Guidance

Breast screening: reporting, classification and monitoring of interval cancers and cancers following previous assessment

Updated 11 November 2024

Applies to England

1. Overview

This guidance provides advice for services on:

• reporting, classification and monitoring of interval cancers (ICs)
• how to carry out reviews for cancers that occur following a previous assessment

Interval cancers are cancers that are diagnosed in between screening episodes only within the routine population and very high risk (VHR) NHS breast screening programmes. The management of women screened through family history services is governed by National Institute for Health and Care Excellence (NICE) guidance.

There are 3 reasons to report and monitor interval cancers:

1. to monitor rates of interval cancers in conjunction with rates of screen-detected cancer to provide information on whole programme performance

2. to support education and learning objectives for screening services and individuals. This helps improve the quality of the service provided to women

3. to provide information on findings at retrospective review of screening mammograms, so that women with interval cancers who choose to have this information can understand whether any abnormality was present on their previous images

Revisions to previous guidance

This updated guidance includes these revisions:

• a nominated lead is required in services to coordinate the identification of interval cancers and the review process. This is to provide improved oversight of interval cancer monitoring and audit.
• the full pathology dataset for interval cancers (IC collection form 3) is no longer mandatory. These data may be obtained from other sources so duplication is not required.
• data entry of interval cancers must be checked by a second person. This is to ensure that data entry is robust and valid.
• guidance on how to classify interval cancers from previous assessment where the woman has declined diagnostic procedures. This helps to clarify the categorisation of interval cancers.
• clarification of how to classify interval cancers where assessment was done at a different service to where the screening occurred. This helps to clarify the categorisation of interval cancers.
• guidance on how to categorise previously assessed cancers where the interval cancer arose at the same site and side that was assessed but where screening assessment was adequate. This helps to clarify the   categorisation of interval cancers.
• trainees (fellows) are not allowed to undertake previous assessment reviews (PARs)This clarification is to ensure that PARs are performed by experienced staff.
• clarification of the arrangements for interval cancer reviews where support is required from other services. This is to provide guidance for services where capacity for interval cancer review is limited.
• there is a new process for local annual learning reviews of interval cancers led by the director of breast screening. This is to ensure that services regularly audit performance in relation to interval cancers with appropriate oversight by the screening director.
• information on when the duty of candour applies to interval cancers in the programme. This information is for information purposes to allow services to comply with this statutory legislation and had previously been circulated to services as advice but had not been incorporated into guidance.

1.1 Use of interval cancer reports to monitor the NHS Breast Screening Programme

Data from services on interval cancers can be used with the Screening History Information Management (SHIM) system and other data from the NHS Breast Screening Programme (BSP) to calculate sensitivity and specificity for the screening programme. These data are monitored annually within the consolidated standards for breast screening and are made available to services via the breast screening information system (BSIS). This allows comparison of the NHS Breast Screening Programme to international standards of performance for these indicators.

The screening histories manual, which is available to NHS breast screening providers, gives detailed guidance on definitions of types of interval cancer. This allows consistent and accurate reporting so that trends and comparisons can be identified and made.

1.2 Use of interval cancer reviews for education and learning

Image review and the classification of interval cancers in the NHS breast screening programme are particularly valuable for their educational benefit to image readers. By viewing cases where the mammograms show very subtle changes of malignancy, readers have been able to improve their skills in detecting small breast cancers.

Breast cancers can also be diagnosed in women who have been assessed at the previous screening episode then returned to the normal 3-yearly routine recall protocol. These cancers may present in the interval between screens or at the next routine breast screening episode. Timely review of cancers arising in women previously assessed can help determine whether national programme guidance has been followed at assessment. This document provides guidance on the use of the previous assessment review form to undertake these reviews.

For educational purposes, all cancers that occur in between screens should be subject to image review even if they have occurred beyond 36 months. This may occur when the round length is greater than 36 months and the screening invitation has been delayed.

