Page 243 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

222 PART III Therapeutic Modalities for the Cancer Patient

from <10% to >70% depending on histologic grade, completeness the median DFI was 32.7 months. 207 Controversy exists regarding
of excision, tumor size, tumor invasiveness, and tumor location. whether the addition of RT is necessary after incomplete excision
of MCT due to moderate recurrence rates; however, a study of 64
Accordingly, risk assessment for local recurrence and the potential
VetBooks.ir recommendation for adjuvant therapy requires consideration of all dogs showed lower recurrence rates (8% vs. 38%) and significantly
longer MSTs (2930 days vs. 710 days) in dogs receiving adjuvant
of these factors. Based on these risks, combining RT and surgery
may be beneficial in cases where complete excision may not be RT compared with dogs not receiving RT. 208
possible. RT can be administered first with the hope of converting RT also is indicated for cutaneous MCT with regional LN
an inoperable tumor into an operable one. This approach has the metastasis. In one study, 19 dogs with MCT and regional node
benefit of reducing the volume of normal tissues irradiated. More involvement were treated with surgical cytoreduction of the pri-
commonly, surgical excision is performed first as a cytoreductive mary site, radiation to the primary tumor and regional node, and
procedure and then followed by RT to eliminate the residual sub- prednisone. 209 The median disease-free ST was 1240 days. In a
clinical disease. In one study, 48 dogs with STS were treated with study of 21 dogs with grade II, stage II MCTs treated with surgi-
surgical cytoreduction followed by RT; eight dogs (16%) devel- cal resection and adjuvant chemotherapy, the addition of RT for
oped tumor recurrence, and the 5-year survival rate was 78%. 196 locoregional control resulted in a longer MST (2056 days) than
In another study of 38 dogs with STS of the body and extremities, dogs not treated with adjuvant RT (1103 days). 210 Palliative RT is
treatment with surgery followed by RT resulted in an MST of commonly used to treat symptomatic locoregional MCT in dogs
2270 days. 139 Recent publications indicate that many low-grade when systemic spread has occurred.
STSs may not recur despite incomplete excision 197–199 ; however,
the level of surgical training can affect outcome. 200 Radiation Considerations
STS can be treated with RT alone; however, tumor control Many tumors of the limbs and trunk extend very close to the skin
is not as durable as with a combination of RT and surgery. 137 surface. Point calculations or 3D-CRT modalities are commonly
RT alone is useful for tumors near the paw/pads, where surgical and successfully used. Application of an appropriate thickness of
options are limited, and in tumors wrapping around the limbs. a bolus material over the skin is frequently needed when treating
Pads in the radiation field initially may slough; however, if appro- with megavoltage photons to avoid underdosing the superficial
priate fractionation schemes are used, the pads regrow and can region. When an extremity is in a radiation field, a 1- to 2-cm
function normally. strip of tissue should be shielded to avoid the risk of lymphedema,
Injection site sarcomas (ISSs) are a significant problem in cats which can present as painful swelling of the distal limb. For micro-
(see Chapter 22). These tumors are challenging to control locally scopic or macroscopic ISSs in cats, radiation planning is chal-
and seem unresponsive to aggressive RT or conservative surgery lenging with 3D-CRT. Inverse planning allows better sculpting
alone. In one study, 33 cats with histologically confirmed ISS around body curvatures and allows a higher dose to be delivered
were treated with RT followed by surgery. 201 The median DFI to the target structures.
and MST were 398 and 600 days, respectively. In another study,
25 cats with subclinical disease after surgery were treated with Treatment-Related Toxicities
RT alone (57 Gy delivered in 3 Gy fractions) and, in some cases, When treating superficial tumors, early effects to the skin are
with adjuvant chemotherapy. 202 The overall MST was 701 days. expected and are restricted to the radiation field. The severity of
Local recurrence was observed in 28.6% of cats. In cats with local these effects are dose related. Depilation is common and in some
recurrence, one tumor developed outside the treatment field, and cases may be permanent. The hair may not return for several
the remaining tumors recurred in the radiation field. Similar find- months and varies in relation to the dose administered to the skin
ings were reported in 78 cats treated with surgical cytoreduction and the individual patient’s sensitivity. Damage to the melano-
followed by RT. 203 In this study, cats that underwent only one cytes may result in hypopigmentation or hyperpigmentation of
surgery before RT had a lower recurrence rate than cats that had the skin and/or alteration of the coat color when regrowth occurs,
more than one surgery. The ST and DFI shortened as the time often resulting in whitish-gray fur (leukotrichia). Dry desquama-
between surgery and the start of RT lengthened. In a study of 79 tion may accompany epilation; this generally does not cause any
cats treated with either pre- or postoperative RT, PCV >25% was problem or discomfort for the patient and usually is not treated.
associated with better outcome (MST 760 days) than in cats with Moist desquamation, which usually appears 3 to 5 weeks after the
PCV <25% (MST 306 days). 204 start of therapy, is associated with pruritus and pain, which can
Cats with surgically nonresectable disease present a greater vary in severity. Self-mutilation exacerbates the problem and may
challenge. Escalation of the radiation dose by delivering the dose lead to ulceration or necrosis. Pain management is an important
in smaller fractions is probably necessary for these patients. IMRT part of the overall treatment plan. NSAIDs are usually the first
or SRT may be beneficial for obtaining adequate dose to the line of therapy if there are no contraindications. Tramadol, aman-
tumor, because appropriate sparing of the lungs, viscera, and spi- tadine, and gabapentin are often added to the regimen. Severe
nal cord is critical in these patients. SRT was used to treat 11 cats late effects to the skin are rare with fractionated RT but include
with nonresectable ISSs. Acute radiation effects were minimal and fibrosis and necrosis.
cats tolerated the treatment well; however, this modality should be
considered palliative as the overall MST was 301 days. 205 Bone Tumors
Cutaneous mast cell tumors (MCTs) can be treated success-
fully with RT (see Chapter 21). The obvious advantage is that Although OSAs are not considered highly radiation-responsive
greater margins may be obtained with RT than with surgery. The tumors, RT may be considered as part of a multimodality therapy
probability of control may be improved if surgical cytoreduc- when surgical excision is not an option (see Chapter 25). RT can
tion is performed first. In a study of 37 dogs with grade II MCTs be combined with chemotherapy and surgery for limb- sparing
treated with cytoreduction and RT, tumor control at 1 and 2 years protocols. 211,212 SRT is currently being evaluated as a limb-
exceeded 90%. 206 In 56 dogs with incompletely resected MCTs, sparing alternative. In one study, SRS was performed on 11 dogs