What Is Radiotherapy? How It Works, Side Effects, and Common Questions

Radiotherapy (often shortened to RT) uses high-energy rays to damage the DNA of cancer cells so they can no longer divide. It is one of the three main pillars of cancer treatment (alongside surgery and medication), and about half of all cancer patients will receive radiotherapy at some point — whether to cure the cancer, support surgery, or relieve advanced symptoms.

Radiotherapy is a highly precise 'invisible surgery': there is no incision and no anaesthesia, and the rays are aimed at the tumour from outside the body. Modern techniques can concentrate the radiation dose on the tumour while avoiding surrounding normal tissue as much as possible.

How It Works

When radiation passes through a cell, it damages its DNA. Cancer cells divide quickly and are less able to repair DNA, so they are more likely to die from radiation than normal cells. Radiotherapy is given in multiple sessions (for example, once a day for several weeks) precisely to exploit this difference: normal cells have time to repair between treatments, while cancer cells are gradually worn down.

Radiotherapy falls into two main categories: external beam radiotherapy (a linear accelerator delivers radiation from outside the body — the most common type) and internal radiotherapy (brachytherapy, where a radiation source is placed near the tumour, often used for cervical and prostate cancer). Advanced techniques such as intensity-modulated radiotherapy (IMRT) and stereotactic body radiotherapy (SBRT) can further improve precision; proton therapy is a newer option that is especially valuable for children and tumours in sensitive locations.

When It Is Used

Curative radiotherapy: some cancers (such as nasopharyngeal cancer, early laryngeal cancer, cervical cancer, and prostate cancer) can be cured with radiotherapy alone (or radiotherapy plus chemotherapy) without surgery. Adjuvant radiotherapy: after surgery, radiation is directed at the tumour bed to clear any remaining cancer cells (for example, after breast-conserving surgery).

Preoperative radiotherapy can shrink a tumour first (for example, in rectal cancer); palliative radiotherapy is used for patients with advanced disease to quickly relieve bone metastasis pain, brain metastasis symptoms, or tumour-related bleeding and obstruction — just a few sessions can significantly improve quality of life.

Common Side Effects and How to Manage Them

Side effects of radiotherapy depend on the area being treated and are mostly local. The skin in the treatment area may become red, itchy, or even peel, similar to sunburn: keep the skin clean and dry, wear loose clothing, use moisturisers recommended by your medical team, and do not apply home remedies or scratch the area.

Fatigue is the most common whole-body reaction to radiotherapy. It usually appears in the second half of the course and gradually improves over several weeks after treatment ends; keeping a regular routine and doing light activity can help. Other site-specific reactions include: mouth sores and painful swallowing with head and neck radiotherapy (which can be relieved with medicated mouthwash and painkillers), diarrhoea or frequent urination with pelvic radiotherapy, and discomfort when swallowing with chest radiotherapy. Most of these reactions fade within a few weeks after the course ends.

Living Well During Treatment

During radiotherapy you can generally live and work as normal (depending on your energy levels), and showing up at the treatment centre on time each day is the main task. Before treatment begins, you will have a 'simulation' session — a CT scan is used to plan the radiation, and small marks are made on your skin; do not scrub them off vigorously before they are washed away.

There are no special dietary restrictions, though patients having head and neck radiotherapy may choose soft, smooth, high-calorie foods if swallowing is difficult, and a dietitian will follow up with you. Most importantly: after external beam radiotherapy, your body is not radioactive, and you can hug your family as normal. Only internal radiotherapy or radioactive drug treatment (such as iodine-131) requires brief isolation precautions, and your medical team will explain these in detail.

Clinical Trials: Another Treatment Option

Radiotherapy techniques and combination strategies are still evolving: new directions such as proton therapy, FLASH radiotherapy, and radiotherapy combined with immunotherapy are all being advanced through clinical trials.

The CancerLink platform can match you to recruiting clinical studies in Hong Kong based on cancer type, stage, and genetic mutations, and study-related treatment costs are fully covered once you are successfully enrolled. For patients whose cancer has recurred or whose response to standard treatment is limited, trials may offer a new path forward.

Radiotherapy may sound abstract, but it is actually one of the most 'nothing-to-it' cancer treatments — a few minutes of radiation each day, with no pain. Bring your questions to the radiation oncology team; they answer questions like 'Will it hurt?' and 'Will I be radioactive?' every day.