A multidisciplinary team and patient perspective on omission of surgery after neoadjuvant systemic therapy for early breast cancer: A European Society of Surgical Oncology (ESSO) Research Academy survey.

BACKGROUND: Surgical de-escalation aims to reduce morbidity without compromising oncological outcomes. Trials to de-escalate breast cancer (BC) surgery among exceptional responders after neoadjuvant systemic therapy (NST) are ongoing. Combined patient and clinician insights on this strategy are unknown. METHODS: The European Society of Surgical Oncology Young Surgeons Alumni Club (EYSAC) performed an online survey to evaluate the perspective of multidisciplinary teams (MDTs) on omission of surgery ("no surgery") following complete response to NST for early BC. The aim was to identify MDT considerations and perceived barriers to omission of BC surgery. Patient insights were obtained through a focused group discussion (FGD) with four members of the patient advocacy group, Guiding Researchers and Advocates to Scientific Partnerships (GRASP). RESULTS: The MDT survey had 248 responses, with 229 included for analysis. Criteria for a "no surgery" approach included: patient's tumor and nodal status before (39.7 %) and after (45.9 %) NST and comorbidities (44.3 %). The majority chose standard surgery for hypothetical cases with a complete response to NST. Barriers for implementation were lack of definitive trials (55.9 %), "no surgery" not being discussed in MDTs (28.8 %) and lack of essential diagnostic or therapeutic options (24 %). Patients expressed communication gaps about BC surgery, lack of trust regarding accuracy of imaging, fear of regret and psychosocial burden of choosing less extensive surgery. CONCLUSIONS: Before accepting "no surgery" after complete response to NST, MDTs and patients need level 1 evidence from clinical trials, access to standard diagnostic modalities and treatments. Patient's fear of regretting less surgery need to be acknowledged and addressed.

Exploring locoregional treatment reporting in neoadjuvant systemic breast cancer treatment studies: A systematic review.

Accurate information about locoregional treatments in breast cancer neoadjuvant systemic therapy (NST) trials is vital to support surgical decision-making and allow meaningful interpretation of long-term oncological outcomes. This systematic review (PROSPERO registration CRD42023470891) aimed to describe the current practice of outcome reporting in NST studies. A systematic search identified primary research studies published 01/01/2018-08/09/2023 reporting outcomes in patients receiving NST for breast cancer followed by locoregional treatment. Included were randomised controlled trials (RCTs) and non-randomised studies (NRS) with >250 participants reporting at least one locoregional treatment outcome. Outcomes were extracted verbatim and categorised using content analysis. Descriptive statistics were used to summarise results. Of the 3111 abstracts screened, 137 studies (22 RCTs and 115 NRS) reporting at least one locoregional outcome in 575,531 patients were included. The 137 studies reported a total of 510 surgical outcomes with a median of 3 (range 1-12) per study. No single outcome was reported in all studies. Type of breast (n = 129, 94.2 %) and axillary (n = 86, 62.8 %) surgery were reported most frequently. Only 34 % (n = 47) studies reported how treatment response was assessed and if/how this informed surgical decision-making. Only a fifth (n = 28) reported outcomes relating to surgical de-escalation. Only 72 studies (52.6 %) reported any radiation therapy (RT)-related outcome, most frequently whether RT had been received (n = 63/72, 87.5 %). Current reporting of locoregional treatment outcomes in NST studies is poor, inconsistent and urgently needs to be improved. A core outcome set and reporting guidelines may improve the quality and value of future research.

Identifying research priorities for improving information and support for patients undergoing breast cancer surgery: a UK patient-centred priority setting project.

PURPOSE: To use robust consensus methods with individuals with lived breast cancer experience to agree the top 10 research priorities to improve information and support for patients undergoing breast cancer surgery in the UK. METHODS: Research uncertainties related to information and support for breast cancer surgery submitted by patients and carers were analysed thematically to generate summary questions for inclusion in an online Delphi survey. Individuals with lived breast cancer experience completed two Delphi rounds including feedback in which they selected their top 10 research priorities from the list provided. The most highly ranked priorities from the survey were discussed at an in-person prioritisation workshop at which the final top 10 was agreed. RESULTS: The 543 uncertainties submitted by 156 patients/carers were categorised into 63 summary questions for inclusion in the Delphi survey. Of the 237 individuals completing Round 1, 190 (80.2%) participated in Round 2. The top 25 survey questions were carried forward for discussion at the in-person prioritisation workshop at which 17 participants from across the UK agreed the final top 10 research priorities. Key themes included ensuring patients were fully informed about all treatment options and given balanced, tailored information to support informed decision-making and empower their recovery. Equity of access to treatments including contralateral mastectomy for symmetry was also considered a research priority. CONCLUSION: This process has identified the top 10 research priorities to improve information and support for patients undergoing breast cancer surgery. Work is now needed to develop studies to address these important questions.

