Mother-in-law (86) diagnosed with breast cancer. What things will we need to be certain to ask?

First cancer treatments are changing all the time and secondly they grow very slowly in the elderly and I believe 86 is elderly but I may not agree in another 8 years .
There is no basis for making assumptions about the nature of a lump. She needs a mamogram ultra sound, biopsy, CT scan or MRI or whatever combination of the above they choose to do. Some of which may make her anxious but are not intrusive in them selves except the biopsy which will be done with a needle under local anesthetic and she will only feel the initial prick no worse than an injection. If she is willing any of these procedures will cause her minimal distress and at least you will know the answer and can go from there. All are outpatient procedures.
Once you have an answer MIL can decide where to go from there. They can do a simple lumpectomy, a radical mastectomy, R/T and Chemo or any of the above.
Cancer when confined to the breast is rarely painful so that is a good sign. Oncologists are very aggressive and always predict the worst.
Any major surgery has significant risks both from the anesthetic and the actual operation. At 86 it could take 6-12 months to recover. Does she want to waste a year of whatever time she has left. As it is so painful she may have a simple abscess which is easily treated with antibiotics.
Try not to think of the worst and get other opinions if it proves to be cancer. If MIL decides to do nothing which is her right and may be the sensible thing to do please support her decision. As was said above don't do the tests if you are not going to do the treatment and above all don't panic, she does not need to go for surgery this week. Give her and yourselves time.

At this point the doctor (oncologist?) and others who reviewed the tests "believe" it's a cancerous mass, and "believe" it's aggressive.

I wouldn't make any decisions until the tests have been reviewed by an oncologist. An outpatient biopsy might be recommended, and that would be more determinative of whether the cancer is an aggressive one, and what stage it is.

Then you can make decisions whether to proceed with treatment, if at all.

Chemo and radiation are absolutely horrible on anyone, and would be harder on an 86 year old woman. If the cancer is aggressive, and is in a later stage, it would be unkind and not fruitful to put her through the slash and burn regimen of treatment.

In the meantime, research Gilda's Club and CURE magazine online to see if you can find a local Club which will offer support groups for your family, and the magazine to look for articles on similar situations - knowing how others proceeded might help you in making this tough decision.

If it were me, unless the cancer was stage I and the treatment was merely medicine (no slash and burn), I wouldn't consider putting any of my family through the hell of chemo, radiation and/or surgery.

Years ago, a very wise geriatrician told me " don't do the test if you're not going to do the treatment". ( this was in reference to my mom; they'd found some suspicious cells in a chest tap and wanted to do a bone marrow biopsy. She was 90, with dementia. We declined. The oncologist was horrified. Mom is still with us 4 years later).

Would your mil be able to cope with radiation, chemo, mastectomy? How will they determine if this is cancer and what stage? Can the biopsy ( if that's what they need to do be done in the hospital and not outpatient ( i would have opted for leaving her in the hospital, but that's another story).

Does MiL have dementia? Can she make treatment choices?

That would be my list.

Teaka this must be among the most miserable weekends you've ever spent. I'm so sorry.

The only advice I can offer is not to jump the gun. Sit tight and wait until you have more certain information about what exactly your poor MIL is facing; and make today as pleasant and light-hearted as you can. If it does turn out to be bad news - prepare for the worst, hope for the best - you will all know soon enough; so while I know this is easier said than done try to put the worry aside so that at least you can enjoy the moment.

If your MIL decides she does want to talk about it, though, that's different. But all the same the only truthful answer you can give her, to just about every question, is "we don't know yet." I hope her bladder is more comfortable today, anyway - that's all she needs, poor love.

Thank you. I'll be prepared with a pad & pen - good suggestion.

I'm so sorry
Waiting and worrying is hard but it seems you MIL is not yet worrying which is the most important thing
If you have a referral to an oncologist from the hospital then I'd suggest getting a notepad that you can take to the appointment - write down a few questions before the appointment and try to take notes during the appointment if a lot of information is thrown your way