Variation in management of lymph node positive prostate cancer after radical prostatectomy within a statewide quality improvement consortium.

BACKGROUND: Patients with lymph node positive (pN+) disease found at the time of radical prostatectomy with pelvic lymphadenectomy for clinically localized prostate cancer (CaP) are at high risk of disease persistence and progression. Contemporary management trends of pN+ CaP are not well described. MATERIALS AND METHODS: Patients in the Michigan Urologic Surgery Improvement Collaborative (MUSIC) with clinically localized prostate cancer who underwent radical prostatectomy between 2012 and 2023 with cN0/pN+ disease were identified. The primary outcome was to evaluate patient and practice-level factors associated with time to secondary post-RP treatment. Secondary outcomes included practice-level variation in management of pN+ CaP and rates of secondary treatment modality. To assess factors associated with secondary treatment, a Cox proportional hazards model of a 60-day landmark analysis was performed. RESULTS: We identified 666 patients with pN+ disease. Overall, 66% underwent secondary treatment within 12 months post-RP. About 19% of patients with detectable post-RP PSA did not receive treatment. Of patients receiving secondary treatment after 60-days post-RP, 34% received androgen deprivation therapy (ADT) alone, 27% received radiation (RT) alone, 36% received combination, and 4% received other systemic therapies. In the multivariable model, pathologic grade group (GG)3 (HR 1.5; 95%CI: 1.05-2.14), GG4-5 (HR 1.65; 95%CI: 1.16-2.34), positive margins (HR 1.46; 95%CI: 1.13-1.88), and detectable postoperative PSA ≥0.1 ng/ml (HR 3.46; 95%CI: 2.61-4.59) were significantly associated with secondary post-RP treatment. There was wide variation in adjusted practice-level 12-month secondary treatment utilization (28%-79%). CONCLUSIONS: The majority pN+ patients receive treatment within 12 months post-RP which was associated with high-risk pathological features and post-RP PSA. Variation in management of pN+ disease highlights the uncertainty regarding the optimal management. Understanding which patients will benefit from secondary treatment, and which type, will be critical to minimize variation in care.

Can MRI Help Inform Which Men With a History of Multifocal High-Grade Prostatic Intraepithelial Neoplasia or Atypical Small Acinar Proliferation Remain at an Elevated Risk for Clinically Significant Prostate Cancer?

PURPOSE: We investigated the association of MRI findings in men with a previous diagnosis of atypical small acinar proliferation (ASAP) or multifocal high-grade intraepithelial neoplasia (HGPIN) with pathologic findings on repeat biopsy. MATERIALS AND METHODS: We retrospectively reviewed patients with ASAP/multifocal HGPIN undergoing a repeat biopsy in the Michigan Urological Surgery Improvement Collaborative registry. We included men with and without an MRI after the index biopsy demonstrating ASAP/multifocal HGPIN but before the repeat biopsy. Men with an MRI prior to the index biopsy were excluded. We compared the proportion of men with ≥ GG2 CaP (Grade Group 2 prostate cancer) on repeat biopsy among the following groups with the χ2 test: no MRI, PIRADS (Prostate Imaging-Reporting and Data System) ≥ 4, and PIRADS ≤ 3. Multivariable models were used to estimate the adjusted association between MRI findings and ≥ GG2 CaP on repeat biopsy. RESULTS: Among the 207 men with a previous diagnosis of ASAP/multifocal HGPIN that underwent a repeat biopsy, men with a PIRADS ≥ 4 lesion had a higher proportion of ≥ GG2 CaP (56%) compared with men without an MRI (12%, P < .001). A lower proportion of men with PIRADS ≤ 3 lesions had ≥ GG2 CaP (3.0%) compared with men without an MRI (12%, P = .13). In the adjusted model, men with a PIRADS 4 to 5 lesion had higher odds (OR: 11.4, P < .001) of ≥ GG2 CaP on repeat biopsy. CONCLUSIONS: MRI is a valuable diagnostic tool to triage which men with a history of ASAP or multifocal HGPIN on initial biopsy should undergo or avoid repeat biopsy without missing clinically significant CaP.

MyProstateScore in men considering repeat biopsy: validation of a simple testing approach.

BACKGROUND: Men with persistent risk of Grade Group (GG) ≥ 2 cancer after a negative biopsy present a unique clinical challenge. The validated MyProstateScore test is clinically-available for pre-biopsy risk stratification. In biopsy-naïve patients, we recently validated a straightforward testing approach to rule-out GG ≥ 2 cancer with 98% negative predictive value (NPV) and 97% sensitivity. In the current study, we established a practical MPS-based testing approach in men with a previous negative biopsy being considered for repeat biopsy. METHODS: Patients provided post-digital rectal examination urine prior to repeat biopsy. MyProstateScore was calculated using the validated, locked model including urinary PCA3 and TMPRSS2:ERG scores with serum PSA. In a clinically-appropriate primary (i.e., training) cohort, we identified a lower (rule-out) threshold approximating 90% sensitivity and an upper (rule-in) threshold approximating 80% specificity for GG ≥ 2 cancer. These thresholds were applied to an external validation cohort, and performance measures and clinical outcomes associated with their use were calculated. RESULTS: MyProstateScore thresholds of 15 and 40 met pre-defined performance criteria in the primary cohort (422 patients; median PSA 6.4, IQR 4.3-9.1). In the 268-patient validation cohort, 25 men (9.3%) had GG ≥ 2 cancer on repeat biopsy. The rule-out threshold of 15 provided 100% NPV and sensitivity for GG ≥ 2 cancer and would have prevented 23% of unnecessary biopsies. Use of MyProstateScore >40 to rule-in biopsy would have prevented 67% of biopsies while maintaining 95% NPV. In the validation cohort, the prevalence of GG ≥ 2 cancer was 0% for MyProstateScore 0-15, 6.5% for MyProstateScore 15-40, and 19% for MyProstateScore >40. CONCLUSIONS: In patients who previously underwent a negative prostate biopsy, the MyProstateScore values of 15 and 40 yielded clinically-actionable rule-in and rule-out risk groups. Using this straightforward testing approach, MyProstateScore can meaningfully inform patients and physicians weighing the need for repeat biopsy.

