Page 393 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 20 Melanoma 371

control, although this is somewhat controversial. 142 Most pub-
lished protocols in dogs and cats have used higher dose per frac-
tion protocols, although the relative radiosensitivity of melanoma
VetBooks.ir in companion animal species has not been determined and nor
has the ideal dose per fraction in hypofractionated protocols been
determined.
Hypofractionated protocols have the advantage of fewer treat-
ments with fewer anesthetic episodes, lower cost, and less time
commitment for the owner. These protocols also result in less
severe acute effects. The main disadvantage of hypofractionated
protocols is a lower overall and biologic equivalent dose, which
theoretically results in lower rates of local control and an increased
risk for late side effects. For dogs with a better prognosis, lower
dose per fraction protocols can be used to decrease the risk of
late effects. For a discussion of radiation effects, see the section on
A radiation side effects in Chapter 13.
When planning for RT, most radiation oncologists will treat
both the primary tumor and the regional LN, including the man-
dibular and medial retropharyngeal LNs (Fig. 20.8). These LNs
are included because, as noted earlier in this chapter, normal sized
LNs do not preclude the possibility of metastasis. 109
The reported range of partial and complete responses to RT
are 25% to 67% and 19% to 69%, respectively, yielding an
overall response rate of 82% to 94%, although one recent study
reported a 0% complete and partial response rates and a 73%
overall response rate when including stable disease in this mea-
sure. 107,136–138,140,141 When treating gross disease, responses are
generally rapid and dramatic decreases in tumor volume can be
seen within several weeks of starting therapy (Fig. 20.9). The PFS
ranges from 3.6 to 8.6 months. 107,108,135,136 Interestingly, the lon-
gest reported PFS was from a study in which all dogs had surgery
as part of their treatment protocol. 135 Local recurrence rates after
B RT vary and are confounded by different radiation protocols and
adjuvant therapies used in these studies: local recurrence rates of
• Fig. 20.6 (A) Intraoperative photograph of a melanoma confined to the 26% and 27% have been reported after the treatment of micro-
lateral aspect of the caudal dental arcade. Surgical margins in this case scopic disease, whereas local progression was reported in 45% of
were approximately 1.5 cm around the rostral, dorsal, and caudal borders 107,135
of the tumor. The incision along the hard palate was made just medial to dogs after the irradiation of macroscopic disease.
the dental arcade. (B) Surgical margins are inked with a green surgical The reported MSTs for dogs treated with RT range from 4.5
ink and histopathologic assessment showed complete histologic excision. to 14.7 months (see Table 20.1). 106–108,135-138,139–141 The majority
of these studies are retrospective and use different RT equipment,
protocols, and adjuvant therapies, making it difficult to determine
an ideal treatment regimen. Reported protocols include 2 to 4 Gy
fractions daily for 12 to 19 treatments (45.6–57 Gy), 107,108 4 Gy
per fraction three times weekly for 4 weeks (48 Gy), 6 Gy per frac-
tion twice weekly for 2.5 (30 Gy) or 3 weeks (36 Gy), 135,136,141
6 Gy per fraction once weekly for 4 weeks (24 Gy), 135 6 Gy once
weekly for 6 fractions (36 Gy), 106,141 8 Gy per fraction once
weekly for 3 (24 Gy) or 4 fractions (32 Gy), 135,137,141 9 Gy per
fraction once weekly for 4 fractions (36 Gy), 107,138,139 and 10 Gy
per fraction once weekly for 3 fractions (30 Gy). 107
Five cats with oral melanoma and treated with RT have been
26
reported. There was one complete response and two partial
responses with an MST of 146 days. Three cats were treated adju-
vantly: one cat with carboplatin, one cat with carboplatin and
mitoxantrone, and one cat with a DNA-based vaccine as an adju-
vant to RT.
Chemotherapy may have a role in the management of dogs
and cats with MMs treated with RT, either as a radiation sensitizer
and/or because of the risk of metastatic disease. In one study, 39
• Fig. 20.7 Malignant melanoma in the interdigital webbing between digits
3 and 4 (same dog as in Fig. 20.2). Surgical margins were between 1 and dogs with incompletely excised oral melanomas were treated with
2
2
2 cm. Complete histologic excision was achieved in this case after partial either cisplatin (10–30 mg/m IV) or carboplatin (90 mg/m IV)
foot amputation. once weekly approximately 1 hour before receiving RT. 106 The