Page 244 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 13 Radiation Oncology 223

with appendicular OSA 213 ; although radiation dose varied and 3, 6, and 9 months, respectively. 101 Area of bone involvement was
not all dogs received adjuvant chemotherapy, good limb function prognostic for fracture as dogs with involvement of subchondral
bone had a median time to fracture of 4.2 months versus 16.3
and tumor control were observed in some of the patients. A recent
VetBooks.ir study of 46 dogs with OSA treated with SRT showed a median months in dogs without. 101 Occasional grade III acute and late
skin toxicity was also seen in this study, particularly for tumors
survival of 9.7 months, which is comparable to reports of amputa-
tion plus chemotherapy, but a high rate of pathologic fracture. 101 involving the radius. 101 Proximity of the tumor to the skin must
Treatment of axial OSA has been particularly challenging be considered, particularly in locations such as the distal radius
because of the proximity to important normal tissue structures, where minimal soft tissue exists between the bone and skin.
particularly when treating vertebral tumors. In a retrospective Bisphosphonates at time of diagnosis or before SRT may be help-
study of multimodality therapy for axial OSAs, dogs that under- ful for bone maintenance. 220,221 Prophylactic stabilization with a
went curative-intent RT protocols had a longer duration of tumor bone plate or interlocking nail has been investigated, but this was
control (265 days) than those treated with a palliative regimen associated with high rates of severe complications. 222,223
(79 days). 214 SRT was delivered to nine dogs with primary or
96
secondary vertebral OSA. The protocols and doses varied based Lymphoma
on proximity to spinal cord. Five of six dogs had improved pain RT can be important in the treatment of localized lymphoma, and
control, but the overall MST was only 139 days. 191 As more infor- it has an emerging role as an adjuvant therapy in the treatment of
mation becomes available regarding dose constraints and optimal systemic lymphoma. Lymphocytes are exquisitely radiation sensi-
fractionation, RT may have value, but it should currently be con- tive 224 and may undergo apoptosis in addition to classic mitotic
sidered a palliative procedure. death after exposure to radiation. 225 Humans with non-Hodgkin
The mechanism of the amelioration of pain caused by bony lymphoma commonly receive combined modality treatment with
neoplasia is not completely understood, but decreases in intratu- RT and chemotherapy, primarily for stage I or stage II disease, 226
moral pressure, local antiinflamatory effects, tumor cell apoptosis, although this treatment may have a role in more advanced stages
and cytotoxicity to osteoclasts have been proposed. 215 Relief of as well. 227 Combined modality treatment or RT alone is also
pain may occur almost immediately or may be delayed, sometimes beneficial in humans for primary lymphomas of bone, cutaneous
as long as 2 weeks after the start of RT. Human studies have indi- B-cell lymphoma, and mycosis fungoides. 228–231
cated that a single large dose may be comparable or superior to In one study of 10 cats, extranodal lymphomas (nasal cavity,
multifraction protocols using more conventional doses per frac- retrobulbar area, mediastinum, subcutaneous tissue, maxilla, and
tion. 216 In a recent study of 58 dogs, 8 Gy was administered on 2 mandible) were treated with megavoltage RT with or without
consecutive days, providing onset of pain relief within 2 days in concurrent chemotherapy. 232 Complete responses were achieved
91% of patients. 217 Commonly in veterinary medicine, 7 to 10 in 80% of cats and partial responses in two cats. The median
Gy fractions have been administered on days 0, 7, and 21. 218,219 remission time for the cats that experienced complete responses
In one study, 12 of 15 dogs with appendicular bone tumors treated was 114 weeks, and three cats were alive and disease free at 131
with palliative RT had improved limb function, and the MST was weeks. Three cats developed disease outside the radiation field,
130 days. 219 In another study, dogs with appendicular OSA were which suggests that adjuvant chemotherapy or extending the field
given either three fractions of 10 Gy or two fractions of 8 Gy 118 ; for part of the treatment to include additional nodes may improve
74% of 95 dogs experienced pain relief for a median duration of 73 locoregional control.
days. No difference in response was found between the two treat- RT has also been used to treat cutaneous lymphoma in dogs.
ment groups. Sometimes localized pain recurs before metastatic RT of localized lymphoma has been reported to result in pro-
disease becomes life limiting. Palliative RT can be readministered longed remission times. 233 Humans with cutaneous B-cell lym-
as long as the owners understand that continued administration phoma are commonly treated with RT, which generally results in
of large doses per fraction eventually leads to late effects. Clearly, long-term control of disease. The extent of skin involvement and
most palliative OSA RT protocols help relieve pain; the preference the presence of extracutaneous disease are prognostic. 231 Mycosis
of the protocol chosen by the attending radiation oncologist will fungoides is sometimes treated with total skin electron irradia-
likely be based on the geographic location of the clients and the tion in humans. As with cutaneous B-cell lymphoma, the stage of
availability of staffing. disease is prognostic, but human patients with disease confined to
the skin may have prolonged remission times.
Radiation Considerations Half-body RT has been investigated as an adjuvant multicen-
RT is unlikely to have a substantial palliative effect if a displaced tric canine lymphoma. An 8 Gy dose to lymphoma cells reduces
pathologic fracture from a bone tumor is already present. Addi- the surviving fraction to 0.005, which is much greater than the
tional monitoring during anesthesia recovery is recommended to estimated cell kill from one cycle of chemotherapy. 224 In one
prevent the patient from trying to stand prematurely. Nonslick study, RT was administered at the end of the chemotherapy pro-
flooring may be helpful in preventing injury to the affected limb tocol, whereas another sandwiched the RT after the first chemo-
while recovering from anesthesia. therapy cycle. 234,235 Although the protocols were well tolerated,
the additional expense was not justified by patient outcome.
Treatment-Related Toxicities Whole abdomen RT has also been used as a salvage technique
All dogs with bone tumors are at risk of developing pathologic for feline GI lymphoma. 117 In a study of 11 cats with relapsed or
fracture unless the affected limb is amputated. This risk may be resistant alimentary lymphoma treated with 4 Gy fractions on 2
increased with radiation therapy due to palliative benefit, result- consecutive days, 10 cats had either partial or complete clinical
ing in increased weight-bearing load and other potential effects of remission. Acute effects associated with RT were minimal. The
radiation on bone maintenance. For limb sparing using SRT/SRS, post-RT MST was 214 days. 117
patient selection is crucial. A study of 46 dogs treated with SRT An emerging approach in the management of dogs with
for OSA had pathologic fracture rates of 27%, 56%, and 62% at hematologic malignancies, such as lymphoma and leukemia, is