Strategies to accelerate the elimination of cervical cancer in British Columbia, Canada: a modelling study.

BACKGROUND: To eliminate cervical cancer in Canada by 2040, defined as an annual age-standardized incidence rate (ASIR) lower than 4.0 per 100 000 women, the Canadian Partnership Against Cancer (CPAC) identified 3 priorities for action: increasing human papillomavirus (HPV) vaccine coverage, implementing HPV-based screening and increasing screening participation, and improving follow-up after abnormal screen results. Our objective was to explore the impact of these priorities on the projected time to elimination of cervical cancer in British Columbia. METHODS: We used OncoSim-Cervical, a microsimulation model led and supported by CPAC and developed by Statistics Canada that simulates HPV transmission and the natural history of cervical cancer for the Canadian population. We updated model parameters to reflect BC's historical participation rates and program design. We simulated the transition to HPV-based screening and developed scenarios to explore the additional impact of achieving 90% vaccination coverage, 95% screening recruitment, 90% ontime screening, and 95% follow-up compliance. We projected cervical cancer incidence, ASIR, and year of elimination for the population of BC for 2023-2050. RESULTS: HPV-based screening at current vaccination, participation, and follow-up rates can eliminate cervical cancer by 2034. Increasing on-time screening and follow-up compliance could achieve this target by 2031. Increasing vaccination coverage has a small impact over this time horizon. INTERPRETATION: With the implementation of HPV-based screening, cervical cancer can be eliminated in BC before 2040. Efforts to increase screening participation and follow-up through this transition could potentially accelerate this timeline, but the transition from cytology- to HPV-based screening is fundamental to achieving this goal.

Human papillomavirus-based screening at extended intervals missed fewer cervical precancers than cytology in the HPV For Cervical Cancer (HPV FOCAL) trial.

While cervix screening using cytology is recommended at 2- to 3-year intervals, given the increased sensitivity of human papillomavirus (HPV)-based screening to detect precancer, HPV-based screening is recommended every 4- to 5-years. As organized cervix screening programs transition from cytology to HPV-based screening with extended intervals, there is some concern that cancers will be missed between screens. Participants in HPV FOr CervicAL Cancer (HPV FOCAL) trial received cytology (Cytology Arm) at 24-month intervals or HPV-based screening (HPV Arm) at 48-month intervals; both arms received co-testing (cytology and HPV testing) at exit. We investigated the results of the co-test to identify participants with cervical intraepithelial neoplasia grade 2 or higher (CIN2+) who would not have had their precancer detected if they had only their arm's respective primary screen. In the Cytology Arm, 25/62 (40.3%) identified CIN2+s were missed by primary screen (ie, normal cytology/positive HPV test) and all 25 had normal cytology at the prior 24-month screen. In the HPV arm, three CIN2+s (3/49, 6.1%) were missed by primary screen (ie, negative HPV test/abnormal cytology). One of these three misses had low-grade cytology findings and would also not have been referred to colposcopy outside of the trial. Multiple rounds of cytology did not detect some precancerous lesions detected with one round of HPV-based screening. In our population, cytology missed more CIN2+, even at shorter screening intervals, than HPV-based screening. This assuages concerns about missed detection postimplementation of an extended interval HPV-based screening program. We recommend that policymakers consider a shift from cytology to HPV-based cervix screening.

A Systematic Review of Risk Factors for Development, Recurrence, and Progression of Vulvar Intraepithelial Neoplasia.

OBJECTIVE: Vulvar intraepithelial neoplasia (VIN) is a premalignant condition with high recurrence rates despite treatment. Vulvar intraepithelial neoplasia develops through separate etiologic pathways relative to the presence or absence of human papillomavirus (HPV) and TP53 mutations. This systematic review was conducted (1) to identify historical risk factors for the development, recurrence, and progression of VIN and (2) to critique these risk factors in the context of advances made in the stratification of VIN based on HPV or TP53 status. MATERIALS AND METHODS: A systematic search was performed on MEDLINE, Embase, Cochrane Database, PsychInfo, and CINAHL from inception to July 5, 2021. Three gynecologic oncologists independently evaluated the eligibility of studies based on predetermined inclusion and exclusion criteria, abstracted data, and then analyzed the relevant data. RESULTS: A total of 1,969 studies (involving 6,983 patients) were identified. Twenty-nine studies met inclusion criteria. The quality of evidence was low; primarily level 2b (Oxford Centre for Evidence-Based Medicine). Risk factors associated with the development of VIN include: smoking and coexisting vulvar dermatoses. Risk factors associated with recurrence include: smoking, multifocal disease, and positive surgical margins. Recent studies identified the presence of differentiated VIN/TP53 mutation as the most significant risk factor for both VIN recurrence and malignant progression. CONCLUSIONS: The current body of evidence consists primarily of small retrospective observational studies. Well-designed retrospective case-control series and/or prospective observational studies are urgently needed. Ideally, future studies will collect standardized data regarding associated risk factors and stratify women with VIN based on HPV and TP53 status.

