Page 437 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 22 Soft Tissue Sarcomas 415

Hypofractionated RT after either incomplete or close complete
(<3 mm) histologic excision of STSs has also been reported with
encouraging results.
Protocols reported have utilized weekly
140,141
VetBooks.ir fractions of 6 to 9 Gy to a total dose of 24 to 36 Gy. 140 An 18% to
21% local recurrence rate was reported in these studies; however,
the majority of STSs were initially small (median size 3.6 cm) and
either grade I or II (83%). 140,141 Acute toxicity was mild, but late
toxicities, although uncommon, were noted in both studies. 140.141
An increased risk of late toxicity generally occurs with larger radia-
tion doses per fraction and this risk must be considered in a group
of patients who are expected to survive for prolonged periods
after RT. Progression-free intervals (PFIs) in these studies were
698 days to not reached, and the probability of being free of local
tumor recurrence at 1, 2, and 3 years was 81%, 73%, and 73%,
respectively. 140,141 In one study, delaying hypofractionated RT for
more than 4 weeks after surgery was associated with an improved
• Fig. 22.11 Planned marginal resection of a soft tissue sarcoma in a dog. outcome, with local recurrence nine times more likely in dogs in
Marginal resection followed by full-course postoperative radiation therapy which RT was started less than 4 weeks after surgical excision. 140
provides excellent local tumor control and preserves both the limb and limb Histologic grade was prognostic for both median PFS and OST
function. Radiation therapy should not involve the limb circumferentially to in one study. 141 Median PFS times were 1904 days, 582 days, and
preserve both lymphatic and venous drainage of the distal extremity. If 292 days for dogs with grade I, II, and III STSs, respectively; the
close but clean margins were obtained for a grade I soft tissue sarcoma, median tumor-specific OST was not reached for dogs with grade
observation alone may be an acceptable alternative.
I and II STSs, but was 940 days for dogs with grade III STSs. 141

resection combined with full-course postoperative RT is an attrac- Radiation Therapy for Gross Disease
tive alternative to limb amputation for extremity STS (Fig. 22.11).
This multimodality approach requires additional planning and RT can also be used a single modality, usually for palliation of
costs, but preserves the limb and limb function. Surgery involves clinical signs. RT alone, using a cumulative dose of 50 Gy, resulted
completely removing all grossly visible tumor and then marking the in 1- and 2-year tumor control rates of 50% and 33%, respec-
lateral, proximal, and distal extents of the surgical field with radi- tively. 142 Measurable and palpable (i.e., macroscopic) STSs are
opaque clips to assist in planning of RT. 133 Migration of the radi- resistant to long-term control with conventional doses of radia-
opaque clips has been reported but does not significantly influence tion alone (40–48 Gy). 143,144 Although one study reported a 30%
the planned radiation field. 133 complete response rate with RT alone, 145 these tumors do not rap-
RT should be started a minimum of 7 days postoperatively idly regress after RT and, if there is significant tumor shrinkage, it
to minimize the risk of radiation-induced complications with the is not usually a durable response.
surgical wound, such as delayed healing and dehiscence. 134 Full- Hypofractionated RT has been reported for the treatment of
course fractionated protocols are recommended, with reported macroscopic STSs using a number of different protocols, includ-
schedules including 3.0- to 4.2-Gy fractions on a Monday-to-Fri- ing 3 to 4 fractions of 8 Gy once weekly for a total dose of 24 to
day or Monday–Wednesday–Friday schedule for a total dose of 42 32 Gy, 5 to 6 fractions of 6 Gy once to twice weekly for a total
to 63 Gy. 135–138 The optimal fractionation and total dose schemes dose of 30 to 36 Gy, and 5 fractions of 4 Gy for a total dose of
for canine STS have not been determined, but cumulative doses 20 Gy. 146–150 The results of hypofractionated RT are similar to
greater than 50 Gy are recommended because local tumor control those of full-course RT in the gross disease setting. In two studies,
is better with higher cumulative doses. 135 Acute side effects of RT, the overall response rate was 46% to 50%, 146,148 whereas stable
such as moist desquamation, are relatively mild and transient. 136 disease was more common in two other studies. 147,149 The median
Local tumor control and ST are very good when incompletely PFI ranged from 155 to 419 days. 146–150 Prognostic factors for
resected STSs are treated with postoperative, definitively frac- median PFI include tumor location and previous surgeries. The
tionated RT with local tumor recurrence reported in 16.7% to median PFI is significantly better for limb STSs (466 days) than
36.8% of dogs, 135–137 and 1-, 2-, 3-, and 4-year local control rates STSs located on the head or trunk (110 days), and dogs treated
of 71% to 84%, 60% to 81%, 57% to 81%, and up to 81%, with more than one surgery had a significantly decreased median
respectively. 135,136,138 The local tumor recurrence rates may have PFI (105 days) compared with dogs treated with one or no surgery
been adversely affected by the inclusion of incompletely excised (420 days). 148 The MST after hypofractionated RT for macro-
oral FSAs in one study. 135 The median time to local recurrence scopic STS is 206 to 513 days. 146–150 STS location has a signifi-
is 412 days to more than 798 days. 105,135–137 The median time to cant effect on MST, with STSs located on the limbs (579 days)
recurrence was significantly shorter in dogs with grade III STSs having a better outcome than those on the head (195 days) or
(78 days) compared with dogs with grade I and II STSs (>1416 trunk (190 days). 148 In this study, 40% of dogs were also treated
days) in one study. 135 The overall MST for incompletely resected with metronomic chemotherapy. Metronomic chemotherapy did
nonoral STSs treated with postoperative RT is 2270 days, with not improve median PFIs, but this adjunctive treatment did sig-
survival rates ranging from 80% to 87% at 1 year, 72% to 87% at nificantly improve MSTs (757 days compared with 518 days for
2 years, 92% at 3 years, and 76% at 5 years. 135–139 In one study, dogs not treated with metronomic chemotherapy). 148
dogs with STSs with a mitotic index greater than 9 per 10 high- Preoperative RT is becoming commonplace in veterinary
power fields (HPFs) were more likely to have local recurrence and oncology. The rationale and advantage of administering RT before
shorter ST. 139 surgery are that (1) the radiation field is smaller because, after