Vacuum-assisted excision: a safe minimally invasive option for benign phyllodes tumor diagnosis and treatment-a systematic review and meta-analysis.

Synopsis: This is a systematic review and meta-analysis comparing surgical excision with percutaneous ultrasound-guided vacuum-assisted excision (US-VAE) for the treatment of benign phyllodes tumor (PT) using local recurrence (LR) as the endpoint. Objective: To determine the frequency of local recurrence (LR) of benign phyllodes tumor (PT) after ultrasound-guided vacuum-assisted excision (US-VAE) compared to the frequency of LR after surgical excision. Method: A systematic review and meta-analysis [following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) standard] was conducted by comparing LR in women older than 18 years treated for benign PT by US-VAE compared with local surgical excision with at least 12 months of follow-up. Studies were retrieved from PubMed, Scopus, Web of Science, and Embase. The pooled effect measure used was the odds ratio (OR) of recurrence. Results: Five comparative prospective or retrospective observational studies published between January 1, 1992, and January 10, 2022, comparing surgical excision with percutaneous US-VAE for LR of benign PT met the selection criteria. Four were retrospective observational cohorts, and one was a prospective observational cohort. A total of 778 women were followed up. Of them, 439 (56.4%) underwent local surgical excision, and 339 (43.6%) patients had US-VAE. The median age of patients in the five studies ranged from 33.7 to 39 years; the median size ranged from 1.5 cm to 3.0 cm, and the median follow-up ranged from 12 months to 46.6 months. The needle gauge ranged from 7G to 11G. LR rates were not statically significant between US-VAE and surgical excision (41 of 339 versus 34 of 439; OR 1.3; p = 0.29). Conclusion: This meta-analysis suggests that using US-VAE for the removal of benign PT does not increase local regional recurrence and is a safe minimally invasive therapeutic option. Systematic review registration: https://www.crd.york.ac.uk/prospero/, identifier CRD42022309782.

Understanding Patient Experiences to Inform Future Studies to Optimize Personalization of Treatment for Early Breast Cancer.

BACKGROUND: Breast cancer treatment is multimodal, but not all patients benefit from each treatment, and many experience morbidities significantly impacting quality of life. There is increasing interest in tailoring breast cancer treatments to optimize oncological outcomes and reduce treatment burden, but it is vital that future trials focus on treatments that most impact patients. This study was designed to explore patient experiences of treatment to inform future research. METHODS: An online survey was co-developed with patient advocates to explore respondents' experiences of breast cancer treatment. Questions included simple demographics, treatments received, and views regarding omitting treatments if that is deemed safe. The survey was circulated via social media and patient advocacy groups. Responses were summarized by using simple statistics; free text was analyzed thematically. RESULTS: Of the 235 participants completing the survey, 194 (82.6%) would choose to omit a specific treatment if safe to do so. The most commonly selected treatments were chemotherapy (n = 69, 35.6%) and endocrine therapy (n = 61, 31.4%) mainly due to side effects. Fewer respondents would choose to omit surgery (n = 40, 20.6%) or radiotherapy (n = 20, 10.3%). Several women commented that survival was their "absolute priority" and that high-quality evidence to support the safety of reducing treatment would be essential. CONCLUSIONS: Patients with breast cancer are individuals who may wish to optimize different components of their treatment. A portfolio of studies co-designed with patients is needed to establish an evidence base for greater treatment personalization with studies focused on reducing avoidable chemotherapy and endocrine therapy a priority.

The MARECA (national study of management of breast cancer locoregional recurrence and oncological outcomes) study: protocol for a prospective, multicentre cohort study.

