“Ask Dr. HKSH” – Nuclear Medicine

Dr. WONG Yuet Hung, Catherine

Specialist in Nuclear Medicine

Q1. What is nuclear medicine?

Nuclear medicine involves the safe administration of small amounts of radioactive substances—commonly referred to as "tracers"—via inhalation, injection, or oral ingestion. Once inside the body, these tracers emit signals that can be detected by specialised imaging devices, such as PET/CT and SPECT scanners. This enables Nuclear Medicine physicians to evaluate a range of conditions, including reduced blood flow to the heart, thyroid dysfunction, neurodegenerative changes, cancer metastasis, etc.

Compared with conventional X-rays or standard blood tests, nuclear medicine allows for faster and more precise disease detection at the molecular level. Beyond its diagnostic applications, nuclear medicine also offers therapeutic utilisation; for example, high-dose radioactive substances are used in targeted radiotherapy. These agents can precisely targeted on cancer cells and deliver localised radiation over a very short distance, making them particularly effective for patients with thyroid and prostate tumours. As a comprehensive medical specialty, nuclear medicine uniquely integrates molecular imaging for diagnosis with internal radionuclide therapy.

Q2. What are the differences between CT, MRI and Nuclear Medicine (NM) scans?

As conventional medical imaging techniques, computed tomography (CT) and magnetic resonance imaging (MRI) are mainly used to examine body structures. For example, they can help assess tumour size and shape, as well as local invasion. Nuclear medicine (NM) scans, in contrast, are a form of molecular imaging that uses radioactive substances as tracers. A distinctive feature of nuclear medicine is the wide variety of tracers available, all of which are designed based on the biochemical processes of the human body. These tracers can be thought of as detectives: once inside the patient's body, they can track the progression of disease in various sites, thereby facilitating the assessment of biochemical activity and organ function.

One example is the glucose-based tracer fluorine-18 fluorodeoxyglucose (F-18 FDG). Since cancer cells require glucose to support their growth, this tracer helps determine whether a tumour has metastasised and how aggressive it is. Other tracers, such as fluorine-18 prostate-specific membrane antigen (F-18 PSMA) and gallium-68 prostate-specific membrane antigen (Ga-68 PSMA), are used for assessing prostate cancer, while gallium-68 DOTATATE (Ga-68 DOTATATE) is employed to detect neuroendocrine tumours.

Thus, nuclear medicine goes beyond merely observing the surface manifestations of a disease; it also delves into the cellular level, providing insight into underlying biochemical changes and enabling earlier detection of medical conditions.

Q3. How can nuclear medicine help patients?

Nuclear medicine is widely used in both diagnosis and therapy. In terms of diagnosis, its applications can be broadly divided into two main categories. The first involves conventional nuclear medicine imaging, such as bone scans for the detection of bone metastases and other infective foci, as well as myocardial perfusion imaging, which can assess whether reduced blood flow to the heart muscle in case of coronary artery plaque, and can also be used to evaluate thyroid function.

The second category involves more advanced imaging techniques, namely positron emission tomography/computed tomography (PET/CT), which has three main areas of application. The first is oncological imaging, including cancer staging, post-treatment response assessment, and regular follow-up to monitor for disease recurrence. The second is cardiac assessment, which evaluates the viability of ischaemic myocardium—a factor that directly influences the potential benefit of cardiac revascularisation procedures. The third area pertains to neurodegenerative diseases, for which radiotracers are now available to enable early detection of Alzheimer's disease and Parkinson's disease via PET/CT.

On the therapeutic front, the applications of nuclear medicine are also well established. Radioactive iodine is used to treat hyperthyroidism and thyroid cancer; Lutetium-177 PSMA (Lu-177 PSMA) can be used to treat patients with metastatic prostate cancer; and Lutetium-177 DOTATATE (Lu-177 DOTATATE) is used for patients with neuroendocrine tumours—all of which have demonstrated remarkable therapeutic efficacy.

Q4. What is nuclear medicine therapy?

Nuclear therapy, commonly known as PRRT (Peptide Receptor Radionuclide Therapy), is a form of precision targeted radiotherapy. The therapeutic agent consists of two main components: one is a peptide, which enables precise identification of tumour cells; the other is a high-energy radioactive substance, with Lutetium-177 (Lu-177), yttrium-90 (Y-90), and radioactive Iodine being the most commonly used at present.

This therapy can be likened to a "smart missile," as the drug can precisely recognise and trap into tumour cells. Once the drug enters the cancer cells, it releases beta particles or alpha particles, destroying them over an extremely short distance. This treatment offers numerous advantages: firstly, it can address multiple metastatic lesions in a single treatment session; secondly, because the drug specifically targets cancer cells, it has relatively little impact on normal organs and tissues. Moreover, the side effects are minimal—unlike chemotherapy, it does not cause hair loss or nausea. In addition, the intervals between treatment sessions are relatively long, and patients do not need to undergo surgery, making this therapy generally well-accepted by most patients. Currently, this treatment is primarily used for thyroid cancer, prostate cancer, and neuroendocrine tumours.

Q5. Can nuclear medicine help early detection of dementia?

In the past, dementia was often diagnosed only when patients presented with relatively obvious symptoms, such as memory loss or behavioural changes, and the diagnosis was usually confirmed through clinical assessment, questionnaires, and conventional imaging scans. By that time, most patients were likely already at a relatively advanced stage of the disease.

Currently, two novel tracers are available for PET/CT imaging, which facilitate dementia detection by targeting amyloid protein and tau protein, respectively. Studies have shown that these two proteins accumulate abnormally in the brains of patients with dementia. With these tracers, early protein deposition can now be visualised with high clarity and precision, enabling earlier detection of dementia. Importantly, these changes can still be detected even in individuals who present with only mild symptoms, or in those who have a family history of dementia but have not yet developed obvious clinical manifestations. This allows patients to receive timely treatment and undergo follow-up scans for disease monitoring and treatment response assessment.