Radiation Therapy for Melanoma

Oral melanoma in dogs is typically a very aggressive tumor. The local tumor is usually very invasive and destructive. In addition, melanoma also has a high rate of metastasis to local lymph nodes and the lungs. Development of the Oncept ® DNA melanoma vaccine may help slow down progression of the disease and prolong survival for these dogs. However, the initial studies of these types of immunotherapy all show that these treatments only work well when the local tumor is controlled.

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Figure 1 – CT scan of maxillary melanoma, showing bone destruction

Local tumor control for melanomas typically requires surgery. However, in some cases, surgery may not be possible or it may not be indicated. For dogs where the tumor cannot be easily removed with surgery, radiation therapy can be a very effective option.

What is the Best Dose?
Early studies in the treatment of melanomas in people found that melanomas may not respond well to the traditional fractionated radiation, which uses small doses of radiation for a large number of treatments. In the early 70s it was discovered that some melanomas are able to repair radiation damage very well, so that they would only respond to large individual doses of radiation (Barranco, Cancer Research 1971). Based on this, more recent studies have used a small number of doses with a large dose per treatment.

Two veterinary studies from the mid 90s showed that this type of hypofractionated radiation can be very successful. Dogs had a complete response rate between 50 and 70% and overall response rate of 83 to 94% (Bateman, JVIM 1994; Blackwood, JAVMA 1996). A more recent study looked at the effect of radiation and chemotherapy for dogs with incompletely excised tumors and showed that local tumor control was excellent with minimal side effects (Freeman JVM 2003).

Another added benefit of this type of radiation is that using hypofractionated radiation typically results in very limited side effects. Dogs treated with this type of radiation typically have mild to moderate mucositis, which resolves within one to two weeks after radiation.

Cyberknife offers the ability to treat with a very large dose to the tumor with minimal normal tissue affected. For more information regarding Cyberknife see http://vetcyberknife.com/

Locoregional Disease Control
Another major issue for dogs with melanoma is the high risk of tumor spread. One of the first places melanoma’s typically spread is to the local lymph nodes. The most common sites are the mandibular and retropharyngeal nodes, but it also can spread to tonsils.

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Figure 2 CT scan of a dog with an enlarged retropharyngeal lymph node

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Figure 3 Enlarged, pigmented tonsil due to metastatic melanoma

4Figure 4 Three – dimensional radiation plan for treatment of melanoma showing 2 beams treating the primary tumor (blue and green)
and one electron beam (yellow) treating the left mandibular node (shown in 3-D as a green volume)

Another concern is that metastatic melanoma can be present in the lymph nodes, even when they are normal size. One study showed that as many as 40% of dogs with normal sized lymph nodes can have metastatic disease. One great benefit of radiation therapy for melanoma is that it allows for treatment of both the primary tumor and all of the local lymph nodes, tonsil etc. Although there haven’t been any studies yet looking at the effectiveness of including lymph nodes for dogs with melanoma, this may help control the loco-regional disease.

What about Cats?
Unfortunately, there is very little information about cats with oral melanoma. One study showed that they can have repsonses to radiation. (Farrelly, VRUS 2004) However, these responses were not very long lived. Future options may include adding in immunotherapy or increasing the doses of radiation used.

Submittted by Dr. John Farrelly DVM, MS,
ACVIM (Oncology), ACVR (Radiation Oncology)
Radiation Oncologist/Medical Oncologist at The Veterinary Cancer Center

References

Barranco SC, Romsdahl MM, Humphrey RM. The radiation response of human malignant melanoma cells grown in vitro. Cancer Res. 1971 Jun;31(6):830-3.

Bateman KE, Catton PA, Pennock PW, Kruth SA. 0-7-21 radiation therapy for the treatment of canine oral melanoma. J Vet Intern Med. 1994 Jul-Aug;8(4):267-72.

Blackwood L, Dobson JM. Radiotherapy of oral malignant melanomas in dogs. J Am Vet Med Assoc. 1996 Jul 1;209(1):98-102.

Freeman KP, Hahn KA, Harris FD, King GK. Treatment of dogs with oral melanoma by hypofractionated radiation therapy and platinum-based chemotherapy (1987-1997). J Vet Intern Med. 2003 Jan-Feb;17(1):96-101.

Williams LE, Packer RA. Association between lymph node size and metastasis in dogs with oral malignant melanoma: 100 cases (1987-2001). J Am Vet Med Assoc. 2003 May 1;222(9):1234-6.

Farrelly J, Denman DL, Hohenhaus AE, Patnaik AK, Bergman PJ. Hypofractionated radiation therapy of oral melanoma in five cats. Vet Radiol Ultrasound. 2004 Jan-Feb;45(1):91-3.

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