Abstract
Canine malignant melanoma (CMM) is a locally and systemically aggressive cancer that shares many biological and clinical characteristics with human mucosal melanoma. Hypofractionated radiation protocols have been used to treat CMM but little is known about its radiation biology. This pilot study is designed to investigate response of CMM cell lines to various ionizing radiations and cytotoxic agents to better understand this canine cancer. Four CMM cell lines were evaluated by clonogenic survival assay under aerobic and hypoxic conditions and parameters such as alpha beta (α/β) ratio, oxygen enhancement ratio (OER), and relative biological effectiveness (RBE) were calculated after 137Cs, 6 megavoltage (MV) photon, or carbon ion irradiation. Six cytotoxic agents (cisplatin, camptothecin, mitomycin C, bleomycin, methtyl methanesulfonate and etoposide) were also assessed for their efficacy. Under aerobic condition with 6 MV photon, the α/β ratio of the four cell lines ranged from 0.3 to >100, indicating a wide variation of cellular sensitivity. The ratio increased under hypoxic condition compared to aerobic condition and this was more dramatic in 137Cs and 6 MV photon treatments. OER of carbon was lower than 137Cs at D10 in 3 of the 4 cell lines. The RBE values generally increased with the increase of LET. Different cell lines showed sensitivity/resistance to different cytotoxic agents. This study revealed that CMM has a wide range of radiosensitivity and that hypoxia can reduce it, indicating that widely used hypofractionated protocols may not be optimal for all CMM patients. Several cytotoxic agents that have never been clinically assessed can improve treatment outcome.
Highlights
Canine malignant melanoma (CMM) is a common oral and cutaneous tumor in dogs, and recently CMMs have been considered as a large animal model for human mucosal melanomas [1]
In a previously reported study that evaluated clinical outcomes after different radiation therapy protocols to treat CMM, no significant advantage in treatment outcome was found in dogs that were treated with a fractionated protocol compared to those that were treated with a hypofractionated protocol
Cell survival was highest in all cell lines for the lowest linear energy transfer (LET), 137Cs, treatment,2f.1o.lSluorwviveadl Cburyves6MV photon carbon
Summary
Canine malignant melanoma (CMM) is a common oral and cutaneous tumor in dogs, and recently CMMs have been considered as a large animal model for human mucosal melanomas [1]. In a previously reported study that evaluated clinical outcomes after different radiation therapy protocols to treat CMM, no significant advantage in treatment outcome was found in dogs that were treated with a fractionated protocol compared to those that were treated with a hypofractionated protocol. This report may be part of the reason why hypofractionated radiation protocols with a large dose per fraction are most commonly used to treat canine CMM [8,9,10]. Parameters that help understand a cell’s radiobiology include alpha beta ratio, survival fraction (SF) at a specific radiation dose (i.e., SF at 2 Gy; SF2), and radiation dose that results in a specific cell survival rate (i.e., 10% survival; D10) Knowing those parameters may help to better understand radiobiology of CMM and create a more effective radiation therapy protocol
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