Cancer- "a condition associated with the uncontrolled growth of abnormal cells within the body." 

These abnormal cells can create tumors which can grow to cause several different "painful and serious" complications. 

Several factors can contribute to the "likelihood" of you dog developing cancer. These factors include: age, breed, gender, and environment. 

Age: "Nearly half of all dogs over the age of 10 years old will develop cancer."
Breed: "Certain tumors are more common in specific breeds." For example: Boxers are pron to skin tumors, German Shepherds are pron to spleen tumors, and giant breeds are pron to bone cancer. 
Gender: Believe it or not; "some cancers develop under the influence of sex hormones", causing gender to be a contributing factor to cancer.
Environment: "Exposure to chemicals, such as: some pesticides and herbicides, and radiation" can cause cancer. 

Information taken from Hill's pamphlet for Cancer: Cancer. N.p.: Hill's Pet Nutrition, 2001. Print.

1) My life is likely to last 10-15 years. Any separation from you is likely to be painful. Give me time to understand what you want of me.

2) Place your trust in me. It is crucial for my well-being.

3) Don't be angry with me for long, and don't lock me up as punishment. You have your work, your friends, your entertainments; but I have only you.

4) Talk to me. Even if I don't understand your words. I do understand your voice when speaking to me.

5) Be aware that however you treat me, I will never forget it.

6) Before you hit me, before you strike me, remember that I could hurt you, and yet...I choose not to.

7) Before you scold me for being lazy or uncooperative, ask yourself if something might be bothering me. Perhaps I'm not getting the right food, I have been in the sun too long, or my heart might be getting old or weak.

8) Please take care of me when I grow old. You, too, will grow old.

9) On the ultimate difficult journey, go with me, please. Never say you can't bear to watch. Don't make me face this alone. Everything is easier for me if you are there, because I love you so.

10) Let my last memory be of your kind face, gentle voice, and of your loving arms holding me.

My instructor passed out a handout of the Pet's Ten Commandment's. I read it, and absolutely adored it. Every one of them is so true, and should be followed just as a human's ten commandments are. Commandment's 8-10 are especially important when it comes to cancer. Stay strong for your beloved pet, as it would stay strong for you.