I'm very sorry for your loss. It is likely the osteosarcoma (bone cancer) was present prior to her leg injury. Often OSC is diagnosed because of an injury or limp. Generally, it is thought that OSC has nearly always metasticized to the lungs even when it is not visible on x-ray, because it is a very aggressive type of cancer. I lost my Simon to OSC less than a month ago. We got a little more than 2 months with him post-diagnosis.
Unfortunately, Dobermans have some pretty big health issues that are common to the breed. Cancer is one, cardio (dilated cardiomyopathy) another, and CVI (Wobbler's Disease) another.
It is always a risk with this breed, but if you love the breed and are willing to take the risk of health problems, the best thing you can do is "stack the deck" in your favor by choosing an ethical breeder who is breeding for health and longevity in addition to correct structure and temperament. One thing I recommend to start with is to look for breeders who have dogs on the DPCA (Doberman Pinscher Club of America) Longevity listings. You can read about the longevity program here: The DPCA | Longevity Program
. On the left side of the page you can click on the longevity listings to see dogs who have made that list.