Background: Despite a UK 5-year breast cancer survival rate of 86.6%, patients may develop breast cancer recurrence within the same breast after breast conserving surgery, as well as in the remaining skin or chest wall after mastectomy or in the ipsilateral lymph glands. These recurrences, collectively termed locoregional recurrence (LRR), occur in around 8% of patients within 10 years of their original diagnosis. Currently, there is a lack of robust information on the presentation and prevalence of LRR with no UK-specific clinical guidelines available for the optimal management of this patient group. Additionally, there is a need to identify patterns of LRR presentation and their progression, which will enable prognostic factors to be determined. This will subsequently enable the tailoring of treatment and improve patient outcome. Methods: The MARECA study is a prospective, multicentre cohort study recruiting patients diagnosed with breast cancer LRR +/- associated distant metastases. Over 50 UK breast units are participating in the study with the aim of recruiting at least 500 patients over a recruitment period of 24 months. The data collected will detail the tumour pathology, imaging results, surgical treatment, radiotherapy and systemic therapy of the primary and recurrent breast cancer. Study follow-up will be for up to 5 years following LRR diagnosis to determine subsequent oncological outcomes and evaluate potential prognostic factors. Discussion: This study will address the current knowledge gap and identify subgroups of patients who have less successful treatment outcomes. The results will determine the current management of LRR and the prognosis of patients diagnosed with breast cancer LRR +/- distant metastases in the UK, with the aim of establishing best practice and informing future national guidelines. The results will direct future research and inform the design of additional interventional trials and translational studies.

Minimally invasive treatment of early, good prognosis breast cancer-is this feasible?

Breast cancer screening programmes frequently detect early, good prognosis breast cancers with significant treatment burden for patients, and associated health-cost implications. Emerging evidence suggests a role for minimally invasive techniques in the management of these patients enabling many women to avoid surgical intervention. Minimally invasive techniques include vacuum-assisted excision, cryoablation, and radiofrequency ablation. We review published evidence in relation to the risks and benefits of each technique and discuss ongoing trials. Data to date are promising, and we predict a trend towards minimally invasive treatment for early, good-prognosis breast cancer as technical skills, suitability criteria, and follow-up protocols are established.

Digital pathology for reporting histopathology samples, including cancer screening samples - definitive evidence from a multisite study.

AIMS: To conduct a definitive multicentre comparison of digital pathology (DP) with light microscopy (LM) for reporting histopathology slides including breast and bowel cancer screening samples. METHODS: A total of 2024 cases (608 breast, 607 GI, 609 skin, 200 renal) were studied, including 207 breast and 250 bowel cancer screening samples. Cases were examined by four pathologists (16 study pathologists across the four speciality groups), using both LM and DP, with the order randomly assigned and 6 weeks between viewings. Reports were compared for clinical management concordance (CMC), meaning identical diagnoses plus differences which do not affect patient management. Percentage CMCs were computed using logistic regression models with crossed random-effects terms for case and pathologist. The obtained percentage CMCs were referenced to 98.3% calculated from previous studies. RESULTS: For all cases LM versus DP comparisons showed the CMC rates were 99.95% [95% confidence interval (CI) = 99.90-99.97] and 98.96 (95% CI = 98.42-99.32) for cancer screening samples. In speciality groups CMC for LM versus DP showed: breast 99.40% (99.06-99.62) overall and 96.27% (94.63-97.43) for cancer screening samples; [gastrointestinal (GI) = 99.96% (99.89-99.99)] overall and 99.93% (99.68-99.98) for bowel cancer screening samples; skin 99.99% (99.92-100.0); renal 99.99% (99.57-100.0). Analysis of clinically significant differences revealed discrepancies in areas where interobserver variability is known to be high, in reads performed with both modalities and without apparent trends to either. CONCLUSIONS: Comparing LM and DP CMC, overall rates exceed the reference 98.3%, providing compelling evidence that pathologists provide equivalent results for both routine and cancer screening samples irrespective of the modality used.

Cost-Effectiveness Analysis of Digital Breast Tomosynthesis Added to Synthetic Mammography in Breast Cancer Screening in Brazil.

