skin (desquamation) keratoconjunctivitis sicca, cataract formation, and blindness in
(Fig. 24.8). 104,105,119,122,127 Acute effects develop during the course the irradiated eye if radiation doses are not limited. 122 The lens
of RT and resolve within 2 to 8 weeks after treatment. 104,105,119,127 is particularly sensitive to radiation damage, with cataracts form-
Oral antibiotics, pain medications, and/or ocular medications ing in people at doses as low as 2 to 5 Gy. 131 Other late effects
including artificial tears may be needed to support the patient are rare when standard radiation doses per fraction are used. In
during this period. Rarely, temporary esophagostomy or gastros- general, the risk of late effects increases when doses per fraction
tomy tube feedings may be indicated if oral mucositis is severe to are increased. The use of highly conformal RT delivery techniques
maintain adequate nutritional intake. allows for avoidance of critical structures including the eyes and
Late radiation effects, although less common than acute effects, brain. 122
are more detrimental and long lasting. Their development is also Overall, although the majority dogs with sinonasal neoplasia
dose-dependent and should be prevented with thoughtful RT experience a favorable tumor response to RT with resolution of
planning. Tissues that may be affected include the ocular lens clinical signs, the long-term prognosis is poor. Even when treated
(cataracts), cornea (keratitis, atrophy, keratoconjunctivitis sicca), with a definitive RT protocol, most dogs die or are euthanized
anterior uvea (uveitis), retina (hemorrhage and degeneration), as a result of local disease progression. An investigation of treat-
neuronal tissue (brain necrosis, causing neurologic changes and/ ment failure patterns after full-course MV irradiation showed that
the median duration of local control in 24 dogs was 312 days. 132
Marked tumor regression (90% reduction in size) was observed
using CT in 46% of cases and was associated with a longer dura-
tion of local control than that seen in dogs in which tumor response
to radiation was less favorable (389 vs. 161 days). 132 Most of the
dogs in that series experienced local progressive disease, which
affirms the need for more effective treatment strategies.
The following approaches have been investigated to improve
local control using conventionally fractionated RT.
1. Definitive-intent RT (4.2 Gy × 10 daily fractions) followed by
surgical exenteration of residual or recurrent disease showed
promise in a small series of dogs (n = 13), with a MST of 47
months, compared with 19 months for dogs treated with radia-
tion alone. 121 In this study, complete exenteration of the nasal
cavity involving removal of all turbinates and periosteal lin-
ing in the nasal cavity was performed. The combination treat-
A ment was associated with an increased incidence of late effects,
including rhinitis (bacterial and fungal), osteomyelitis, and
fistula formation, but MST was the longest reported for dogs
with nonlymphomatous sinonasal tumors. 121 The risk of these
complications may be reduced with a more limited and focused
extirpation of intranasal tissue in the region of the tumor and
with the use of highly conformal radiation delivery techniques
that minimize dose to normal tissues. A larger group of dogs
must be treated in this manner to confirm the safety and effi-
cacy of this modified surgical approach. Another study (n = 16)
evaluating exenteration after definitive-intent preoperative RT
(median 3 Gy × 18 daily fractions) resulted in a MST of 457
days (15 months) with no long-term side effects observed. 133
The surgical approach used in these cases was not described
and some dogs had advanced metastatic disease at the time of
surgery, potentially contributing to the less favorable MST.
B 2. A logical and intuitive approach to improve local control is to
increase radiation dose. This was investigated in an older study
• Fig. 24.8 Potential acute effects of radiation therapy (RT) in dogs. (A) using a boost technique in which the total radiation dose was
Resolving desquamation 1 month after definitive RT for a nasal tumor. escalated to 57 Gy without an increase in overall treatment
Note the hair loss in the radiation field. (B) Oral mucositis on the last day
of a definitive radiation protocol. Note the yellowish material on the inner time. The treatment, delivered using nonconformal radiation
aspect of the upper lip. This represents dead epithelium that is being shed and without the benefit of computerized treatment planning
from the mucosa. in most dogs, proved too toxic with respect to acute effects and