Heart scans and cancer risk

MR A saw me for a second opinion. Within two years, he had undergone five invasive coronary angiograms (ICA) and multiple percutaneous coronary interventions (PCI) for blockage of his three major heart arteries.

ICA involves the insertion of plastic tubes through arteries in the leg or wrist and injecting contrast into heart arteries to obtain X-ray images. PCI refers to the procedure of opening narrowed heart arteries using balloons mounted on plastic tubes or cylindrical meshes called stents. As his doctor could only open one out of three major heart arteries that were blocked during each PCI procedure, the treatment was spaced out over multiple procedures.

It is not an uncommon practice for many heart specialists to only perform PCI for one artery at a time. Hence, for complex heart disease involving all the three major heart arteries, at least two to three procedures may be required to complete the opening of the heart arteries. As both ICA and PCI involve X-ray radiation, the concern is whether the benefits of opening the blocked heart arteries outweigh the potential risk of cancer.

Radiation safety

In September 2014, the American Heart Association (AHA) issued a statement on enhancing radiation safety in the imaging of the heart by reducing radiation exposure to patients. For patients such as Mr A, technological developments and increasing experience have made it possible to perform PCI of all three major heart arteries in one single procedure with acceptable radiation exposure, less cost and less risks.

Whenever testing of the heart is required based on the high likelihood of heart disease, the potential benefit will outweigh any theoretical risk arising from exposure to ionising radiation. Even when there is appropriate indication for testing of the heart, if a comparable test is available that does not require the use of ionising radiation, this should be the preferred choice.

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Secondly, patient preferences should be considered in the decision-making process. Thirdly, once the decision to proceed to imaging is confirmed, the imaging protocol should be tailored to the patient. Finally, in making the decision on the choice of imaging, the result should be able to provide definitive conclusions for treatment plans and, wherever possible, avoid the need for serial heart testing for more confirmation.

In taking this patient-centred approach, the potential benefits for any heart testing done will far outweigh any risk from ionising radiation exposure.

Cancer risk

The lifetime risk of cancer for various heart-imaging procedures varies by age and sex. For women, a computed tomography (CT) scan of the heart arteries (CT coronary angiogram, or CTCA) provides interesting lessons. For a woman at age 30 who has a CTCA performed using a standard protocol with a 64-slice CT scanner, the estimated lifetime risk of cancer is almost 0.5 per cent with a radiation exposure of 15 to 25 millisievert (mSv, the unit used to denote amount of radiation absorbed by the region of body exposed to ionising radiation). This risk is reduced to 0.3 per cent for those aged 50.

If an optimal radiation reduction protocol is carried out using the same scanner, the risk for the 30-year-old drops sixfold to about 0.07 per cent. For the 50-year-old, the risk decreases 10-fold.

For men, the risk for a 30-year-old is only about one quarter of the risk of a 30-year-old woman using a standard protocol, and about 0.02 per cent using an optimal protocol. For a 50-year-old, the risk is about 0.08 per cent for a standard protocol and negligible when using an optimal protocol. Hence, young women should generally avoid any investigation that requires exposure to ionising radiation.

If there is a need to do so, the AHA guidelines recommend that the test should result in not more than 3 mSv of radiation. Generally, for those 50 years or more, especially men, the impact of radiation from heart tests is low as the age alone significantly increases the risk of cancer. The newest generation of CT scanners using dual source technology is able to routinely perform CTCA with very low radiation - less than 3 mSv in optimal patients.

Accuracy of radiation dose

Radiation exposure for heart tests and procedures is generally from three main sources: CT scans, nuclear scans and fluoroscopy. For CTCA, the X-ray exposure estimation varies within a

20 per cent range and hence the estimates are reasonably accurate. Furthermore, CTCA is done within seconds with a clearly defined and limited area of the chest being exposed to the X-rays. Hence, the X-ray exposure is mainly in the region of the heart and a part of the lungs. For women, there is also X-ray exposure to the breasts. The heart as an organ is generally resistant to the X-ray radiation, so the main impact is on the lung and, in women, the breasts.

Nuclear scans of the heart include single-photon emission CT (SPECT) and positron emission tomography (PET) scans of the heart. The type and radioactivity of the agent being injected into the body is the main determinant of exposure to radiation.

The differences with CT scans include:

• It is difficult to accurately estimate the radiation exposure when radioactive agents are injected into the body
• As the radioactive agents circulate throughout the body, the radiation exposure is not limited to just one part of the body but all of the organs
• It is difficult to be certain how long the radioactive agent remains in the body as the way the agent is handled by the body varies from patient to patient
• It shows the distribution of the radioactive agent in the heart muscle and cannot provide any information on the heart arteries. A good centre will have well-trained doctors to individualise the scanning protocol, select the appropriate radioactive agent, and have the latest imaging equipment with the latest technology.

Fluoroscopy is a method where physicians use X-ray machines to produce real-time, low-dose X-ray images of a part of the body during which a diagnostic or therapeutic procedure is performed. ICA and PCI procedures are done under fluoroscopy. The amount of X-ray radiation delivered during ICA or PCI is driven mainly by the behaviour of the heart specialist performing the procedure.

Making good heart decisions

Here are a few tips to get the best outcomes with the smallest risk of radiation exposure. Firstly, if you are checking for heart disease, always look for an option where there is no exposure to ionising radiation. For example, instead of a CTCA or a nuclear scan (SPECT or PET) of the heart, a magnetic resonance (MR) scan, which has no ionising radiation, may be an option.

Secondly, if an MR scan is not possible, choose a heart scan that is non-invasive, has the lowest radiation dose, and gives you sufficient information to avoid further scans.

Hence, if the aim is to determine whether there is significant blockage of the heart arteries, instead of an ICA, a CTCA is a possible option. It only exposes a limited and specific region of the body to seconds of radiation, and new-generation scanners provide high resolution images of heart arteries, allowing doctors to obtain a definitive result.

Thirdly, if CTCA is required, ensure that the centre uses a low radiation dose protocol and is highly experienced to avoid the possibility of needing a repeat scan (double radiation dose), which may be necessary if the images from the initial scan were affected by artefacts.

Fourthly, whatever scans you have to do, go to a highly experienced centre. It can be frustrating to get an inconclusive report where you are advised to go for further testing, instead of a definitive report that allows clear treatment decisions.

Finally, remember that whether it be diagnosis or treatment, you should always aim for the lowest exposure to radiation.

• This article is produced on alternate Saturdays in collaboration with MWH Heart, Stroke & Cancer Centre