I have had radiotherapy & lympth nodes removed. I have been told if I cut under my arm I will suffer? Is true?

Yes, it's true. That side of your body is going to be more suseptible to cuts, irritations and a whole slew of other problems. You'll get easily infected since the nodes aren't there to do their job, so infections and problems have to be nipped in the bud before they can cause problems.

From now on, you need to be really careful with what you do with your arm.
If you cut yourself, thoughly wash with warm water and soap and then apply an antibiotic ointment (like Bacitracin)
Always use an electric razor for shaving under your arms.

Don't cut your cuticles, but use an orange stick to gently push them back

Wear rubber gloves when you're using chemicals like cleaning products or using steel wool, and heavy gloves for gardening. Always use oven mitts when checking anything in the oven.

Carry your purse or heavy objects with your other arm - but make sure you don't baby the arm that has had the nodes removed. You DO need to maintain strenth in the arm, but just don't over-do it.

Wear sunblock at all times when going outside.

When you need bloodwork done, blood pressure taken, injections or vaccinations, always use the other arm. If your lymph nodes have been removed from both side, have the doctor or nurse alternate the sides that they draw blood from, check pressure, etc..

Avoid tight sleeves and wear things that don't cause pressure to the arm.

Hang in there.... and good luck !

You have lots of lymph nodes so you will be OK, there are some in your back and neck close to the underarm. So if you get cut and get an infection the antigen will get processed in the next nearest node. As long as it gets in the lymphatic pathway the infectious agent will be processed.

Radiotherapy effects will eventually go away the your immune system will soon be restored.