RADIATION ONCOLOGY PERSPECTIVE
Nathan Lee, DVM, DACVR (radiation oncology)
Dr. Nathan Lee
It has been well-established that adjuvant radiation therapy can be used for local control of incompletely excised grade II
mast cell tumors.34,35 A study of 32 dogs with incompletely excised grade II mast cell tumors reported an 88% disease-free survival at two to five
years after treatment with definitive radiation therapy with a total dose of 54 Gy over 39 days.34 Another report of 37 dogs treated for incompletely excised grade II mast cell tumors showed a disease-free survival of 93%
at three years for dogs treated with megavoltage or orthovoltage radiation at doses of 46.2 to 48 Gy, using several different
fraction sizes and schedules.35,36
Definitive radiation therapy
Grade II mast cell tumors that have metastasized to regional lymph nodes can also benefit from definitive radiation therapy.
In one report, 19 dogs with various grade tumors (16 were grade II) and regional lymph node metastasis that were treated with
a radiation dose of 48 to 57 Gy to the primary tumor and the regional lymph node had a median disease-free survival time of
Prophylactic radiation therapy
If the regional lymph node is not involved, based on cytology of the draining lymph nodes, there is some controversy whether
prophylactic irradiation of the regional lymph node is warranted. In a recent report, 45 dogs with stage 0 incompletely excised
grade II mast cell tumors underwent definitive radiation therapy, with 24 of these patients receiving prophylactic radiation
to the primary draining node. When comparisons were made between the dogs that received prophylactic irradiation of the regional
lymph node and dogs that did not, no statistically significant differences were observed in disease-free survival rate, time
to local recurrence, or overall survival rate.37
Large mast cell tumors that cannot undergo cytoreduction by surgery can be treated with hypofractionated radiation therapy
and either prednisolone alone38 or prednisone and toceranib.39 In a study from 2004, dogs treated with hypofractionated radiation therapy and prednisolone alone had an overall response
rate of 88.5% and a median progression-free interval of 1,031 days.38 A newly published study looking at hypofractionated radiation therapy, prednisone, and toceranib for dogs with measurable
mast cell tumors had an overall response rate of 76.4% and a median progression-free interval of 316 days.39 Unfortunately, no studies have investigated hypofractionated radiation therapy alone for gross mast cell tumors.
The current radiation protocol at The University of Tennessee for incompletely excised grade II mast cell tumors is 48 Gy
given in 3 Gy fractions (Monday through Friday). We prophylactically treat the draining lymph node in dogs with stage 0 disease.
If stage II disease is present, we extend the prophylactic treatment to the next lymph node in the chain (e.g. if the popliteal lymph node is involved, the internal and external iliac lymph nodes and bed are treated prophylactically).