The personal health revelations in Congress leader Sonia Gandhi’s memoir, Belonging: A Journey of Love, have drawn attention to an unsettling reality of cancer. Gandhi writes about undergoing surgery for breast cancer in 2010 and the discovery of a growth in her pancreas during a follow-up examination a year later. The pancreatic growth was found to be at a precancerous stage and was surgically removed.Her account, however, raises an important question that troubles many cancer survivors and their families: If the original cancer has been treated successfully, how can cancer appear in another organ, sometimes years later? The answer lies in the complex and often unpredictable behaviour of cancer cells.Cancer can spread even when the original tumour has been successfully treated“Breast cancer can recur in other organs like bones, lungs, liver or even pancreas because some of the breast cancer cells can escape from the breast and travel elsewhere through the lymphatic system or bloodstream and then become dormant or enter a sleeping stage,” says Dr Ankit Jain, Senior Consultant, Medical Oncology, Indraprastha Apollo Hospitals, New Delhi.“This process is called micro metastasis. At this stage, these cells cannot be identified with routine investigations like scans and blood tests and can, therefore, escape detection.” In other words, a patient may complete surgery, chemotherapy, radiation, targeted therapy or hormonal treatment and show no evidence of cancer, while a few microscopic cancer cells may still survive elsewhere in the body.“However, changes in these cells or their surrounding environment, or the development of new mutations, can result in these cells becoming active again. They start to multiply, leading to cancer recurrence in the future,” explains Dr Jain.This explains why a person treated for breast cancer may develop a tumour in the bones, lungs or liver years later, even when there is no recurrence in the breast itself. “Please note that recurrence does not necessarily mean that the original treatment has failed because these microscopic cells usually survive outside the original tumour,” he says.What causes cancer cells to spread?Story continues below this adDr Jain explains that several biological changes can contribute to this process. “Development of new genetic mutations can lead these cancer cells to multiply in an uncontrolled fashion, resulting in abnormal numbers, shape and size of the cells. These cells can invade surrounding tissues and develop their own blood supply.”He adds that cancer cells can also acquire the ability to become more mobile. “Cancer cells learn ways to break away from the original tumour, becoming more mobile, invading surrounding tissues or entering the bloodstream. Some of these cells settle in various organs like the lungs, liver, bones or even the brain.”Once they reach a distant organ, some cancer cells may remain dormant for years, while others begin multiplying and eventually form secondary tumours.Not every cancer cell that enters the circulation succeeds in establishing a new tumour. Many are destroyed by the immune system or fail to survive in the new environment. However, a small proportion may persist and eventually become active.Story continues below this adHow is recurrence diagnosed?According to Dr Jain, patients may experience symptoms such as loss of appetite, unexplained weight loss, shortness of breath, persistent cough or pain at a particular site, including the bones. “Other symptoms may include jaundice, which is the yellow discolouration of the white part of the eyes, generalised itching, abdominal pain and pale stools,” he says.However, these symptoms are not necessarily indicative of cancer recurrence and can occur in several other medical conditions. Persistent or unexplained symptoms should nevertheless be evaluated by a doctor, particularly in someone with a history of cancer.“Such recurrence can be diagnosed based on patients’ symptoms or through blood tests, rebiopsy or scans, including PET-CT, ultrasound of the abdomen, chest X-ray and MRI of the brain, depending on the symptoms of the patient,” Dr Jain explains.A biopsy of the suspected tumour may be particularly important. It helps establish whether the new growth is a metastasis from the original cancer or an entirely new primary cancer developing in another organ. The distinction is crucial because treatment decisions can differ significantly.Story continues below this adHow to treat secondary cancer?“Treatment for recurrence depends on the site of metastasis, the patient’s previous co-morbidities, fitness level and the choices of the patient and family,” says Dr Jain. “This includes chemotherapy, targeted therapy, immunotherapy, hormonal therapy and radiotherapy.”Why follow-up after cancer treatment mattersFor cancer survivors, completing treatment is an important milestone, but it does not mean that medical follow-up can be discontinued. Regular consultations, recommended surveillance and prompt reporting of persistent or unexplained symptoms remain important parts of long-term cancer care.At the same time, patients should not interpret every ache, cough or change in appetite as a sign that cancer has returned. Follow-up investigations should be guided by the treating oncologist and the individual’s risk profile.