A Scoping Review of Treatment Outcome Measures for Vulvar Intraepithelial Neoplasia.

OBJECTIVE: The goal of this study is to identify a list of clinician-reported outcome measures (CROMs) and patient-reported outcome measures (PROMs) through a review of published studies reporting on any therapeutic interventions for vulvar intraepithelial neoplasia (VIN). MATERIALS AND METHODS: A systematic search of published studies reporting on any therapeutic interventions for VIN was performed on MEDLINE, Embase, Cochrane Database, PsychInfo, and CINAHL from inception to September 20, 2021, based on predetermined study selection criteria. Data were extracted and analyzed by 2 authors independently using Covidence software. RESULTS: Thirty two of 2386 studies identified met study selection criteria. None of the 32 studies provided an explicit definition of VIN treatment "success." The most common CROM was "clinical response to treatment." The most common scale used to measure this outcome was "complete response/partial response/no response"; however, 17 of 23 studies (73.9%) did not define these values. Laboratory CROMs were reported in 12/32 (37.5%) studies. Patient-reported outcome measures were reported in only 10 of 32 studies(31.3%) -the most common PROM was "symptoms." Only 2 of 32 studies measured PROMs related to "quality of life" domains. Adverse events/treatment-related adverse effects were reported in 24 of 32 studies (75%), although 71% of studies provided no details on how these data were collected. CONCLUSIONS: There is a large variation in outcome measures, instruments, and scales used for any clinician-reported treatment outcome such as "clinical response." Most studies do not include patient-reported outcome measures assessing quality of life domains. A Core Outcome Set for the treatment of VIN is needed to improve the quality of VIN research.

Molecular characterization of invasive and in situ squamous neoplasia of the vulva and implications for morphologic diagnosis and outcome.

Human papillomavirus (HPV)-independent vulvar squamous cell carcinoma (VSCC) is an aggressive clinical entity. Current diagnostic guidelines for premalignant lesions are ambiguous, and their molecular profile and progression events are still unclear. We selected 75 samples, from 40 patients, including 33 VSCC, 8 verrucous carcinomas (VC), 13 differentiated-type vulvar intraepithelial neoplasia (dVIN), 11 suspicious for dVIN (?dVIN), 6 differentiated exophytic vulvar intraepithelial lesions (DE-VIL), 2 vulvar acanthosis with altered differentiation (VAAD), and 2 usual-type vulvar intraepithelial neoplasia (uVIN/HSIL). Invasive and precursor lesions were matched in 29 cases. Clinical information, p16 immunohistochemistry, and mutation analysis were performed on all lesions. All dVIN, ?dVIN, DE-VIL, and VAAD were p16 negative, all uVIN/HSIL were p16 positive. In the HPV-independent group, mutations were identified in 6 genes: TP53 (n = 40), PIK3CA (n = 20), HRAS (n = 12), MET (n = 5), PTEN (n = 4), and BRAF (n = 1). TP53 mutations occurred in 73% (22/30) VSCC, 85% (11/13) dVIN, 70% (7/10) ?dVIN and no VC (0/8), DE-VIL (0/6) nor VAAD (0/2). Basal atypia was the only reliable feature of TP53 mutations. ?dVIN lesions that were non-acanthotic and atypical but obscured by inflammation, all harbored TP53 mutations. In lesions without TP53 mutations, PIK3CA (50% VC, 33% DE-VIL, 100% VAAD, 40% VSCC) and HRAS (63% VC, 33% DE-VIL, 0% VAAD, 20% VSCC) mutations were found. Mutational progression from in situ to invasive was seen (7/26, 27%) and usually involved TP53 (4/26, 15%). Cases with TP53 and PIK3CA co-mutations had the worse clinical outcomes (p < 0.001). We recommend testing for p53 in all HPV-independent lesions suspicious for dVIN, even in the presence of marked inflammation or non-acanthotic skin, particularly when close to a margin. VC, VAAD, and DE-VIL, were almost never mutated for TP53, but instead often harbored PIK3CA and HRAS mutations. In VSCC, combined TP53 and PIK3CA mutations may inform prognosis.

p53 Immunohistochemical patterns in HPV-related neoplasms of the female lower genital tract can be mistaken for TP53 null or missense mutational patterns.

We have recently encountered p53 immunohistochemical (IHC) patterns in human papillomavirus (HPV)-associated carcinomas of the gynecologic tract, which were confused with absent (null) or overexpression TP53 mutational staining. We therefore evaluated p53 and p16 IHC in 25 squamous cell carcinomas (SCC) (16 vulva, 4 Bartholin's gland, and 5 cervix), 20 endocervical adenocarcinomas (EDAC), 14 high-grade squamous intraepithelial lesions (HSIL), 2 adenocarcinoma in situ (AIS), all of which exhibited morphologic features of HPV. Only cases showing diffuse/strong block-like p16 staining were included for further study. All EDACs underwent TP53 sequencing and HPV in situ hybridization (ISH) was performed in selected cases. p53 IHC staining fell into two main patterns. The most common was designated as "markedly reduced (null-like)" (absence or significantly attenuated staining in >70% of cells), which could be confused with true null mutational pattern. This was present in 14/25 (56%) SCCs, 7/14 (50%) HSILs, and 18/20 (90%) EDACs. The second notable pattern was "mid-epithelial (basal sparing)" (distinct absence of staining in basal cells juxtaposed with strong staining in parabasal cells), seen in 10/25 (40%) SCC, 7/14 (50%) HSIL, and none of the EDACs. There was scattered weak to moderate p53 staining (conventional wild type) in 1/25 (4%) SCC and 2/20 (10%) EDAC. No cases showed strong/diffuse overexpression. One EDAC had a TP53 missense mutation and exhibited "markedly reduced (null-like)" staining. HPV ISH revealed an inverse relationship with p53, cells positive for HPV mRNA were negative for p53. Knowledge of these patterns can help pathologists avoid misinterpreting p53 status in the setting of HPVA cancers.

Major p53 immunohistochemical patterns in in situ and invasive squamous cell carcinomas of the vulva and correlation with TP53 mutation status.

The recent literature has shown that vulvar squamous cell carcinoma (VSCC) can be stratified into two prognostically relevant groups based on human papillomavirus (HPV) status. The prognostic value of p53 for further sub-stratification, particularly in the HPV-independent group, has not been agreed upon. This disagreement is likely due to tremendous variations in p53 immunohistochemical (IHC) interpretation. To address this problem, we sought to compare p53 IHC patterns with TP53 mutation status. We studied 61 VSCC (48 conventional VSCC, 2 VSCC with sarcomatoid features, and 11 verrucous carcinomas) and 42 in situ lesions (30 differentiated vulvar intraepithelial neoplasia [dVIN], 9 differentiated exophytic vulvar intraepithelial lesions [deVIL], and 3 high-grade squamous intraepithelial lesions or usual vulvar intraepithelial neoplasia [HSIL/uVIN]). IHC for p16 and p53, and sequencing of TP53 exons 4-9 were performed. HPV in situ hybridization (ISH) was performed in selected cases. We identified six major p53 IHC patterns, two wild-type patterns: (1) scattered, (2) mid-epithelial expression (with basal sparing), and four mutant patterns: (3) basal overexpression, (4) parabasal/diffuse overexpression, (5) absent, and (6) cytoplasmic expression. These IHC patterns were consistent with TP53 mutation status in 58/61 (95%) VSCC and 39/42 (93%) in situ lesions. Cases that exhibited strong scattered staining and those with a weak basal overexpression pattern could be easily confused. The mid-epithelial pattern was exclusively observed in p16-positive lesions; the basal and parabasal layers that had absent p53 staining, appeared to correlate with the cells that were positive for HPV-ISH. This study describes a pattern-based p53 IHC interpretation framework, which can be utilized as a surrogate marker for TP53 mutational status in both VSCC and vulvar in situ lesions.

Association of human papilloma virus status and response to radiotherapy in vulvar squamous cell carcinoma.

INTRODUCTION: Vulvar squamous cell carcinoma develops through two separate pathways, associated with the presence or absence of high-risk human papilloma virus (HPV). The objective of this study was to evaluate treatment response and clinical outcomes in women with HPV-associated versus HPV-independent vulvar squamous cell carcinoma treated with primary radiation therapy, in order to determine the ability to use HPV status as a predictor of response to radiation therapy. METHODS: This was a retrospective cohort study combining data from British Columbia Cancer, Canada and Duke University, USA. Patients were included who had been treated with radiation therapy but excluded if they had received major surgical interventions. Immunohistochemistry for p16 (as a surrogate for high-risk HPV infection) and p53 was performed. We analyzed the univariable association between p16 status and clinico-pathological features and performed univariable survival analysis for p16. RESULTS: Forty-eight patients with vulvar squamous cell carcinoma treated with primary radiation therapy were identified: 26 p16 positive/HPV-associated patients and 22 p16 negative/HPV-independent patients. p16 positive vulvar squamous cell carcinoma demonstrated a significantly improved overall survival (HR 0.39, p=0.03) and progression-free survival (HR 0.35, p=0.02). In women treated with definitive radiation therapy, p16 positivity was associated with improved overall survival (HR 0.29, p<0.01) and progression-free survival (HR 0.21, p<0.01). Among patients who received sensitizing chemotherapy, a significant association was observed with p16 positive tumors and overall survival (HR 0.25, p=0.03) and progression-free survival (HR 0.09, p<0.01). CONCLUSION: This study suggests that HPV status in vulvar squamous cell carcinoma has both prognostic and predictive implications, with increased radiosensitivity demonstrated in HPV-associated vulvar squamous cell carcinoma. Implications may include radiation dose de-escalation for HPV-associated vulvar squamous cell carcinoma and increased surgical aggressiveness for HPV-independent vulvar squamous cell carcinoma.

Screening for Anal Cancer in Women With a History of Vulvar High-Grade Squamous Intraepithelial Lesions.

OBJECTIVES: The primary obkective was to determine the prevalence of (a) a positive anal cancer screen and (b) histological anal high-grade squamous intraepithelial lesion (HSIL) in women undergoing surveillance for previously diagnosed and treated human papillomavirus (HPV)-associated vulvar HSIL. The secondary objective was to determine the patients' acceptability of the screen. MATERIALS AND METHODS: This is a single-institution, cross-sectional pilot study. Women, aged 30 to 80 years, with a history of biopsy-proven vulvar HSIL were invited to undergo screening for anal cancer. Positive screen characterized by abnormalities in any of the following: anal high-risk HPV (HR-HPV); anal cytology; and digital anorectal examination. All women with an abnormal screen were referred for high-resolution anoscopy. All women completed a postscreen questionnaire. RESULTS: Fifty-seven patients were recruited. The median (interquartile range) age was 61.5 (51.0-68.0) years. The prevalence of a positive screen was 56.1% (95% CI = 43.3%-68.2%). Of the 32 screen-positive patients, 12 had both abnormal cytology and HR-HPV, 3 had positive HR-HPV alone, and 17 had abnormal cytology alone. Of the 29 patients with a positive screen who went on to anoscopy, the prevalence of anal HSIL was 33.3% (95% CI = 19.2%-51.2%). The prevalence of anal HSIL among all of those who had screening (N = 57) was 18.2% (95% CI = 10.2%-30.3%). The examination was well tolerated with 100% of patients, indicating that they would have the screening again. CONCLUSIONS: Women with vulvar HSIL have an increased risk of developing anal HSIL. Larger studies are needed to define optimal screening protocols as well as algorithms for management in high-risk populations.

Evidence of Decreased Long-term Risk of Cervical Precancer after Negative Primary HPV Screens Compared with Negative Cytology Screens in a Longitudinal Cohort Study.

BACKGROUND: The growing use of primary human papillomavirus (HPV) cervical cancer screening requires determining appropriate screening intervals to avoid overtreatment of transient disease. This study examined the long-term risk of cervical precancer after HPV screening to inform screening interval recommendations. METHODS: This longitudinal cohort study (British Columbia, Canada, 2008 to 2022) recruited women and individuals with a cervix who received 1 to 2 negative HPV screens (HPV1 cohort, N = 5,546; HPV2 cohort, N = 6,624) during a randomized trial and women and individuals with a cervix with 1 to 2 normal cytology results (BCS1 cohort, N = 782,297; BCS2 cohort, N = 673,778) extracted from the provincial screening registry. All participants were followed through the registry for 14 years. Long-term risk of cervical precancer or worse [cervical intraepithelial neoplasia grade 2 or worse (CIN2+)] was compared between HPV and cytology cohorts. RESULTS: Cumulative risks of CIN2+ were 3.2/1,000 [95% confidence interval (CI), 1.6-4.7] in HPV1 and 2.7/1,000 (95% CI, 1.2-4.2) in HPV2 after 8 years. This was comparable with the risk in the cytology cohorts after 3 years [BCS1: 3.3/1,000 (95% CI, 3.1-3.4); BCS2: 2.5/1,000 (95% CI, 2.4-2.6)]. The cumulative risk of CIN2+ after 10 years was low in the HPV cohorts [HPV1: 4.7/1,000 (95% CI, 2.6-6.7); HPV2: 3.9 (95% CI, 1.1-6.6)]. CONCLUSIONS: Risk of CIN2+ 8 years after a negative screen in the HPV cohorts was comparable with risk after 3 years in the cytology cohorts (the benchmark for acceptable risk). IMPACT: These findings suggest that primary HPV screening intervals could be extended beyond the current 5-year recommendation, potentially reducing barriers to screening.

2023 Canadian Colposcopy Guideline: A Risk-Based Approach to Management and Surveillance of Cervical Dysplasia.

This guideline provides evidence-based guidance on the risk-based management of cervical dysplasia in the colposcopy setting in the context of primary HPV-based screening and HPV testing in colposcopy. Colposcopy management of special populations is also discussed. The guideline was developed by a working group in collaboration with the Gynecologic Oncology Society of Canada (GOC), Society of Colposcopists of Canada (SCC) and the Canadian Partnership Against Cancer (CPAC). The literature informing these guidelines was obtained through a systematic review of the relevant literature via a multi-step search process led by information specialists. The literature was reviewed up to June 2021 with manual searches of relevant national guidelines and more recent publications. Quality of the evidence and strength of recommendations was assessed using the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) framework. The intended users of this guideline include gynecologists, colposcopists, screening programs and healthcare facilities. Implementation of the recommendations is intended to promote equitable and standardized care for all people undergoing colposcopy in Canada. The risk-based approach aims to improve personalized care and reduce over-/under-treatment in colposcopy.

Colposcopy referral rates post-introduction of primary screening with human papillomavirus testing: evidence from a large British Columbia cohort study.

Background: Shifting from cytology to human papillomavirus (HPV)-based cervical cancer screening will initially increase colposcopy referrals. The anticipated impact on health systems has been raised as a concern for implementation. It is unclear if the higher rate of colposcopy referrals is sustained after initial HPV-based screens or reverts to new lower baselines due to earlier detection and treatment of precancer. This study aimed to investigate long-term rates of colposcopy referrals after participation in HPV-based screening. Methods: Participants of HPV for Cervical Cancer Screening trial (HPV FOCAL) received one (HPV1, N = 6204) or two (HPV2, N = 9540) HPV-based screens. After exit, they returned to British Columbia's (BC) cytology screening program. A comparison cohort from the BC screening population (BCS, N = 1,140,745) was identified, mirroring trial inclusion criteria. All participants were followed for 10-14 years through the provincial screening registry. Colposcopy referral rates per 1000 screens were calculated for each group. Trial colposcopy referrals for HPV1 and HPV2 were calculated under two referral scenarios: (1) all HPV positive referred to colposcopy; (2) cytology triage with ASCUS or greater referred to colposcopy. Colposcopy referrals from post-trial screens in HPV1 an HPV2 and all screens in BCS were based on actual recommendations from the screening program. A multivariable flexible survival regression model compared hazard ratios (HR) throughout follow-up. Findings: Scenario 2 referral rates were higher during initial HPV screen(s) vs cytology screen (HPV1: 28 per 1000 screens (95% CI: 24, 33), HPV2: 32 per 1000 screens (95% CI: 29, 36), BCS: 8 per 1000 screens (95% CI: 8.9)). However, post-trial rates in HPV1 and HPV2 were significantly lower than in BCS. Cumulative rates in HPV1 and HPV2 approached the cumulative rate in BCS 11-12 years after HPV-based screening (HPV1: 11 per 1000 screens (95% CI: 10, 12), HPV2: 16 per 1000 screens (95% CI: 15-17), BCS: 11 per 1000 screens (95% CI: 10, 11)). Adjusted models demonstrated reductions in referral rates in HPV1 (HR = 0.6, 95% CI: 0.5, 0.7) and HPV2 (HR = 0.7, 95% CI: 0.6, 0.8) relative to BCS by 54 and 72 months post-final HPV screen respectively. Interpretation: Reduced colposcopy referral rates were observed after initial rounds of HPV-based screening. After initial HPV screening, referral rates to colposcopy after cytology triage were below the current rates seen in a centralized cytology program after approximately four years. Any expected increase in referrals at initiation of HPV-based screening could be countered by staged program implementation. Funding: This work was supported by the National Institutes of Health (R01 CA221918), Michael Smith Health Research BC (RT-2021-1595), and the Canadian Institutes of Health Research (MCT82072).