BACKGROUND: Literature meta-analysis results show that digital breast tomosynthesis (DBT) combined with synthesized two-dimensional (s2D) mammograms can reduce recalls and improve breast cancer detection. Uncertainty regarding the screening of patients with breast cancer presents a health economic challenge, both in terms of healthcare resource use and quality of life impact on patients. OBJECTIVE: This study aims to estimate the cost effectiveness of DBT + s2D versus digital mammography (DM) used in a biennial breast cancer screening setting of women aged 40-69 years with scattered areas of fibroglandular breast density and heterogeneous dense breasts in the Brazilian supplementary health system. METHODS: A cost-effectiveness analysis was performed on the basis of clinical data obtained from a systematic review with meta-analysis performed to evaluate the analytical validity and clinical utility of DBT + s2D compared with DM. The search was conducted in the PubMed, Cochrane Library and Embase databases, with the main descriptors of the technology, a comparator, and the clinical condition in question, on 9 June 2022. The hybrid economic model (decision tree plus Markov model) simulated costs and outcomes over a lifetime for women aged 40-69 years with scattered areas of fibroglandular breast density and heterogeneous dense breasts. We analyzed incremental cost-effectiveness ratio (ICER) to measure the incremental cost difference per quality-adjusted life year (QALY) of adding DBT + s2D to breast cancer screening. RESULTS: DBT + s2D incurred a cost saving of € 954.02 per patient, in the time horizon of 30 years, compared with DM, and gained 5.1989 QALYs, which would be considered a dominant intervention. These results were confirmed in sensitivity analyses. CONCLUSION: Switching from DM to biennial DBT + s2D was cost effective. Furthermore, reductions in false-positive recall rates should also be considered in decision making.

Vaginal Estrogen Therapy Use and Survival in Females With Breast Cancer.

Importance: Genitourinary syndrome of menopause can be treated with vaginal estrogen therapy. However, there are concerns about the safety of vaginal estrogen therapy in patients with breast cancer. Objective: To determine whether the risk of breast cancer-specific mortality was higher in females with breast cancer who used vaginal estrogen therapy vs females with breast cancer who did not use hormone replacement therapy (HRT). Design, Setting, and Participants: This cohort study analyzed 2 large cohorts, one each in Scotland and Wales, of females aged 40 to 79 years with newly diagnosed breast cancer. These population-based cohorts were identified from national cancer registry records from 2010 to 2017 in Scotland and from 2000 to 2016 in Wales and were followed up for breast cancer-specific mortality until 2020. Females were excluded if they had a previous cancer diagnosis (except nonmelanoma skin cancer). Data analysis was performed between August 2022 and August 2023. Exposure: Use of vaginal estrogen therapy, including vaginal tablets and creams, was ascertained from pharmacy dispensing records of the Prescribing Information System for the Scotland cohort and from general practice prescription records for the Wales cohort. Main Outcomes and Measures: The primary outcome was time to breast cancer-specific mortality, which was obtained from national mortality records. Time-dependent Cox proportional hazards regression models were used to calculate hazard ratios (HRs) and 95% CIs for breast cancer-specific mortality, comparing vaginal estrogen therapy users with HRT nonusers and adjusting for confounders, including cancer stage and grade. Results: The 2 cohorts comprised 49 237 females with breast cancer (between 40 and 79 years of age) and 5795 breast cancer-specific deaths. Five percent of patients with breast cancer used vaginal estrogen therapy after breast cancer diagnosis. In vaginal estrogen therapy users compared with HRT nonusers, there was no evidence of a higher risk of breast cancer-specific mortality in the pooled fully adjusted model (HR, 0.77; 95% CI, 0.63-0.94). Conclusions and Relevance: Results of this study showed no evidence of increased early breast cancer-specific mortality in patients who used vaginal estrogen therapy compared with patients who did not use HRT. This finding may provide some reassurance to prescribing clinicians and support the guidelines suggesting that vaginal estrogen therapy can be considered in patients with breast cancer and genitourinary symptoms.

Global trends in psycho-oncology research investments 2016-2020: A content analysis.

OBJECTIVE: An estimated one-third of cancer patients experience a clinically significant psychological disorder, however it is unclear to what extent this is reflected in research funding. To address this a systematic analysis the allocation of psycho-oncology research funding globally between 2016 and 2020 was conducted. METHODS: A global dataset of 66,388 cancer research awards, from 2016 to 2020 inclusive and totalling $24.5 billion USD was assembled from public and philanthropic funders. Each award was previously categorised by cancer site type and research theme, including psychosocial research and these awards were further sub-categorised for this analysis. RESULTS: There was $523m of funding awarded for psychological research across 1122 studies: 2.14% of all cancer research funding during this period ($24.5 billion). Median funding per award was $97,473 (IQR $36,864 - $453,051). Within psychological research, mental health received most funding ($174m, 33.5% of psychological funding). Cognitive behavioural therapy (CBT) focused research was the specific psychological support with the highest proportion of funding at $14 million. By country of funder, the USA provided most investment ($375.5 m, 71.8%). CONCLUSIONS: Psycho-oncology research received relatively little funding, for example, when compared with pre-clinical cancer research. There needs to be a shift from pre-clinical science to research that benefits cancer patients in the shorter-term. Low- and middle-income countries, and ethnic minorities in higher-income settings, were underrepresented despite having a large cancer burden, indicating inequities that need to be addressed.

