Page 1398 - Clinical Small Animal Internal Medicine

1336 Section 11 Oncologic Disease

was 23.3 months. Another study evaluating 35 dogs with stable disease in 87% of the tumors treated with a median
VetBooks.ir soft tissue sarcomas that were excised to microscopic time to progression of 8.7 months.
As an alternative to once weekly fractionation, a retro-
disease followed by daily radiation to a total dose of
42–57 Gy reported an overall median survival of 61.7
consisting of 4 Gy fractions given over five consecutive
months. spective study evaluated a palliative radiation protocol
Interestingly, a recent study evaluated intentional mar- days. In this study, 10 soft tissue sarcomas were treated.
ginal excision of canine STS of the limb followed by a A measurable overall response rate of 80% was reported,
coarsely fractionated radiation protocol consisting of with tumor control achieved in 100% of patients and a
four, once weekly, 8–9 Gy fractions. Fifty‐six dogs were median progression‐free survival of 5.7 months. With
included. The results compared favorably to studies this protocol, the risk for late side‐effects is about half
using more fractionated “definitive” protocols, as the that of the traditional 8 Gy × 4 fraction protocol.
one‐, two‐, and three‐year disease‐free intervals were Many advancements have been made in the field of
82%, 74%, and 70%, respectively, with a local recurrence radiation oncology, and with these advancements and
rate of 18%. The main disadvantages of using coarsely improved technology, stereotactic radiation can now be
fractionated protocols are reduced efficacy and the used to treat STSs. Stereotactic radiation consists of high
increased risk of late radiation toxicity, which may be sig- doses of radiation precisely targeted at the tumor with
nificant and include bone necrosis and radiation‐induced rapid dose fall‐off outside the tumor to help spare nor-
tumors at the site. In the study, the risk of developing late mal tissues and limit side‐effects. Stereotactic protocols
radiation side‐effects was low when the fraction size did generally consist of 1–3 fractions of radiation given on a
not exceed 8 Gy. consecutive‐day basis. The goal of stereotactic radiation
is to deliver an ablative dose of radiation to the tumor
Radiation Therapy and achieve long‐term control of the disease. Currently,
If surgical excision of a soft tissue sarcoma is impossible there are no published veterinary studies on stereotactic
because of tumor location or size, radiation therapy can radiation and STS, but it is expected that this will be a
be applied; however, measurable and bulky tumors tend very viable treatment option for nonresectable STS.
to have a poor long‐term response to primary radio-
therapy. Tumor control with conventional doses of radi- Chemotherapy
ation alone (40–48 Gy) are difficult to obtain because of Adjuvant chemotherapy at times is useful in the treat-
their low growth fraction, relatively long doubling time, ment of STS. It is best utilized when combined with radi-
and tendency to develop hypoxic regions within the ation therapy and/or surgery. Chemotherapy alone does
tumor parenchyma. Reported one‐year control rates are not seem to be effective, with the exception of providing
30% at 35 Gy (10 × 3.5 Gy), 35% at 40 Gy (10 × 4 Gy), 48% palliation in some cases. Doxorubicin‐based protocols,
at 45 Gy (10 × 4.5 Gy) and 67% at 50 Gy (10 × 5 Gy). At either alone or in combination with cyclophosphamide,
two years, control rates vary from 12% at 40 Gy to 33% have shown the most promise with an overall response
at 50 Gy. rate of 23%.
As a single modality, radiation has generally been con- Despite the fact that STS are slow to metastasize, the
sidered palliative for STS with control defined as a slowly metastatic rate for cutaneous STS is grade dependent
regressing or stable tumor mass. The goal of palliative or and varies from less than 15% (grade I and II) to 41%
coarsely fractionated radiation therapy is to alleviate (grade III). Due to the higher metastatic rate of grade III
pain, swelling, and inflammation associated with the soft tissue sarcomas, postoperative chemotherapy should
tumor, in the hope of improving quality of life. Typical be considered as it may prevent or delay metastasis.
palliative protocols involve fewer fractions of larger radi- Single‐agent doxorubicin, mitoxantrone, or combination
ation doses (4–10 Gy), often given on a weekly basis. The protocols are most commonly used.
delivery of a small fraction size once per week is unlikely Metronomic chemotherapy (continuous administra-
to lead to a durable response because tumor cells have a tion of chemotherapy drugs at doses that are significantly
significant amount of time to repair damage and prolifer- lower than conventional maximally tolerated dose ther-
ate between treatments. Therefore, larger doses of radia- apy) has been evaluated in canine STS. A retrospective
tion are often delivered to optimize killing of tumor cells, study evaluated 85 dogs with incompletely resected
with the increased risk of late radiation toxicity accepted STSs. Thirty dogs received continuous cyclophospha-
2
as part of the treatment. Since palliative radiation therapy mide (10 mg/m ) and standard‐dose piroxicam therapy
is administered to improve quality of life, not to prolong (0.3 mg/kg). Fifty-five control dogs did not receive addi-
it, patients likely will not live long enough to experience tional therapy. When comparing these two groups,
permanent late effects. A study evaluating nonresectable treated dogs had a disease‐free interval of 13.7 months
STS treated with a palliative protocol (3 × 8 Gy) reported which was significantly longer compared to 7.0 months