Different breast types cause different breast cancer risk
New knowledge has confirmed that mammographic density is an important factor in breast diagnostics.
Mammographic density is a term that says something about the glandular tissue in a breast. The glandular tissue is presented as different shades of grey on the mammography images. Areas with dense glandular tissue appear white or light grey on mammograms, while fatty tissue appears black or dark grey. (See image increasing degree of density from A to D, where A is most fat-rich, and D has the densest glandular tissue)
The challenge is that tumors in the breast can also be white or light in the images, so that when the glandular tissue is clogged, the tumor can be hidden, and the possibility of detecting a tumor can be reduced.
In addition, it has now also been established that women with very high mammographic density have a significantly higher risk of developing breast cancer than women with low density.
It is not possible to know from the outside whether the breasts have dense glandular tissue or if they are rich in fat, nor is density related to the size of the breast.
A recent doctoral thesis from doctor and PhD candidate Nataliia Moshina from the Mammography Programme and the Cancer Registry of Norway attempts to investigate the role of mammographic density in a population-based screening programme such as the Mammography Programme, and whether the programme should be changed as a result of the increased knowledge about density.
"There is much to suggest that screening for breast cancer can be improved in the long term when we take mammographic density into account – but unfortunately we still lack some important pieces of knowledge. For example, we still don't have a good, objective way to measure density. A good, standardised tool is needed here before it will be effective and safe enough to distinguish women on the basis of breast density," says PhD candidate Nataliia Moshina.
Nataliia Moshina on the day of the disputation. Photo: Tone Rise, Hold your breath
Knowledge about mammographic density is necessary in «all» women
Recently, it has been discussed whether women with mammographically dense breasts should be offered more frequent screening or other screening methods in addition to mammography. This is called stratified screening.
Until now, knowledge about mammographic density has mainly been linked to women who have already been diagnosed with breast cancer. There has therefore been a great need to obtain more knowledge about mammographic density among women in general, i.e. among healthy women participating in the screening programme, in order to investigate whether mammographic density can be used to stratify the Mammography Programme in order to improve the programme.
The aim of Moshina's doctoral project was to obtain such knowledge among women who participated in the Mammography Programme in the period 1996-2015.
The project was carried out at the Cancer Registry of Norway with support from Extrastiftelsen. Professor Solveig Hofvind, who also works at the Cancer Registry of Norway as head of the Mammography Programme, was Moshina's main supervisor.
- The time is still not ripe
Moshina's research shows that mammographic density is an important factor to take into account in screening because the tumors detected in the women with dense breasts had a larger average tumor diameter, and a greater proportion of these women had spread to lymph nodes in the armpit compared to women with high-fat breasts.
Tumor diameter and spread to lymph nodes are both factors that have a major impact on prognosis. However, the research also shows that only 5% of screened women were assessed as having mammographically dense breasts.
Despite the fact that the degree work has provided a lot of new and important knowledge, Moshina concludes in her thesis that the time is not yet right to offer women with mammographically dense breasts more frequent examinations or other screening methods in addition to mammography.
"There are several reasons for this, but one of the most important is that the measurement methods for distinguishing between those with high or low mammographic density are still not good enough," says Moshina.
The method currently used is based on the X-ray doctor who evaluates the mammography images visually estimating the proportion of the glandular tissue of the breast.
This is a subjective measurement method that results in variation in the results, depending on who performs the assessment.
Nor is the method particularly suitable in a screening programme if the assessment is to be made for everyone who participates, since a screening programme is entirely dependent on X-ray doctors being able to work quickly and focus on looking for signs of breast cancer.
"We wanted to explore the use of an automated, and thus more standardized, measurement method for mammographic density, in the form of software that reads the density of the mammograms. Our results show that the automated method corresponds to the subjective methods based on X-ray doctors assessing the images, but that the automated method is not without challenges either," explains Moshina.
The conclusion of the doctoral work is therefore that even more knowledge is needed about measurement methods for mammographic density before the variable can be considered to be used to stratify the screening programme in Norway.
Important to find the tumors that kill
Solveig Hofvind, head of the Mammography Programme, says that the results from both Moshina and others' studies indicate that stratified screening should most likely be based on more risk factors than just mammographic density alone.
"However, Moshina's studies have provided new knowledge about density needed to include density as one of several risk factors when assessing stratified screening for breast cancer," says Hofvind.
She emphasises that cost-effectiveness must also be considered.
- What do women gain by being screened more frequently and/or with other methods, and what are the disadvantages? It is important to find the tumors that kill if the intervention is to be cost-effective," she says.
Hofvind is sober and believes that there is much we still do not know enough about to be able to introduce stratified screening on the basis of mammographic density.
She emphasises that we already have a stratified screening programme in Norway, in that we only screen the age group 50-69 years.
"Age is a very strong risk factor for breast cancer. Other countries screen both younger and older women – perhaps it is a better idea to start with such an expansion, to age groups where it is well documented that the benefits of screening are greater than the disadvantages, concludes Hofvind.