1.3 Information for women

This document should be read alongside:

• guidance on applying duty of candour and disclosing audit results: this provides NHS Screening Programmes advice on how to share results of the audit with women should they wish to receive this information
• interval cancers and applying the duty of candour toolkit: this provides breast screening programme specific advice including details of the e-learning for health module and patient information

Breast screening and interval cancers

The NHS breast screening programme is organised to detect cancers at an earlier stage than would present symptomatically so that treatment outcomes will be more successful. Interval cancers arise in all organised breast screening programmes, as not all cancers are detectable on mammography. In the NHS Breast Screening Programme some cancers are not detectable at screening and will grow to become visible on mammography or clinically detectable in the period between routine screens. In a very small number of cases, cancers are undetected due to reader misinterpretation.

For every 1,000 women screened in England, around 9 women will have breast cancer detected at their screening appointment. Approximately 3 in every 1,000 women screened in England will have an interval cancer diagnosed. Of these women with interval cancers, approximately:

• 80% have no cancer visible on their previous screening mammogram
• 13% have cancers that most mammogram readers would not have detected due to very subtle changes on the mammogram
• 7% have cancers that most mammogram readers would have detected on the screening mammogram (false negatives)

1.4 Interval cancer definition

A screening interval cancer is a breast cancer diagnosed in the interval between scheduled screening episodes in women who have been screened and issued with a normal screening result.

If cancer is diagnosed after a final routine screening attendance by a woman aged between 68 and her 71st birthday, it is only defined as an interval cancer if it is diagnosed within 36 months of that attendance.

Both invasive and non-invasive cancers can be classified as interval cancers, but only invasive cancers are included in the calculation of interval cancer rates.

1.5 Measuring and comparing rates of interval cancers

The NHS Breast Screening Programme requires all eligible women aged 50 up to their 71st birthday to be invited for routine breast screening appointments at intervals of 36 months. In practice, women may be invited just before the age of 50 and they may be reinvited for a subsequent screen at either less or more than 36 months following their last screen. To ensure comparability between all screening services, there are strict rules when calculating rates of interval cancers.

Numerator is the number of women eligible for screening presenting with an invasive interval cancer within 36 months of a previous screen. The numerator only counts women who have been screened and developed an interval cancer.

Denominator is the total number of eligible women screened within a screening year

Interval cancers arising within 36 months of the specified period are expressed as a rate per 1,000 screened. In circumstances where service rates are being compared, interval cancers may be calculated over a complete round of 3 years.

Women will have their final routine screening appointment between the ages of 68 and 70. Some women may have had an additional invitation between the ages of 71 and 73 if they were randomised as part of the age extension trial (AgeX). Recruitment into the AgeX trial stopped in March 2020.

Any invasive cancers diagnosed in the 36 months following their last screening episode are defined as an interval cancer and should be counted in the rates of interval cancers. For example, if a woman is screened routinely at age 70 and is not diagnosed with cancer, she will count as having an interval cancer if later diagnosed with cancer up to the age of 73, even though the programme would not routinely invite her at this age (if it is within 36 months of her previous screen).

1.6 Monitoring interval cancers for educational and service improvement purposes

To learn from past and present performance, we need to identify and review as many women as possible diagnosed with an interval cancer.

The AgeX trial means some women below the age of 50 or up to 73 years had breast screening. This means that women can be diagnosed with an interval cancer outside of the core age bands of the routine programme.

Screening slippage (where services do not screen all of their eligible population within 36 months of their previous screen) happens for a variety of reasons. This means that women can be diagnosed with cancer beyond 36 months from their previous screen even though they have not yet been invited for their next routine screening appointment.

To ensure that we examine all possible interval cancers we require that:

• all women who have a cancer diagnosed within 40 months of their previous screen should be reviewed as an interval cancer case – this includes women who self-referred
• interval cancers should be examined for all women screened (as part of the routine screening programme, very high risk (VHR) breast screening programmes or the AgeX trial), whatever their age at screening
• invasive and non-invasive cancers should be included in reviews, but only invasive cancers should be included in the calculation of rates

More detailed information on the analysis of interval cancers is in the screening histories manual that is available to breast screening providers on the National Breast Screening System (NBSS) website.

2. Radiological review of interval cancers

Image review and the classification of interval cancers in a breast screening programme are particularly valuable for their educational benefit to image readers. By viewing cases where the mammograms show very subtle changes in malignancy, readers have been able to improve their skills in detecting small breast cancers.

Image reading review of interval cancers also provides helpful information for women diagnosed with breast cancer who request the results of the review of their previous images.

