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skfinkel's avatar

Did doctors really not know or even suspect that CT scans would cause cancer until now?

Asked by skfinkel (13511points) December 16th, 2009

The studies showing dangers of such “wonder technologies” always seem to come chugging up years after the doctors have been urging them on their patients. They are now predicting thousands more cancer cases for otherwise healthy patients. Where is the “do no harm” credo?

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13 Answers

JLeslie's avatar

That would be totally embarrasing. How can someone not know that multiple xrays and radiation exposure can lead to some cancers? Isn’t it obvious? I saw in the full abstract about the study that prompted the recent breast cancer screening debate that they did consider negative affects of mammography, but did not have any data, only sighting that people with conditions that require regular chest x-rays like TB, do have more incidents of breast cancer. Duh. I only get dental xrays every 2–3 years, although I will be willing to step that up as I get older, and I wil not be getting mammography every year from the age of 40 on.

FireMadeFlesh's avatar

We have known for as long as CT scanners have existed that they require a far higher dose of radiation than plain film x-rays, but if prescribed based on good clinical judgement then the risks posed by the scan are outweighed by the risks of not having the scan when it is required.

The researchers cited in the article are irresponsible, as it is impossible to cite a single source of cancer except in extreme circumstances such as Chernobyl. For example, lung cancer occurs for many different reasons. You cannot assume a smoker with lung cancer would not have cancer without the cigarettes, only that it would be far more likely. CT scans do not directly cause cancer, they only increase the probability.

New technologies in low-dose scanning are improving these risks, but all new developments take time to implement. The article you cite says 15,000 excess deaths per year, but when you consider the number of lives that are saved by scans the problem is not nearly so bad. I would support any move to better equip doctors to make clinical judgements and not rely on imaging so heavily, but at the same time undue panic does not achieve any good.

This is quite a good source on the issue. Note that “conventional” CT does not exist any more, and only helical is used. Also note that there is only a 0.03% increase in the incidence of fatal cancer across a population.

skfinkel's avatar

@FireMadeFlesh : If cancers of the breast or lung are so much higher in the face of having had a CT scan (even if not the sole “cause”), should not this risk be mentioned to patients? Then, at least, there will be a choice. There have been so many mistakes made with x-rays: on women ready to deliver whose babies got cancer twenty years later, to mammograms, and now the news about CT scans. In all cases, the doctors imply that these tests “must” be done and it is only years later that it is “discovered” that the diagnostic test probably killed more people than the test saved.

JLeslie's avatar

@FireMadeFlesh Regarding lung cancer, they can tell by the cell if it is caused by smoking or something else.

FireMadeFlesh's avatar

@skfinkel I am quite sure that in my country at least patients are warned. They are certainly warned of the possible side effects of contrast media, so I cannot imagine why they would not be warned about the risks of a CT scan. There are very strict legal restrictions around medical radiography. Every patient must give informed consent, which may be verbal, to express their desire to have the test.

This is an issue I take seriously, as many doctors are inclined to order CT scans when a plain x-ray would be sufficient because they are frightened of the implications of an incorrect diagnosis. Radiologists have been taken to court for missing information, so CT scans are often performed when they are not required because the information provided is far greater. The practice I have been working in for the last six weeks had poor dose awareness in my opinion, as they would often perform a plain x-ray and a CT scan of the same area. One or the other should be done, but both is useless excess.

I don’t think there is any reason for the public to be alarmed, although being informed is never a bad thing. There is good reason for the medical community and the legal system to be alarmed though, because dose regulation is largely dependent on which doctor prescribes the scan, and how studious the technologist is. Legally, if less cases were presented to the courts, doctors would be far less likely to prescribe unnecessary scans.

The fact is that the danger is negligible. A CT scan of the chest gives less dose than an aeroplane flight from Sydney to London, yet people do that every day when the only motivation is a holiday or a business trip. A routine chest CT scan gives an effective dose of 7mSv (2000, US), while a routine plain x-ray chest gives an effective dose of 0.11mSv (1988, US). The average annual dose from natural sources is 3mSv, so a chest CT scan is equivalent to about 2 years of life – considering the number of carcinogens people are regularly exposed to, this is negligible. Medical x-rays only account for 11% of the population dose in a year.

I apologise for the length of my response. Basically patients should be informed and make their own decisions, but CT scans save thousands more lives than the number that are put at risk. GPs should also be given courses in radiobiology so they are not so liberal in prescribing x-rays of any sort.

As for mammograms, from 1975 to 2000 the dose dropped by a factor of 7, while the image quality almost doubled. Here in Australia, screening for breast cancer uses ultrasound until the patient reaches a less radiosensitive age (I think around 40 years).

skfinkel's avatar

@FireMadeFlesh thank you for your intelligent and informative response. Every time an x-ray is given, or in this case a CT scan, they always tell you it is negligible and won’t harm you. Yet, what if a person flies every week across the country, has a couple CT scans a year, an x-ray or two—these things are cumulative. Who is watching, keeping track? Does anyone ever ask? I didn’t read the study about CT scans in the journal itself, but the article said that the scans would account for 29K more cases of cancer and 15K more deaths. Frankly, I suspect that the lives saved would not be at that level—but that information should be accessible. I would be surprised if it is even a wash. Even if the CT scan catches cancer, how many of those with cancer will survive?