Global funding for cancer research between 2016 and 2020: a content analysis of public and philanthropic investments.

BACKGROUND: Cancer is a leading cause of disease burden globally, with more than 19·3 million cases and 10 million deaths recorded in 2020. Research is crucial to understanding the determinants of cancer and the effects of interventions, and to improving outcomes. We aimed to analyse global patterns of public and philanthropic investment in cancer research. METHODS: In this content analysis, we searched the UberResearch Dimensions database and Cancer Research UK data for human cancer research funding awards from public and philanthropic funders between Jan 1, 2016, and Dec 31, 2020. Included award types were project and programme grants, fellowships, pump priming, and pilot projects. Awards focused on operational delivery of cancer care were excluded. Awards were categorised by cancer type, cross-cutting research theme, and research phase. Funding amount was compared with global burden of specific cancers, measured by disability-adjusted life-years, years lived with disability, and mortality using data from the Global Burden of Disease study. FINDINGS: We identified 66 388 awards with total investment of about US$24·5 billion in 2016-20. Investment decreased year-on-year, with the largest drop observed between 2019 and 2020. Pre-clinical research received 73·5% of the funding across the 5 years ($18 billion), phase 1-4 clinical trials received 7·4% ($1·8 billion), public health research received 9·4% ($2·3 billion), and cross-disciplinary research received 5·0% ($1·2 billion). General cancer research received the largest investment ($7·1 billion, 29·2% of the total funding). The most highly funded cancer types were breast cancer ($2·7 billion [11·2%]), haematological cancer ($2·3 billion [9·4%]), and brain cancer ($1·3 billion [5·5%]). Analysis by cross-cutting theme revealed that 41·2% of investment ($9·6 billion) went to cancer biology research, 19·6% ($4·6 billion) to drug treatment research, and 12·1% ($2·8 billion) to immuno-oncology. 1·4% of the total funding ($0·3 billion) was spent on surgery research, 2·8% ($0·7 billion) was spent on radiotherapy research, and 0·5% ($0·1 billion) was spent on global health studies. INTERPRETATION: Cancer research funding must be aligned with the global burden of cancer with more equitable funding for cancer research in low-income and middle-income countries (which account for 80% of cancer burden), both to support research relevant to these settings, and build research capacity within these countries. There is an urgent need to prioritise investment in surgery and radiotherapy research given their primacy in the treatment of many solid tumours. FUNDING: None.

Bridging pre-surgical endocrine therapy for breast cancer during the COVID-19 pandemic: outcomes from the B-MaP-C study.

PURPOSE: The B-MaP-C study investigated changes to breast cancer care that were necessitated by the COVID-19 pandemic. Here we present a follow-up analysis of those patients commenced on bridging endocrine therapy (BrET), whilst they were awaiting surgery due to reprioritisation of resources. METHODS: This multicentre, multinational cohort study recruited 6045 patients from the UK, Spain and Portugal during the peak pandemic period (Feb-July 2020). Patients on BrET were followed up to investigate the duration of, and response to, BrET. This included changes in tumour size to reflect downstaging potential, and changes in cellular proliferation (Ki67), as a marker of prognosis. RESULTS: 1094 patients were prescribed BrET, over a median period of 53 days (IQR 32-81 days). The majority of patients (95.6%) had strong ER expression (Allred score 7-8/8). Very few patients required expedited surgery, due to lack of response (1.2%) or due to lack of tolerance/compliance (0.8%). There were small reductions in median tumour size after 3 months' treatment duration; median of 4 mm [IQR - 20, 4]. In a small subset of patients (n = 47), a drop in cellular proliferation (Ki67) occurred in 26 patients (55%), from high (Ki67 ≥ 10%) to low (< 10%), with at least one month's duration of BrET. DISCUSSION: This study describes real-world usage of pre-operative endocrine therapy as necessitated by the pandemic. BrET was found to be tolerable and safe. The data support short-term (≤ 3 months) usage of pre-operative endocrine therapy. Longer-term use should be investigated in future trials.