In all circumstances clinicians should follow disclosure of audit guidance. This includes applying duty of candour regulations where applicable.

There should be a nominated lead within the breast screening service to coordinate the identification of interval cancers and the review process.

2.1 Identification of interval cancers

Correctly identifying interval cancer cases is an important function of local services and the Screening Quality Assurance Service (SQAS). The Screening History Information Management (SHIM) system will also support this process.

Services can increase the correct identification of interval cancers by identifying women who are in one of the following categories:

• in the screening age group, have newly diagnosed breast cancer, and who are discussed at a symptomatic multidisciplinary team meeting
• receiving very high risk (VHR) screening within the NHS Breast Screening Programme

Practical information on the process of collection of interval cancer data and categorisation of screening histories is given in detail in the screening histories manual on the NBSS website. This document is designed for breast screening services and SQAS to use.

Any cancer found during the same episode of patient care along the screening or treatment pathway is considered to be a screen-detected cancer and should not be categorised as an interval cancer so long as the time between diagnoses is less than 3 months.

2.2 Recording interval cancers in screening services

Screening services must record all episodes of interval cancers on the national breast screening system (NBSS) in a timely way to allow adequate monitoring.  

The screening service must create an interval episode on NBSS as soon as an interval cancer is identified.

All imaging data relating to the interval cancer including radiological category (for example, ‘satisfactory’ or ‘unsatisfactory’) should be entered on NBSS within 6 months of the initial registration.

Women who have requested the results of an interval cancer review should receive them in a timely fashion. This should be within 4 months of the notification of the cancer to the service.

An interval cancer that occurs in a woman who has been previously assessed at breast screening for the side where the interval cancer subsequently presented,  should be reviewed by the service using the previous assessment review form. This should be completed within 4 months of the symptomatic presentation.

The inputting of pathology data on to NBSS is not mandated (interval cancer collection form 3). However, pathology data may be recorded on NBSS if a local decision is made that this information will be useful to support breast screening service audits and review processes. Where needed, it is easier to collect these data prospectively rather than try to capture it at a later date.

Once all required information has been correctly entered on NBSS and double checked by another person, the paper forms (numbered 1 to 3) can be confidentially destroyed.

2.3 Undertaking local review of interval cancers

NHS Breast Screening Programme requirements for interval cancer review are that:

• a minimum of 2 reviewers should review the images
• the previous screening mammograms, with all prior images available at the time of screening, should be reviewed by the readers independently – this should be done without sight of the mammograms taken at diagnosis
• the presence of any abnormal mammographic sign or feature on the previous screening image should be recorded and the radiological level of suspicion for malignancy reported
• the diagnostic images should then be reviewed to confirm that any subtle or suspicious signs detected on the previous screening images match the site of the confirmed breast cancer on the diagnostic images
• there should be very few images that fall into the ‘unsatisfactory’ category – these are images where the appearance is obviously malignant and all readers reviewing the images agree that they would recall
• if the diagnostic symptomatic images are unavailable for review, these are regarded as unclassifiable and should be reported as such on NBSS
• if a bilateral breast cancer is diagnosed, a separate review should be undertaken for each breast
• if an interval cancer is diagnosed following previous screening assessment (on the same side), the screening images should be regarded as unclassifiable, as an abnormality was reported at the time of initial screening (see section 4 on Review of ‘same side’ assessments)

Interval cancer collection forms are available to support these reviews. These forms reflect the fields in NBSS, to assist with data collection prior to data entry. Form 3 relates to the optional pathology information. 

2.4 Classifying screening interval cancers

All interval cancers should be classified into the following categories:

• Satisfactory (1)

Radiological: normal or benign mammographic features.

Readers found no reason to recall.

Breast screening service should follow disclosure of audit process.

• Satisfactory with learning points (2)

Radiological: seen with hindsight, difficult to perceive, not obviously malignant.

Not all readers would recall. Case may provide learning.

Breast screening service should follow disclosure of audit process.

• Unsatisfactory (3)

Radiological: appearance is obviously malignant.

All readers reviewing the images agree that they would recall. Woman should have been recalled.

Breast screening service should follow the duty of candour process.

