Why does cancer cause pain?

The first thing to understand is that the intensity of cancer pain is not necessarily related to the severity or progression of the disease - a small tumour in a particular location can compress a nerve, giving rise to greater pain than a larger and more severe tumour in a different position.
Physical and pharmacological interventions to cure cancer can also cause pain and many cancer survivors have pain that continues after the cancer is eradicated and treatment ends.

Pain in general - not just cancer pain - is divided into three categories: nociceptive, neuropathic, mixed pain.

Nociceptive pain: in scientific language, pain receptors are called "nociceptors". Nociceptive pains originate from irritation of these nociceptors that are capable of picking up various stimuli, such as touch, cold or heat and also pain. Pain receptors are found in our skin, bones, joints, muscles and internal organs (except the brain and lungs).
Nociceptive pain is of two kinds: "somatic" and "visceral”.
- Somatic pain typically manifests as pulling, stinging or drilling sensations in a specific point and is caused by irritation of skin, muscular, joint or bone receptors.
- Visceral pain is caused by receptors in the organs of the chest or abdomen. They are crampy and vague, making them more difficult to locate and can radiate to other parts of the body.

Neuropathic pain: originates directly in the nerve, not from a pain receptor. The cause of the pain is due to a neuropathy (inflammation or injury of a nerve). In this case, the pain can manifest itself as a sensation of heat, burning or stabbing pain attacks, often they are not perceived at the point from which they originate but in the sector of the body innervated by that nerve. For example, pain in a lower limb caused by a herniated disc compressing a sciatic nerve root at the lumbar level.

Mixed pain: both neuropathic and nociceptive pains are contemporarily present. The treatment of this kind of pain is especially challenging as it is often difficult to understand that both types of pain are present.

Consequently, cancer pain can be nociceptive, where a tumour mass causes compression or irritation of nearby tissues or organs (bone metastases, infiltration of tissues and muscles, ulceration of skin or mucous membranes), or neuropathic, where cancer treatment causes nerve damage or the tumour itself releases chemical substances(cytokines) that induce or increase neuropathic pain. These can also cause resistance to analgesic drugs and abnormally acute perception of pain, a phenomenon known as hyperalgesia. Cancer pain can also be mixed, with both nociceptive and neuropathic causes.

Finally, as already mentioned, in some cases it is the cure (for example, surgery, radiotherapy or chemotherapy), not the cancer itself, that can cause pain.