Acute on chronic pain description is to explain the pain which was chronic and has now become suddenly severe, sharp and unbearable. It could be due to acute exacerbation of the original process.
Aetiologically it could be due to sudden increase in the size of the primary or due to deposition of metastases into long bones, vertebrae, lungs, liver.
It could be a referred pain of visceral involvement by the primary malignancy or secondaries and being now seen by the patient as a newly originating pain after acute on chronic pain .
It could be due to therapeutic intervention like acute postoperative pain due to either resection being carried out or palliative surgery as debulking of tumour has been done.
Acute on chronic pain result tells us that it could be the acute pain of thoracotomy or laparotomy; it could be an inflammatory response in algesic form as a result of radiation, like radiation dermatitis, cystitis, and oropharyngitis.
It might be as a result of chemotherapeutic agents leading to acute gastrointestinal colicky syndromes, myalgias or myositis.
Usually, acute on chronic pain of such origin is self-limiting and can easily be controlled with help of nonsteroidal anti inflammatory drugs (NSAIDs) like aspirin. ibubrufen or diclofenac.