• Unclassifiable (0)

Cases where the diagnostic imaging is not available

Cases where the woman was recalled to assessment at the last screen for the same side where the interval cancer was later diagnosed; a previous assessment review form should be completed for these cases

Images in the ‘unsatisfactory’ category should have obvious lesions. If there is any disagreement or it is felt this is only obvious with hindsight, they should be placed in the ‘satisfactory with learning points’ category.

Interval cancers that are classified as ‘unsatisfactory’ are subject to duty of candour guidance. These cases should be recorded as notifiable safety incidents in the provider’s risk management system (for example DATIX) and locally agreed procedures for duty of candour should be undertaken.

Additionally, when there is a classification of ‘unsatisfactory’, a screening incident assessment form (SIAF) should be completed and sent to the screening and immunisation team (SIT) and SQAS. This is so that SQAS can be kept informed of any learnings from the reviews. However, these cases will not normally be classified as a screening incident and therefore additional investigation or actions are not usually required.

It may not be possible to confirm an interval cancer classification for a small proportion of cases. This may occur where it has not been possible to obtain the required set of images, despite best efforts, or where a woman underwent screening  assessment prior to interval cancer diagnosis (see Review of ‘same side’ assessments section).

2.5 Recording interval cancer categories on NBSS

The NBSS system records the category of interval cancer. The system has not been updated to reflect the revised categories (as above). It is programme policy to retain the existing categories on NBSS. From August 2017, the categories converted to:

Category of interval cancer	Entry on NBSS
Satisfactory	Normal
Satisfactory with learning points	Uncertain
Unsatisfactory	Suspicious

This interval classification should be used for reviews and when inputting this on to NBSS. Where it has not been possible to determine an interval cancer classification, this should be recorded on NBSS as ‘unclassifiable’.

3. Cancers following assessment

Assessment cancers are breast cancers diagnosed in women who have been assessed and returned to routine screening following their previous breast screening appointment. This includes both interval and screen-detected cancers which have been assessed at the previous screening episode. Assessment cancers include all cancers regardless of where in the breast they occur and includes those cancers occurring in the opposite breast to the side on which assessment took place.

Assessment cancers diagnosed in the same breast (ipsilateral) are further classified as:

• same side as previously assessed
• same side and same site as previously assessed

The local service should review all assessment cancers, including those that happen after 36 months and up to 40 months after their previous screen. Breast cancers diagnosed in follow-up non-attenders (women who were either recalled for assessment or placed on short-term recall at the previous screening episode but failed to attend) are defined as interval cancers in assessment non-attenders.

Assessment cancers occurring in the opposite breast that are interval cancers should be reviewed according to the interval cancer review protocol above.

4. Review of ‘same side’ assessments

The review protocol for both interval and screen detected cancers that occur following assessment is described in the previous assessment review form (previously called Form 4). Use of this form will help produce consistency and objectivity in the review process.

However well assessment is conducted in a screening programme, cancers will occasionally occur at the same site as that previously assessed, either presenting as interval cancers or screen detected at the next screening appointment. Audit and review of these cases is essential for education, ongoing service improvement, and to establish and monitor the background rate of such events.

As these are known cancer cases, reviewers should be aware that hindsight can bias the review. This should be acknowledged when categorising assessment cancers.

When previous assessment was at the same site as the subsequent cancer diagnosis, it does not necessarily mean the previous assessment was flawed. Audit has shown that, on review, the majority of such cases had undergone adequate assessment. Cases falling into this category should have their interval cancer classification recorded as ‘unclassifiable’ on NBSS, as an area of concern was identified on the screening images.

4.1 Undertaking local review of ‘same side’ assessment cancers

The NHS Breast Screening Programme requirements for a review are that:

• the review of the previous assessment episode and current diagnostic images should be undertaken by 2 practitioners identified as eligible responsible assessors within an assessment clinic (such as a radiologist, breast clinician or radiographic consultant practitioner). Where a locum is in post, their involvement in these reviews would need to be agreed by the director of breast screening in advance. As trainees, fellows are not able to complete previous assessment reviews
• the review should be undertaken on the most recent assessment episode in the last 3 years
• the individual involved with the previous assessment should not undertake the review, but should be made aware of the outcome
• if a bilateral breast cancer is diagnosed, a separate review should be undertaken for each breast
• the following information should be available
  • original screening images
  • all documentation from assessment
  • all additional views and/or ultrasound images from assessment
  • documentation of multidisciplinary team opinion
  • symptomatic images and/or ultrasound
  • pathology reports

Where services require support from other breast screening services in order to complete previous assessment reviews, this must be subject to a formal agreement between services. This must clarify the necessary roles, responsibilities and governance arrangements in place and must be endorsed by the screening directors within the host organisations of both the responsible and the supporting breast screening service.

4.2 Completing a previous assessment review form

The previous assessment review form (previously called Form 4) should be completed in all cases where a cancer has been diagnosed on the same side following a previous assessment attendance. This includes:

• interval cancers
• screen detected (when the woman is diagnosed with cancer at the next routine screening appointment)
• when the woman is on short-term recall

When completing a previous assessment review form, consider the following points:

1. The site assessed at the initial assessment and the site where cancer is diagnosed at the time of diagnosis should be clearly marked on the pictograms. A decision is then made regarding whether this is the same or a different site within the breast.
2. If the cancer has been diagnosed at the same site, the case should be carefully reviewed using the previous assessment review form to document whether each aspect of the assessment was carried out according to expected processes and met national guidance on breast screening assessment or not.
3. If the cancer has been diagnosed at a different site to the previous assessment, the question on the form ‘Was this cancer at the same site?’ should be answered ‘No’. The rest of the form may then be left blank. The local service should still submit the form to SQAS.
4. If there is any aspect of assessment that is felt to be imperfect, the reviewer(s) need to consider whether this is a slight difference to what they would have done/expected. In this circumstance it should be classified as ‘satisfactory with learning points’ (see section 6.1).
5. If all reviewers agree that there was a clear failure either to follow guidance, or in the interpretation or investigations or multidisciplinary team decision-making, this should be classified as ‘unsatisfactory’.
6. If there is insufficient information to be sure about an aspect of a previous assessment, this should be accurately recorded as ‘don’t know’ or ‘uncertain’. The reason why this is unclear may be important also, for example if there is poor or inadequate documentation of what has been done or not done at assessment. Even if the information available is incomplete, a decision should be made to the best of the reviewers’ ability as to whether the assessment was satisfactory or not.
7. All previous assessment review forms should give an outcome of the review for ‘same side, same site’ reviews. This should be either satisfactory assessment, satisfactory assessment with learning points or unsatisfactory assessment.

Women may attend an assessment appointment and further examinations may indicate that biopsy is required in accordance with the guidance. Occasionally, the woman may decline to have a biopsy or other diagnostic procedure that would allow the detection or exclusion of malignancy.  In this scenario, the subsequent interval cancer classification following ‘same side, same site’ assessment, should be deemed ‘satisfactory (A)’, because the clinical intention was to follow the guidance.

If the screening assessment took place at a different service from where the woman was screened during that episode, the previous assessment review form should be completed by both screening services and then submitted to SQAS by the service where the woman was initially screened during the episode.

4.3 Classifying ‘same side, same site’ assessment cancers

• Satisfactory (A)

Assessment process guidance: guidance followed.

Assessment process: Assessment investigation and procedures carried out to a good standard.

Action: Nil.

Disclosure of audit process.

• Satisfactory with learning points (B)

Assessment process guidance: Minor deviation from guidance.

Assessment process: Some assessors might have interpreted or carried out procedures in a different way that might have resulted in the cancer being detected.

Action: May provide learning.

Disclosure of audit process.

• Unsatisfactory (C)

Assessment process guidance: Clear failure to follow guidance.

Assessment process: On review, all assessors would have interpreted the findings differently or performed the procedure to a higher standard.

Action: Learning required and possible retraining.

Duty of candour process.

