Sometimes genuine emergencies happen to someone in cancer treatment, and sometimes they are unrelated to the cancer or the treatment. As in, just because you are receiving chemotherapy for lung cancer, you are not protected from the "normal" risks of a stroke or a heart attack. Of course, the fact of the cancer and the drugs may complicate things and surely will make any emergency even more frightening.
If you are receiving active treatment (chemotherapy or radiation) or if you are recovering from surgery, your doctor will give you a list of possible side effects and will make it clear which ones constitute an emergency. For example, people on chemotherapy are generally told to call 24/7 if they have a fever of more than 101.4 and to call 911 if they experience chest pain, fainting, or trouble breathing.
On the other end of the spectrum, there are many aches or pains or symptoms that seem scary (especially since virtually all of us with cancer jump quickly to the conclusion that any problem=cancer) that really are normal and self-correcting. Most oncologists practice with the "two week rule" for patients who have completed treatment. This means that, with the exception of those things above, it is generally safe to wait for two weeks before calling. Almost always that cough or back pain will have gone away. Clearly, if you are really scared about something, it is much better to go ahead and call. You likely will be reassured and can then put away the fear.
From Cure comes this very good article and specific list of what constitutes oncologic emergencies. I will give you the start of the article and a link to read more, followed by the link to the list:
Preparing for a Cancer-Related Emergency
Know which symptoms call for immediate action and which can be safely ignored.
Carmen Gonzalez, an associate professor of emergency medicine at MD Anderson Cancer Center in
Houston, still remembers a patient who came to the emergency room 15 years ago. An internist by
training, she had just begun working at MD Anderson. The patient was a man in his 40s, a husband and
father with sarcoma who was undergoing chemotherapy and had ignored a fever of more than 101 degrees
Fahrenheit. The fever raged for one day, then two and then three. By the fourth day, he was so drowsy that
his wife brought him to the ER. But it was too late. His cancer was survivable, but he died of sepsis, a
severe blood infection that can result from a chemotherapy-induced low white blood cell count. Gonzalez
could have saved the patient from this oncologic emergency with antibiotics and intravenous fluids if only
he’d come for help earlier. “His wife had to live with the sorrow of thinking his death could have been
prevented, and their children had to live without a father—just because they thought he would get better,”
And here is the list: