This is known as intrathecal chemotherapy, when injections of drugs are given into the spinal tap to destroy any cancer cells that may penetrate the spinal column (anticancer drugs are injected into the fluid-filled space between the thin layers of tissue that cover the brain and spinal cord). The injection is painful and the chemotherapy drug can cause a burning sensation as it is injected.

Does she experience any kind of back pain?
Does she has abnormal sensations or loss of sensation, particularly in the legs?

She does have pain in her spine due to some compression fractures from the bone cancer. Her legs do give out sometimes and she has fallen several times. She had a spinal tap yesterday for some testing and yes she said it was painful.

Metastatic tumors of the spine (those that have spread to the spine from another site) often progress quickly, while primary tumors often progress slowly over weeks to years.
Back pain in any area, that is worse when lying down, worse with strain, cough, sneeze, that may extend to the hip, leg, or feet (or arms) is a symptom of spinal cancer.
Abnormal sensations, like numbness and tingling, particularly in the legs, that may be progressive is another symptom of affected spinal cord with metastatic process.
Muscle weakness, particularly in the legs, that makes walking difficult and even causes falls, is also due to the cancer.

So, I guess your mother in law has secondary cancer of the spinal cord that is usually very progressive.

Best wishes!
Marija

My mom has been going through the same things. It looks like over a year now since your mother in law started her treatments. Did they work?