Better imaging technologies are needed to find some fast-growing breast cancer tumours. (Eric Gaillard/Reuters)

Breast cancers in women over age 50 that are first found between mammogram screenings are more likely to act aggressively and grow quickly, a new Canadian study suggests.

There are two types of breast tumours that can be missed on mammograms — "true" interval cancers that were not actually detectable on a previous mammogram, and "missed" ones that weren't found because of observer error or misinterpretation. 

Breast cancer screening guidelines

  • For women 40 to 49: Have a clinical breast exam performed by a doctor every two years.
  • Talk to your doctor about your risk of breast cancer, and benefits and risks of mammography.
  • If organized screening program in your province or territory does not accept women in their 40s, consult a doctor about the best way to get a mammogram .  

For women 50 to 69:

  • Have a clinical breast exam performed by a trained health-care professional at least every two years.
  • Have a mammogram every two years (all organized breast screening programs in Canada accept women in this age range for mammograms).

For women 70 and older:

  • Talk to your doctor about how often you should be tested for breast cancer.

Source: Canadian Cancer Society

In Tuesday's online issue of the Journal of the National Cancer, Anna Chiarelli of Cancer Care Ontario and her co-authors say true interval cancers were more likely to be aggressive with a poorer prognosis.

The researchers analyzed data for 288 women with true interval cancers and 87 women with missed interval cancers. Both kinds of interval tumours were compared with screen-detected tumours in 450 women who were similar in terms of age, location of the screening centre, and length of time since their last mammogram.

"We expected to find interval cancers to have a worse prognosis than screen-detected cancers," said Chiarelli. "However, very few studies have really looked at them by type of interval cancer. I think it was important to see those differences."

For women over 50, Chiarelli said the findings point to the importance of being "breast aware" to any breast symptoms that could occur between mammograms and getting those checked out by a doctor.

True interval cancers make up 75 per cent of missed tumours, Chiarelli noted. 

Specifically, the researchers found true interval cancers were more likely to have:

  • Grown rapidly in the time between mammograms.
  • Tended to be estrogen receptor negative, which are less common but have a poorer prognosis.
  • Have tumour cells that divide rapidly.
  • Have a less common type and arrangement of cancer cells.

"The aggressive tumour features we observed for interval cancers are likely partly because of the rapid proliferative rate, the delay in diagnosis, and partly reduced tumor detection on mammograms," the study's authors concluded.

"This suggests a need for further advancement in imaging technologies to detect certain types of breast carcinomas and different approaches for early detection of fast-growing tumours."

Chiarelli is now analyzing data on breast cancer cases up to 2009 to look for any differences in detection after better technologies like digital mammography were introduced.

The research looked at mainly Caucasian women.

It was funded by the Canadian Breast Cancer Research Alliance.

With files from CBC's Amina Zafar