Many owners of companion animals with cancer are overwhelmed by having to choose the "right course of action." With the aim of reducing the burden on owners who are forced to act as surrogates for their animals, this work discusses principles that apply to ethical treatment decision-making for animal patients with cancer. Four principles frequently used for ethical decision-making in human medicine will be considered for their potential applicability in veterinary medicine. As a result of these considerations, preliminary guidelines are presented, along which a decision-making discussion can be held. The deliberate integration of the non-maleficence and beneficence principles into the purely empirical facts of what is medically possible helps to maintain a moral perspective in specialized veterinary medicine. At the same time, such guidelines may contribute to individual decision-making in a way that animal patients neither have to endure unnecessarily severe side effects, nor that they are euthanized prematurely.