FireMadeFlesh's avatar

@skfinkel No one keeps track. The only time exposure is recorded is if you are a radiation worker (I think recently this has included pilots), in which case you only wear the monitor during work hours. There are averages in sample groups from which our data are provided, but the general public is not monitored. There are base levels above which the risk is considered unacceptable, but again the public is not monitored.

“Frankly, I suspect that the lives saved would not be at that level” I disagree with this statement. Consider that CT is the only imaging modality that is fast, reliable, and able to provide high quality images of soft tissue. It is used in almost all cases of severe trauma, so I suspect that the quota would be filled from motor vehicle accidents alone. It is the only scan that can quickly and easily detect a bleed on the brain, which is fatal within hours. As well as this, it provides vital information on the stage of a cancer’s progression (it is able to image most types of cancer), which alters the form of treatment for practically every cancer type. The place I have been working at for the past six weeks does 2–3 patients per day for cardiac scans, which provide vital information on heart disease and the probability of a heart attack. CT scans are also used to guide back injections, without which a patient would suffer debilitating disease. Without CT guidance the risk of damaging the spinal cord would be far higher. I don’t have the specific data at my fingertips, as most research deals with specific case types, but I am sure that the numbers of lives saved would be well in excess of 29,000.

One of my lecturers at university is on the advisory board to the Australian government on radiation safety. He always emphasises in class the importance of using plain x-rays instead of CT where possible, but of course that is not in our hands because the doctors have the final say.

JLeslie's avatar

From what I understand pilots and people who work in radiology do have more incidents of cancer. Although, I am also aware that they have found that people around low doses of radiation seem to have less cancer than average population. The pilot/flying comparison for why x-rays should not be a big deal bothers me because x-ray and CT are targeted in a specific area of the body. From what I understand back in the 70’s heavy doses of radiation, to treat breast cancer resulted in heart problems later, damages the cardiovasular system in the adjacent area, and many died 10 years later from heart problems. My mother knew a woman that this happened to. She was worned that she would probably develop heart problems. There is argument from what I understand if any risk still exists with the new radiation tecniques.

@FireMadeFlesh I have a question since you seem so knowledgable. If someone “needs” a CT can an MRI be used just as well? Do they not use MRI only because of the cost?

JLeslie's avatar

Found this link about breast cancer treatment and heart problems if you are interested What occurs to me is that medicine does not think something exists unless it has been proven through a scientific method. So if no one is studying effect of xray over the long term on specific areas of the body, no one is drawing correlations or causations.

FireMadeFlesh's avatar

@JLeslie The most radiosensitive parts of the body are the thyroid, corneas of the eyes, and genitalia. Few CT scans target these areas, since either ultrasound or Nuclear Medicine is usually better for imaging them anyway. However the eyes are included in a brain scan, which is a really common scan, and the genitalia are included in an abdomen scan, which is also really common. A pilot will have all these areas exposed during a flight, but at a lower dose than being in the primary beam of a CT scanner.
Radiotherapy for cancer treatment can cause huge amounts of damage in many different ways. This is not strictly in my field, but I know a little. It doesn’t surprise me that breast cancer treatments lead to heart problems. Radiation burns to the skin are a common side effect of radiation therapy, and I’m quite sure respiratory problems are common too. This is to be expected, because radiation therapy typically delivers a dose to the cancer tissue of 50–70Gy (4.5 is the average lethal dose if delivered to the whole body).

CT and MRI are good for different things. MRI is highly specialised, in that patients with pacemakers, some types of prosthetic implants, and claustrophobic patients cannot have a scan. The patient also needs to keep perfectly still for a long time (about 20 minutes for an average lumbar spine I think), or the scan is ruined. A CT scan has correctional software, and the scan is very fast, so movement isn’t such an issue. The use of the images is also very different. CT scans are great for bony issues, where an MRI is useless, but MRI is great for imaging the spinal canal which CT cannot do. Basically CT relies in the density of tissue, and MRI relies on water and fat content.

JLeslie's avatar

@FireMadeFlesh Thank you for taking so much time to srite that out. Without going through a whole explanation f what I have going on right now…an ENT doctor recommended I get a CAT scan of my head, and I refused at the time, because he said he wanted to evaluate my sinuses, and I feel firmly that is not my problem. Eventually I wound up in a neurologists office for this same problem and he recommended an MRI. I just did the MRI. I honestly don’t think anything is going to be found on either, but I was willing to try the MRI, because again the angle the ENT was coming from made no sense to me with my symptoms and knowledge of my own body. Plus I had regular xrays (it was either two or four done) when my problem first started, which I really did not want to do, and they were perfectly normal, nothing found. So, if you don’t mind one further explanation what exactly is the MRI going to see when scanning the brain?

FireMadeFlesh's avatar

@JLeslie I cannot answer this in too much detail, as I am not yet trained in CT, and MRI is another two years FTE study once I have finished my degree.
Sinus scans are primarily interested in two things – soft tissue, and any fluid that may be in the sinuses, and the bony walls of the sinuses. CT scans are far superior for bony structures, while MRI is usually superior for soft tissue, although it depends on the nature of the tissue.

This article is a very good one. Sorry I can’t be more helpful.

JLeslie's avatar

@FireMadeFlesh Thank you so much for the link. I think I made the right choice.

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