5. After the completion of assessment forms

The breast screening service should process all previous assessment review forms, including those that are ‘same side, different site’, as follows:

• scanning and saving to a secure restricted folder on a shared drive. Once the scanned version has been checked to make sure it is adequate, the original paper copy can be confidentially destroyed
• sent to the local SQAS as soon as possible (and no longer than 4 months after the symptomatic presentation), with both sender and recipient using secure NHSmail

Local services should make sure that:

• reviews with the finding of ‘satisfactory’ or ‘satisfactory with learning points’ are subject to disclosure of audit
• reviews with the finding of ‘unsatisfactory assessment’ are subject to duty of candour guidance. These cases should be recorded as a notifiable safety incident in the provider’s risk management system and locally agreed procedures for duty of candour should be undertaken
• when there is a finding of ‘unsatisfactory assessment’, a screening incident assessment form (SIAF) should be completed and sent to the screening and immunisation team (SIT) and SQAS. SQAS will discuss with the service whether the case should be classified as a screening incident and whether any additional investigations or actions are required

The local SQAS will:

• review and record the results of all previous assessment review forms
• assist as and when requested by a service or trust with further advice on conducting a review or investigation of any unsatisfactory assessment case
• collate all cases and include in the annual data day review conducted by the complete SQAS team
• prepare an annual data summary at a breast screening service level. This will be collated nationally so that national comparisons can be completed
• review in the QA visit for radiology

5.1 Annual learning reviews

The director of breast screening or lead breast screening radiologist should complete an annual learning review. This review has 2 main aims:

• to look for trends in the data that may not be apparent from the review of individual cases completed throughout the year
• to improve the data completeness of the information submitted to SQAS, with incomplete data fields being highlighted and supplied to SQAS

Each director of breast screening or lead breast screening radiologist should submit a response to SQAS to confirm that the previously assessed cases have been shared and discussed with the wider team locally. They should also provide SQAS with a summary of key learning points from the review, including whether they have made any changes to their practice as a result.

6. Disclosure of audit results and applying duty of candour

Guidance on applying duty of candour details best practice in providing information to individuals when they receive a positive diagnosis after a screening result that was reported as negative (normal).

The interval cancer pathway and screen detected pathway show the process that should be followed to enable disclosing outcomes of audit or duty of candour by screening services.

6.1 Dates to apply duty of candour

The duty of candour regulations set out the actions that providers must follow when things go wrong with care and treatment in the NHS Breast Screening Programme. They came into force in November 2014 for NHS bodies and April 2015 for all other organisations.

If an incident happened before the regulations came into force, the Care Quality Commission (CQC) may not be able to take formal regulatory action or prosecute over a breach of the duty.

For interval cancers diagnosed before 27 November 2014, individual trusts should decide themselves whether duty of candour applies.

There are 2 scenarios when duty of candour must apply to interval cancers in the screening programme. These are:

• when a screening test was performed before 27 November 2014 and an interval cancer was diagnosed after that date
• when a screening test was performed after 27 November 2014 and an interval cancer was diagnosed

6.2 Suggested examples/questions in reaching a decision to classify assessment

Was the area that was recalled the area that was assessed?

Was the area that was recalled clearly marked at image reading to aid the assessor?

Did the assessing clinician assess the correct site?

Were additional views performed for a soft tissue abnormality?

If so, were they of the correct site and of appropriate quality?

Were these correctly interpreted?

Ultrasound – is there documentation to indicate that the correct site of the breast was scanned?

For example, a lesion only seen over the pectoral muscle on oblique view may lie in the upper inner breast.

Was definite correlation between the mammographic feature and the ultrasound shown?

Clinical abnormality such as possible tethering or a lump?

Did a suitably qualified clinician examine the woman and was appropriate imaging conducted, such as ultrasound?

Biopsy – was this performed with the correct technique and of the correct area?

Examples might include nodular breast pattern, or one more prominent /dominant nodule recalled

If biopsy was ultrasound guided

Was a tissue marker placed and post procedure images taken to ensure the correct nodule was sampled, or was stereo biopsy used for biopsy guidance?

If an asymmetrical density has not been explained

Was stereo biopsy performed?

Was multi-disciplinary team decision-making robust?

For example:

• a new and radiologically indeterminate 15mm cluster of calcifications sampled by stereo guidance, but no calcification was seen on specimen x-ray and a B2, benign, core biopsy result was accepted by the multi-disciplinary team meeting (sampling in this scenario is inadequate and second line vacuum-assisted biopsy is indicated)
• if histology at the multi-disciplinary team meeting does not appear to correlate with the mammographic feature described, was a further biopsy advised/performed?