When tests detect secondary cancer in the bone they usually show that there are a number of tumour seedlings at various sites rather than a single patch of cancer. Often most of these seedlings will give very little trouble and cause virtually no symptoms. When symptoms do occur the commonest problem is pain. This may need pain killers, radiotherapy or other treatment to ease the discomfort.
The two main other complications from bone secondaries are fractures and a raised calcium level in the blood.
The presence of cancer seedlings in a bone can weaken it. This can cause the bone to break, or fracture, especially if it is a bone that bears a lot of weight or stress, like the thigh bone. When a break in the bone does develop at the site of a cancer deposit it is called a 'pathological fracture'. If doctors feel that x-rays and scans suggest there is a high risk of such a fracture they may give treatment to try and prevent it. This might be with a bone strengthening drug, or some precautionary radiotherapy or even an operation to put a pin or plate in the bone to strengthen it.
Sometimes, particularly when there is a lot of tumour in the bone this can lead to an increase in the level of calcium in the blood. This is called hypercalcaemia. Hypercalcaemia causes thirst, sickness and a general feeling of being unwell. If the calcium level goes higher it can lead to altered behaviour, confusion and even coma. It is usually treated with drugs, given either through a drip, in hospital, or as tablets, which can help to reduce the calcium level back to normal.
As you say, unfortunately, the development of bone secondaries usually means that the cancer cannot be cured. But some people who have bone secondaries are able to survive for years with a very good quality of life and just the occasional treatment . However some cancers do progress more quickly. The length of survival depends on the type of the primary cancer, the rate of progress of the cancer, and the response to treatment. Your mother’s doctors should be able to give you more information about this.
Except for very rare occasions when hypercalcaemia cannot be controlled, people do not die directly as a result of their bone secondaries. It is almost always the progress of the cancer to affect other vital organs like the liver, lungs or brain, or just the general burden of a very extensive cancer spread which will finally prove fatal.