Even at 85, it's difficult to watch a parent manage a cancer diagnosis, and it's natural to be scared. Your first step should be to ask the diagnosing physician or
nurse practicioner specifically what you will see as the disease progresses in your mom.
Generally, because the esophogus connects the mouth and stomach, untreated esophogeal cancer will likely lead to challenges with eating and swallowing. This could lead to further weight loss or dehydration. If this happens, try to give her small quantities of food and drink with frequency.
When she starts experiencing pain, she may also have challenges with swallowing pain medication, so you should ask the medical professionals about options other than pill form. Depending on the severity, they may recommend other forms of pain medicine, such as a pain pump, which she can control in terms of timing of dosages.
She may also have difficulty sleeping, depending on where the tumor is and how she sleeps. This can be tough to solve, as her sleep routine is probably hard to change (and may take some time, when she may be cranky because of lack of sleep), but work with her to find a solution. It could be as simple as rearranging pillows.
I am getting ready to release a much-needed e-handbook (downloadable, less than 50 pages) entitled "A Cancer Patient's End of Life" as part of my "Compassionate Caregiver Series." This can help you, as her caregiver, anticipate what she will likely experience in physical, emotional, and mental terms as she nears death, and how to manage the activity around death. If you would like, I will send you an advance copy free of charge as a member of the Caring.com community. Send another posting directly to me, with an email address, and I will send it on. Good luck.