Page 242 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 13 Radiation Oncology 221

days, respectively; there was no significant difference between the commonly treated tissues, and volume may be an important factor
groups. A control arm of 30 dogs with glial tumors was treated for brain and spinal lesions. Early delayed effects can occur 1 to 3
183
months after treatment and may be due to transient demyelination
medically and their MST was 94 days.
VetBooks.ir radiation, and surgical access is limited. Dogs with pituitary tumors or from edema associated with cytokine release from dying tumor
Pituitary tumors in dogs and cats generally are responsive to
cells. Animals with early delayed effects may have signs similar to
treated with fractionated RT have MSTs varying between 1 and or different from those at the initial presentation or they may be
2 years. 184,185 A study comparing 19 dogs with pituitary tumors stuporous. Early delayed effects occur in up to 40% of humans
receiving RT (48 Gy in 16 daily fractions of 3 Gy) to untreated undergoing brain RT; symptoms include headache, lethargy, and
dogs found 1-, 2-, and 3-year survival rates of 93%, 87%, and 55% exacerbation of focal neurologic signs. 192 In animals, clinical signs
in the RT group and 45%, 32%, and 25% in the unirradiated often are transient, but response time can be slow. Early delayed
group, respectively. 186 Fractionated pituitary irradiation in dogs is effects are treated with the administration of systemic corticoste-
more effective at delaying tumor growth than in controlling adre- roids, and sometimes supportive care is required. MRI may show
nocorticotropic hormone (ACTH) secretion. 186,187 Eucortisolism an apparent increase in tumor size and tumor enhancement dur-
is seen in some patients after RT; however, pre-ACTH and post- ing this time. Focal enhancement in a normal brain associated
ACTH cortisol levels should be monitored at regular intervals so with edema and demyelination may also be present. 193 In a study
that medications can be modified, if indicated. of SRT for canine meningiomas, 37% of dogs showed mild-to-
Cats seem to have marked clinical improvement of associated moderate exacerbation of neurologic signs 3 to 16 weeks after
endocrinopathies when treated with RT. 188 In one study, 53 cats treatment. Most dogs responded to systemic corticosteroids and
8
with functional pituitary adenomas associated with feline acro- supportive care, and dosimetric information relating to the toxic-
megaly were treated with SRT. Diagnosis was based on history, ity helped establish dose constraints for the brain. 8
98
physical examination, laboratory results, and cross-sectional imag- Late effects probably occur in veterinary patients more often
ing of the pituitary region. The overall MST was 1072 days. Of than identified. Late effects generally occur at least 6 months after
the 41 cats where insulin information was available, 95% expe- treatment but can also occur years later. Brain necrosis is the most
rienced a decrease in required insulin dose, with 32% achieving commonly identified effect. The probability of late brain effects
diabetic remission. Hypothyroidism developed in 14% of treated depends on the total dose, fraction size, and the volume of brain
cats. Interestingly, three cats with no pituitary enlargement, but irradiated. In a study of 83 dogs with brain masses treated with
98
who met other criteria, were treated and responded. Previous a hypofractionated protocol (38 Gy administered in five weekly
studies using fractionated RT showed durable response to neuro- fractions), brain necrosis was confirmed or suspected in 14% of
logic disease but limited endocrine responses (Chapter 26). 188,189 dogs. 194 Signs are similar to those associated with early delayed
Two recent manuscripts explored the treatment of trigeminal effects, although the response to corticosteroids is limited. Clini-
nerve sheath tumors with SRT, with MSTs ranging from 441 to cally distinguishing between late effects and tumor recurrence can
745 days. 190,191 In both studies, all tumors had an intracranial often be difficult. CT or MRI evaluation can be misleading. Not
component. Extension into the cranial nerve can involve one to all brain tumors completely regress after treatment, therefore the
three of the branches, and administering dose to involved branches presence of a mass does not always indicate a recurring tumor. A
could be important for tumor control. 191 prudent course is to obtain a CT or MRI evaluation 6 months
after treatment to serve as a reference if clinical signs develop in
Radiation Considerations the future.
The integration of 3-D imaging, patient positioning devices, and
advanced treatment planning techniques have the potential to Superficial Tumors of the Trunk and Extremities
improve tumor targeting and sparing of normal brain tissue. For
many brain tumors, IMRT will not provide an improved dose Many tumors involving the trunk or extremities are amenable to
distribution compared with 3D-CRT; however, dose sculpting treatment by RT. Combining RT and surgery has the potential
may be beneficial for cranial nerve tumors. An additional con- to enhance tumor control and improve the functional outcome
sideration in patients undergoing RT for a brain tumor is the over either modality alone. For many invasive tumors, conserva-
anesthetic risk associated with increased intracranial pressure tive surgery with adjuvant RT can provide a viable alternative to
or brainstem disease; an appropriate anesthetic regimen should radical surgery. For tumors in nonresectable locations, SRT alone
be selected to minimize the risk of complications. Specifically, may provide a good outcome depending on tumor type and vol-
patients should be ventilated while under anesthesia to decrease ume. The surgeon and radiation oncologist should consult before
, and anesthetic agents that decrease (or at least do therapeutic intervention is started to develop an overall treatment
blood Pco 2
not increase) intracranial pressure should be selected. SRT limits approach.
the number of anesthetic episodes, which may be beneficial in Soft tissue sarcomas (STSs) are common tumors arising in a
unstable patients. wide range of locations, most commonly arising as subcutane-
ous masses. Hemangiopericytomas, FSAs, neurofibrosarcomas,
Treatment-Related Toxicities myxosarcomas, and nerve sheath tumors are classified together as
For most brain tumors, acute effects to the skin can be avoided STSs because of their similar biologic behavior (see Chapter 22).
with megavoltage RT. Occasionally, ocular and otic side effects or Metastases are uncommon with grade I and grade II STSs; there-
mucositis to the caudal oral cavity may be seen if these structures fore local tumor control is the primary concern, whereas grade III
are in or adjacent to the treatment field. The radiation tolerance is tumors have more aggressive metastatic potential. STSs are locally
lower when the entire brain is treated; this limits prescription of a invasive, and tumor cells may extend far beyond the bulk of the
dose that is adequate for tumor control but still has an acceptable tumor. Surgery alone can result in long-term tumor control if the
probability of late effects. The radiation tolerance of brain and tumor can be excised completely. 195 When wide excision is not
spinal tissues is generally considered to be less than that of other feasible, recurrence rates after narrow excision are reported to vary