Before treatment (so unrelated to any drug effects) can any cancers cause the symptom of dry mouth?

Dry Mouth: Without knowing if you are referring to any of the head and neck cancers where surgery may have been done, it is difficult to to connect a specific cancer with dry mouth. Many medications, however, can cause dry mouth. This is especially try of some of the pre medications given prior to chemotherapy administration. I would recommend you consult an oncologist.

Saliva glands: If the cancer affects the salivary glands, it could limit production. Also, depending on other causative factors (tobacco, alcohol) there could be an overall dry mouth problem.