Identifying research priorities in breast cancer surgery: a UK priority setting partnership with the James Lind Alliance.

PURPOSE: A James Lind Alliance priority setting partnership was developed to identify research priorities in breast cancer surgery from individuals with lived experience, at high genetic risk of breast cancer, and healthcare professionals (HCPs). METHODS: 'Uncertainties' were collected using an online survey. Following an evidence check and development of summary questions, an interim survey asked participants to rank their top 10 research priorities from the question list. Top-ranked questions from patient/carer, high-risk and professional groups were carried forward for discussion to a final online prioritisation workshop. RESULTS: 260 participants (101 patients/carers, 156 HCPs) submitted 940 uncertainties via the initial survey. These were analysed thematically into 128 summary questions in six topic areas. Following evidence checking, 59 questions were included in the interim survey which was completed by 572 respondents. Marked differences were seen in questions prioritised by patients/carers, HCPs and women at high-risk. The top eight priorities in patient/carer and professional groups and top two priorities for high-risk women were carried forward to the online workshop at which 22 participants discussed and agreed the final top 10. Key themes included de-escalation of breast and axillary surgery, factors impacting the development/detection of locoregional recurrence and optimal provision of support for informed treatment decision-making. CONCLUSION: The top 10 research priorities in breast cancer surgery have been agreed. However, the observed differences in research priorities identified by patients and professional groups were not anticipated. Top priorities from both groups should inform future UK breast cancer surgical research, to ensure that it addresses questions that are important to breast cancer community as a whole.

The DNA damage induced immune response: Implications for cancer therapy.

Genomic instability is a hallmark of tumourigenesis, influencing tumour development and progression. In particular, defects in the DNA damage response (DDR) have been extensively investigated and are known to shape therapeutic response. Since immune checkpoint blockade (ICB) therapy has been approved for treatment of tumours with defective mismatch repair the interplay between DDR pathway deficiency and the immune system has been of particular interest. The cGAS/STING signalling pathway has recently emerged as a key mediator of inflammation in response to DNA damage.This was identified through transcriptional profiling of BRCA1/2 deficient breast cancers and Fanconi Anaemia (FA) patient bone marrow, revealing a common transcriptional subgroup associated with BRCA1/2 and FA deficiency characterised by upregulation of innate immune signalling genes. Additionally, it is now apparent that the DNA damage arising from a multitude of DNA repair defects and DNA damage induced by some classical chemotherapies/radiation also has the ability to induce an innate immune response mediated by cGAS/STING activation. Here we review the role of intrinsic and extrinsic DNA damage in mediating immune activation and its context within tumourigenesis, as well as the potential therapeutic opportunities it represents for the treatment of cancer, such as combining DNA damaging agents with immunotherapies.

Cancer-specific mortality in breast cancer patients with hypothyroidism: a UK population-based study.

PURPOSE: Epidemiological studies have indicated a higher prevalence of hypothyroidism in breast cancer patients, possibly related to shared risk factors and breast cancer treatments. However, few studies have evaluated how hypothyroidism impacts survival outcomes in breast cancer patients. We aimed to determine the association between hypothyroidism and breast cancer-specific and all-cause mortality. METHODS: We conducted a population-based study using the Scottish Cancer Registry to identify women diagnosed with breast cancer between 2010 and 2017. A matched comparison cohort of breast cancer-free women was also identified. Using hospital diagnoses and dispensed prescriptions for levothyroxine, we identified hypothyroidism diagnosed before and after breast cancer diagnosis and determined associations with breast cancer-specific and all-cause mortality. Cox proportional hazards regression was used to calculate hazard ratios (HR) and 95% confidence intervals (CI) adjusted for potential confounders. RESULTS: A total of 33,500 breast cancer patients were identified, of which 3,802 had hypothyroidism before breast cancer diagnosis and 565 patients went on to develop hypothyroidism after. Breast cancer patients had higher rates of hypothyroidism compared with cancer-free controls (HR 1.14, 95% CI 1.01-1.30). Among breast cancer patients, we found no association between hypothyroidism (diagnosed before or after) and cancer-specific mortality (before: HR 0.99, 95% CI 0.88-1.12, after: HR 0.97, 95% CI 0.63-1.49). Similar associations were seen for all-cause mortality. CONCLUSION: In a large contemporary breast cancer cohort, there was little evidence that hypothyroidism, either at diagnosis or diagnosed after breast cancer, was associated with cancer-specific or all-cause mortality.