Breast Cancer Screening in Ontario: Mammograms, Breast Density, and When to Start.
Breast cancer screening can detect some cancers before symptoms develop, when treatment may be less extensive and outcomes may be better. Mammography remains the standard first-line screening test. The appropriate starting age, interval, and need for additional imaging depend on age, personal risk, and breast density.
Ontario expanded the Ontario Breast Screening Program (OBSP) in October 2024. Eligible people aged 40 to 74 can now self-refer for an OHIP-funded screening mammogram without a referral from a physician or nurse practitioner.[1,2]
Access to screening and the best screening plan are related questions. A personalized decision should consider the potential benefits, limitations, breast density, family history, and individual preferences.
Screening Mammography Versus Diagnostic Breast Imaging
Screening mammography is intended for people who do not have new breast symptoms. Its purpose is to identify changes that may represent cancer before they can be felt or otherwise noticed.
A new breast concern requires clinical assessment and may require diagnostic imaging. Symptoms that should be assessed promptly include:
A new breast or underarm lump
Bloody or unexpected nipple discharge
New nipple inversion
Skin dimpling, thickening, redness, or swelling
A new change in breast shape or size
Persistent, localized breast symptoms
A recent normal mammogram does not rule out every breast cancer, particularly in people with dense breasts. New symptoms should be evaluated promptly through a diagnostic pathway, regardless of the date of the last screening appointment.[5,9]
Who Can Self-Refer for a Mammogram in Ontario?
Most Ontario residents aged 40 to 74 can self-refer to the OBSP if they:
Have a valid Ontario health card
Have no new breast cancer symptoms
Have no personal history of breast cancer
Have not had a bilateral mastectomy
Have not had a screening mammogram within the previous 11 months
Have used feminizing hormones for at least five consecutive years, if transfeminine
Eligible individuals can contact an OBSP location directly. A physician or nurse practitioner referral is not required, and screening is covered by OHIP.[1,2]
These eligibility rules apply to organized screening. People with symptoms, a previous breast cancer diagnosis, breast implants, or other clinical considerations may require a different imaging pathway coordinated through a healthcare professional.
When Should Mammograms Start?
Ages 40 to 49
Eligible people in Ontario can begin self-referred mammography at age 40. The OBSP encourages people in this age group to make an informed decision based on their risk factors, preferences, and the benefits and limitations of screening.[1,2]
Evidence supports a mortality benefit from mammographic screening. A systematic review found an approximately 20 percent relative reduction in breast cancer mortality across women at average risk.[3] In the UK Age trial, annual mammography beginning at age 40 or 41 was associated with a 25 percent relative reduction during the first 10 years of follow-up, with a smaller relative effect over longer follow-up.[4]
The absolute benefit is smaller in the 40s because breast cancer is less common at younger ages. Earlier screening can also increase false-positive results, additional imaging, and benign biopsies. Informed decision-making is especially important between ages 40 and 49.
Ages 50 to 74
The Ontario Breast Screening Program recommends mammography every two years for most eligible people in this age range.[1] This group has the most established balance of screening benefits and potential harms.
Some people may be recalled sooner based on their mammogram findings, breast density, or overall risk. The interval stated in the mammography report and OBSP recall letter should be followed.
Age 75 and Older
The OBSP does not routinely recall people after age 74. Continued screening can still be considered following discussion with a physician or nurse practitioner, and a referral is generally required. The decision should account for overall health, life expectancy, previous screening history, and personal preferences.[1,9]
The Benefits and Limitations of Mammography
Mammography is associated with lower breast cancer mortality.[3] Its limitations include:
False-positive results: An abnormal screen may lead to additional imaging or biopsy and ultimately prove benign.
False-negative results: Some cancers are not visible on mammography, especially in dense breast tissue.
Overdiagnosis: Screening can identify cancers that may never have caused illness during a person's lifetime. The exact frequency remains uncertain.
Radiation exposure: Mammography uses a low dose of ionizing radiation. The radiation-related risk from recommended screening is considered small.
Screening frequency affects this balance. In one systematic review, the estimated 10-year probability of a false-positive biopsy after starting mammography at age 40 was approximately 7.0 percent with annual screening and 4.8 percent with biennial screening.[3]
What Is Breast Density?
Breast density describes the proportion of fibrous and glandular tissue compared with fatty tissue on a mammogram. It cannot be determined by breast size, appearance, touch, or clinical examination.[5]
Mammography reports commonly use four Breast Imaging Reporting and Data System categories:
Category A: Almost entirely fatty
Category B: Scattered areas of fibroglandular density
Category C: Heterogeneously dense
Category D: Extremely dense
Categories C and D are considered dense breasts.
Breast density matters for two reasons. Dense tissue can obscure a cancer on a mammogram because both dense tissue and many tumours appear white. Density is also an independent breast cancer risk factor. Compared with predominantly fatty breasts, category D density has been associated with an approximately fourfold to sixfold higher relative risk of breast cancer in published studies.[5]
Density is one part of overall risk. Age, family history, genetic variants, previous breast biopsies, hormone exposure, alcohol intake, and other factors also affect risk.
What Happens if You Have Dense Breasts in Ontario?
