Ann ~
With any type of insurance, it's all in the definitions....and every insurance company can have their own definitions. Although, by law, they have to make the definitions clear in the policy language, there is always a certain amount of subjectivity/ambiguity involved. As Vivian suggested, read the policy language pertaining to tumors to learn what is covered under 'critical illness'. You may need to call the insurance company customer service line (usually toll-free) to get clarification. Be persistent. Don't accept a brush-off by some low-level phone rep. Ask to talk to a supervisor if you aren't happy with what the rep tells you. My wife has had great success with getting claims paid by forcing an insurance company claims supervisor or manager to cogently explain why her claim was rejected when the claim seemed to fall under the policy definitions. Usually, they couldn't, and would agree to pay the claim. Sometimes the fist-in-a-velvet-glove approach works, i.e.
"I really don't want to get the state insurance commissioner involved" (spoken with a smile, of course). I would suggest you avoid asking an HR person to explain the critical illness policy limitations or explain definitions. They usually don't know much about insurance and will likely give you inaccurate information. Remember, they work for your employer, too, and every claim on the company policy can increase next year's premium, which is probably high, already.
I wish you success in your quest as you enter the insurance labyrinth.
JimAddendum 6/18/09 : I just realized that you're in the Canadian health care system, which may alter your options and make most of my post irrelevant. Sorry.