DICER1 and FOXL2 Mutation Status Correlates With Clinicopathologic Features in Ovarian Sertoli-Leydig Cell Tumors.

Sertoli-Leydig cell tumors (SLCTs) are rare ovarian sex cord-stromal neoplasms. The only known recurrent genetic abnormality is DICER1 mutation, with rare mutations reported in FOXL2. We set out to establish a molecular classifier using DICER1 and FOXL2 somatic mutation status and clinicopathologic features in 42 SLCTs. Five tumors (12%) were well differentiated, 31 (74%) moderately differentiated, and 6 (14%) poorly differentiated. Eight (19%) had heterologous elements, and 2 (5%) showed retiform differentiation; all 10 were moderately differentiated. DICER1 RNase IIIb domain mutations were identified in 18/41 (44%; 17 moderately, 1 poorly differentiated), including all cases with retiform or heterologous elements. FOXL2 c.402C>G (p.C134W) mutation was identified in 8/42 (19%) tumors (5 moderately, 3 poorly differentiated). DICER1 and FOXL2 mutations were mutually exclusive. Median age for the cohort was 47 years (range, 15 to 90 y). Patients with DICER1 mutations were younger (median, 24.5 y; range, 15 to 62 y) than patients with FOXL2 mutation (median, 79.5 y; range, 51 to 90 y) (P<0.0001). Nine of 10 tumors with retiform or heterologous elements occurred in premenopausal patients (median, 26.5 y; range, 15 to 57 y). Patients with tumors that were wild type for DICER1 and FOXL2 (15/42, 37%) had an intermediate age (median, 51 y; range, 17 to 74 y). All tumors were FOXL2 positive by immunohistochemistry. Patients with FOXL2 mutation trended toward presenting more often with abnormal bleeding (P=0.13); DICER1-mutant patients trended toward having more androgenic symptoms (P=0.22). Our data suggest at least 3 molecular subtypes of SLCT with distinct clinicopathologic features: DICER1 mutant (younger, more androgenic symptoms, moderately/poorly differentiated, retiform or heterologous elements), FOXL2 mutant (postmenopausal, abnormal bleeding, moderately/poorly differentiated, no retiform or heterologous elements), and DICER1/FOXL2 wild type (intermediate age, no retiform or heterologous elements, including all well-differentiated tumors).

Determinants of magnesium sulphate use in women hospitalized at <29 weeks with severe or non-severe pre-eclampsia.

OBJECTIVE: Magnesium sulphate is recommended by international guidelines to prevent eclampsia among women with pre-eclampsia, especially when it is severe, but fewer than 70% of such women receive magnesium sulphate. We aimed to identify variables that prompt Canadian physicians to administer magnesium sulphate to women with pre-eclampsia. METHODS: Data were used from the Canadian Perinatal Network (2005-11) of women hospitalized at <29 weeks' who were thought to be at high risk of delivery due to pre-eclampsia (using broad Canadian definition). Unadjusted analyses of relative risks were estimated directly and population attributable risk percent (PAR%) calculated to identify variables associated with magnesium sulphate use. A multivariable model was created and a generalized estimating equation was used to estimate the adjusted RR that explained magnesium sulphate use in pre-eclampsia. The adjusted PAR% was estimated by bootstrapping. RESULTS: Of 631 women with pre-eclampsia, 174 (30.1%) had severe pre-eclampsia, of whom 131 (75.3%) received magnesium sulphate. 457 (69.9%) women had non-severe pre-eclamspia, of whom 291 (63.7%) received magnesium sulphate. Use of magnesium sulphate among women with pre-eclampsia could be attributed to the following clinical factors (PAR%): delivery for 'adverse conditions' (48.7%), severe hypertension (21.9%), receipt of antenatal corticosteroids (20.0%), maternal transport prior to delivery (9.9%), heavy proteinuria (7.8%), and interventionist care (3.4%). CONCLUSIONS: Clinicians are more likely to administer magnesium sulphate for eclampsia prophylaxis in the presence of more severe maternal clinical features, in addition to concomitant antenatal corticosteroid administration, and shorter admission to delivery periods related to transport from another institution or plans for interventionist care.