Background/Cancer Screening, Diagnosis And Imaging
Screening is the process of identifying disease in individuals prior to the appearance of signs and symptoms. It is distinct from diagnosis, which often relies on interpreting the signs and symptoms that are already present.
In the UK, as of 2016 there are three screening programmes:
- Bowel cancer: in England, both men and women aged 60-74 are offered bowel-cancer screening every 2 years. Details of the screening process can be read here.
- Breast cancer: this is offered to all women aged 50-70 every 3 years. Women with a higher risk of developing breast cancer (e.g: due to a first degree family relative having the disease, or having faulty genes associated with breast cancer) may be screened annually from the age of 20-40. The exact age at which screening begins for those at a higher risk of the disease will depend on the risk factor. Breast cancer screening involves taking medical images, typically using x-ray radiation (mammogram), but occasionally using MRI (if breast screening starts prior to the age of 50).
- Cervical cancer: Women aged 25-49 are offered a 3-yearly screening test and women aged 50-64 are offered a 5-yearly screening programme. A medical professional will take a small sample of cells from the cervix using a swab or brush and send them off to the laboratory for sampling.
Although it may be possible to screen for a larger number of cancers, this is generally not done for a number of reasons:
- The benefits of screening may not outweigh the harms of screening
- The cancer may be very rare, and the cost of the screening programme may be higher than the money saved by simply treating the disease when it prevents
- Some cancers may not be treatable and screening for the cancer may just cause un-necessary panic to the individual
- The screening test may be unacceptable to the patient (e.g: painful) and so people may not wish to be screened
- The screening test may not be very sensitive at detecting cancer
- Sometimes, the results of the screening test may indicate cancer, when in fact cancer is not present. This may cause un-necessary concern to the individual in question
Although cancer may be indicated by one of the following:
- A screening test
- A medical image
- The appearance of signs/symptoms
they are typically diagnosed following a biopsy or excision of the suspected tumour. This means that (a sample of) the tumour is removed by a clinician and sent off for microscopic examination by a pathologist.
The tumour may be sampled in one of the following ways:
- Needle: a needle may be used to suck out cells/tissue/fluid from the suspected tumour.
- Endoscopically: the clinician may pass a tube through a natural orifice such as the mouth/nose/anus to help visualise and locate the tumour. A sample of the tumour may then be taken using special tools.
- Surgically: the surgeon may remove part of the tumour or all of the tumour for microscopic examination. Although the tumour may be removed from one site, it is still possible tumour cells remain behind in the body, especially if the cancer has spread (metastasised).
Imaging of suspected tumours/cancers is often done to help localise the suspected lesion. This is especially important to guide surgical procedures, but can also help doctors understand the severity of the disease (e.g: by guiding tumour grading or staging). There are several imaging modalities that may be used:
- X-ray imaging: this involves obtaining a 2D image of the body by using radiation. Unfortunately, it is quite difficult to identify many tumours on x-ray images precisely because:
- There is poor soft-tissue contrast. This means it is difficult to visually distinguish two anatomical structures making the process of locating a tumour more difficult.
- There is superposition of anatomy. This means anatomical structures will overlay on top of each other, making it difficult to determine the exact 3D location of a tumour
- CT imaging:
- To overcome the limitations of x-ray imaging, a set of x-ray images may be taken using a CT (computerised tomography) scanner.
- The collection of 2D images will then be reconstructed by the computer to produce a 3D map of anatomy.
- CT images do not suffer from superposition of anatomy, and have a superior contrast resolution than x-ray images.
- The spatial resolution (ability to distinguish to objects very close together) is slightly inferior to x-ray imaging, however it is not sufficiently inferior that CT are of non-diagnostic quality. In fact, CT images are almost always superior to x-ray images.
- However, as more x-ray images are used during a CT scan than for a standard 2D x-ray, the radiation exposure to the patient is higher (this is disadvantageous).
- Sometimes, CT images are taken after being injected with a dye of contrast agent. The contrast agent may enhance the visualisation of certain anatomical structures, such as tumours, making them easier to identify.
- CT scans may last as short as 1-2 minutes.
- MRI: magnetic resonance imaging (MRI) uses strong magnets to form a 3D reconstruction of the anatomical structures in the body.
- It is superior to CT imaging in some respects including better contrast resolution, and lack of ionising radiation.
- However, MRI images take a lot longer to acquire and cannot be used in certain patients such as those with pacemakers (this is because the magnetic fields will interfere with the functioning of the pacemaker).
- MRI images can also be contrast enhanced.
- Nuclear medicine: nuclear medicine scans are taken following the injection of a radioactive tracer. The tracer will typically collect in certain organs, or be uptake in proportion to a particular physiological parameter such as blood flow or tumour metabolism. The tracer is then imaged using SPECT (single positron emission computed tomography) or PET (positron emission tomography).
- SPECT: in SPECT imaging, a radio-imaging camera (called a gamma camera), rotates around the patient and acquires multiple images over a 20-30 minute scan. The computer then recombines all the collected images to produce a 3D distribution of radioactivity throughout the patient
- PET: in PET imaging, the patient is injected with a positron emitting radionuclide. The positrons than annihilate with electrons to produce to photons (light particles). These light particles travel 180o to each other and are detected by the PET scanner. A PET camera will build up an image over time of the distribution of radionuclide throughout the patient.
Nuclear medicine imaging techniques produce images of significantly poorer spatial and contrast resolution to CT and MRI. Hence they are never used to precisely locate a tumour. However, they are much more sensitive than CT/MRI and can be used to carry out quantitative analysis of radionuclide uptake by different areas of the body. This can be useful, for example, in the characterisation of tumours. For example, a tumour may demonstrate significantly higher uptake of a radiotracer than the rest of the body. This relative increase in tracer uptake may be readily detectable on a SPECT / PET image.
PET cameras are superior to SPECT cameras in terms resolution and sensitivity but a PET scan is more expensive than a SPECT scan, and the availability of PET tracers is inferior to the availability of SPECT tracers. As such, both imaging modalities are still commonly used.
Often, two medical imaging modalities are combined (e.g: PET/CT). This is useful to allow both anatomical localisation of the tumour (CT image) and quantitative analysis of radiotracer uptake (PET image).