Ontario mammography reports include a breast-density assessment. People with category D density are recalled by the OBSP for another mammogram in one year instead of the usual two-year interval.[5,6]
Supplemental screening may include breast ultrasound, magnetic resonance imaging (MRI), contrast-enhanced mammography, or digital breast tomosynthesis. These tests may detect additional cancers after a normal mammogram and can increase abnormal recalls, additional testing, and benign biopsies. Current evidence has not established whether supplemental screening based only on breast density reduces breast cancer mortality.[6]
Ontario Health recommended public funding of supplemental screening for people with category D density following a provincial health technology assessment. As of September 2026, supplemental screening is not automatically arranged through the routine OBSP density pathway. Physician- or nurse practitioner-ordered imaging may be OHIP-funded when medically indicated and accepted by the imaging site. Access depends on the modality, local capacity, contraindications, and site criteria.[6]
A 2026 Canadian Society of Breast Imaging position statement recommends offering supplemental screening to people with category C or D density, with MRI preferred where appropriate and available.[7] This professional guidance does not change Ontario program eligibility or guarantee access to a specific test. Decisions should incorporate total breast cancer risk, expected benefit, potential false positives, and patient preferences.
Dense breasts alone do not automatically qualify someone for Ontario's High Risk OBSP.
Who Qualifies for High-Risk Breast Screening?
The High Risk OBSP provides annual mammography and breast MRI, or ultrasound when MRI is medically unsuitable, for eligible people aged 30 to 69. Entry requires a referral from a physician or nurse practitioner and valid OHIP coverage.[8]
Eligibility may include:
A known pathogenic or likely pathogenic gene variant associated with breast cancer, such as BRCA1, BRCA2, TP53, or PALB2
A first-degree relative with a qualifying genetic variant, following genetic counselling and specific program criteria
A lifetime breast cancer risk of at least 25 percent based on personal and family history, formally assessed through a genetics clinic using an accepted model
Chest radiation used to treat another cancer before age 30, with at least eight years since treatment
Personal or family history patterns that meet criteria for genetic assessment
A family history of breast cancer does not always meet the high-risk program threshold. A structured risk assessment can determine whether genetic counselling, earlier screening, or more intensive imaging is appropriate.
Practical Next Steps
Age 40 to 74 with no breast symptoms: Check when your last mammogram was completed. Eligible Ontario residents can contact an OBSP location directly.
Age 40 to 49: Review the potential benefits and limitations of beginning screening with a qualified health professional or Health811 navigator.
After a mammogram: Confirm your breast-density category, recall interval, and whether supplemental imaging should be discussed.
Strong family history or another high-risk feature: Ask whether formal genetic assessment or referral to the High Risk OBSP is indicated.
Any new breast symptom: Arrange prompt clinical assessment, regardless of age or the date of your last mammogram.
The Bottom Line
Ontario residents aged 40 to 74 who meet OBSP eligibility criteria can self-refer for an OHIP-funded screening mammogram. Every two years is the standard interval for most people, while category D density generally leads to annual mammography. Supplemental imaging may be appropriate for some individuals and carries a higher likelihood of additional testing. An appropriate plan incorporates provincial guidance, breast density, family history, genetic risk, overall health, and personal preferences.
References
Government of Ontario. Breast cancer screening and prevention. Accessed September 2, 2026.
Ontario Health. Ontario Increases Access to Breast Cancer Screening by Lowering Starting Age to 40. Published October 8, 2024. Accessed September 2, 2026.
Myers ER, Moorman P, Gierisch JM, et al. Benefits and Harms of Breast Cancer Screening: A Systematic Review. JAMA. 2015;314(15):1615-1634. doi:10.1001/jama.2015.13183.
Duffy SW, Vulkan D, Cuckle H, et al. Effect of Mammographic Screening From Age 40 Years on Breast Cancer Mortality: Final Results of a Randomised, Controlled Trial. Lancet Oncology. 2020;21(9):1165-1172. doi:10.1016/S1470-2045(20)30398-3.
Wilkinson AN. Breast Density Notification: Are Family Doctors Prepared to Counsel Patients on Risks and Management? Canadian Family Physician. 2023;69(11):748-750. doi:10.46747/cfp.6911748.
Ontario Health. Supplemental Screening as an Adjunct to Mammography for Breast Cancer Screening in People With Dense Breasts: A Health Technology Assessment. Ontario Health Technology Assessment Series. 2023;23(9):1-293. PubMed PMID: 39364436.
Kellow Z, Kulkarni S, Gordon PB, Seely JM. Canadian Society Breast Imaging Position Statement on Mammographic Breast Density and Supplemental Screening. Journal of Breast Imaging. 2026;8(1):1-7. doi:10.1093/jbi/wbaf064.
Ontario Health. High Risk Ontario Breast Screening Program Requisition Form and Eligibility Criteria. Accessed September 2, 2026.
Canadian Cancer Society. When Should I Be Screened for Breast Cancer? and Symptoms of Breast Cancer. Accessed September 2, 2026.
Disclaimer
This article is for general educational purposes only and does not constitute medical advice. Breast cancer risk and screening needs vary between individuals. Screening and diagnostic decisions should be made with a qualified healthcare professional who can consider your personal history, family history, symptoms, preferences, and current Ontario guidance. Provincial programs, eligibility criteria, funding, and local imaging access may change over time.
Ontario program information reviewed and current as of September 2, 2026.