Urinary MyProstateScore (MPS) to Rule out Clinically-Significant Cancer in Men with Equivocal (PI-RADS 3) Multiparametric MRI: Addressing an Unmet Clinical Need.

OBJECTIVE: To evaluate the complementary value of urinary MyProstateScore (MPS) testing and multiparametric MRI (mpMRI) and assess outcomes in patients with equivocal mpMRI. MATERIALS AND METHODS: Included patients underwent mpMRI followed by urine collection and prostate biopsy at the University of Michigan between 2015 -2019. MPS values were calculated from urine specimens using the validated model based on serum PSA, urinary PCA3, and urinary TMPRSS2:ERG. In the PI-RADS 3 population, the discriminative accuracy of PSA, PSAD, and MPS for GG≥2 cancer was quantified by the AUC curve. Decision curve analysis was used to assess net benefit of MPS relative to PSAD. RESULTS: There were 540 patients that underwent mpMRI and biopsy with MPS available. The prevalence of GG≥2 cancer was 13% for PI-RADS 3, 56% for PI-RADS 4, and 87% for PI-RADS 5. MPS was significantly higher in men with GG≥2 cancer [median 44.9, IQR (29.4 -57.5)] than those with negative or GG1 biopsy [median 29.2, IQR (14.8 -44.2); P <.001] in the overall population and when stratified by PI-RADS score. In the PI-RADS 3 population (n = 121), the AUC for predicting GG≥2 cancer was 0.55 for PSA, 0.62 for PSAD, and 0.73 for MPS. MPS provided the highest net clinical benefit across all pertinent threshold probabilities. CONCLUSION: In patients that underwent mpMRI and biopsy, MPS was significantly associated with GG≥2 cancer across all PI-RADS scores. In the PI-RADS 3 population, MPS significantly outperformed PSAD in ruling out GG≥2 cancer. These findings suggest a complementary role of MPS testing in patients that have undergone mpMRI.

Initial Findings from a High Genetic Risk Prostate Cancer Clinic.

OBJECTIVE: To improve prostate cancer screening for high-risk men, we developed an early detection clinic for patients at high genetic risk of developing prostate cancer. Despite the rapidly growing understanding of germline variants in driving aggressive prostate cancer and the increased availability of genetic testing, there is little evidence surrounding how best to screen these men. METHODS: We are reporting on the first 45 patients enrolled, men between the ages of 35-75, primarily with known pathogenic germline variants in prostate cancer susceptibility genes. Screening consists of an intake lifestyle survey, PSA, DRE, and SelectMDx urine assay. A biopsy was recommended for any of the following indications: 1) abnormal DRE, 2) PSA above threshold, or 3) SelectMDx above threshold. The primary outcomes were number needed to screen, and number needed to biopsy to diagnose a patient with prostate cancer. RESULTS: Patients enrolled in the clinic included those with BRCA1 (n=7), BRCA2 (n=16), Lynch Syndrome (n=6), and CHEK2 (n = 4) known pathogenic germline variants. The median age and PSA were 58 (range 35-71) and 1.4 ng/ml (range 0.1-11.4 ng/ml), respectively. 12 patients underwent a prostate needle biopsy and there were 4positive biopsies for prostate cancer. CONCLUSION: These early data support the feasibility of opening a dedicated clinic for men at high genetic risk of prostate cancer. This early report on the initial enrollment of our long-term study will help optimize early detection protocols and provide evidence for personalized prostate cancer screening in men with key pathogenic germline variants.

Understanding the Barriers to Neoadjuvant Chemotherapy in Patients with Muscle Invasive Bladder Cancer: A Quality Improvement Initiative.

PURPOSE: Utilization of neoadjuvant chemotherapy (NAC) for the management of muscle-invasive bladder cancer remains low. We sought to understand our practice of NAC use in order to design a quality improvement initiative geared towards optimizing medical oncology referral. MATERIALS AND METHODS: We identified 339 patients with ≥cT2 bladder cancer treated with radical cystectomy between 2012-2017 at our institution. We assessed the rate of referral to medical oncology, rate of NAC administration, as well as medical, patient and provider variables associated with NAC use. Bayesian logistic regression modeling identified variables associated with NAC use and chart review provided granular patient-level data. RESULTS: 85% (n=289) of patients were referred to medical oncology and 62.5% (n=212) received NAC. Renal insufficiency, hearing loss, and treating urologist were conclusively associated with lower odds of NAC use. 46 patients were not referred to medical oncology and 50% of these had medical contraindications to cisplatin cited as the reason for no referral. 38 patients met with medical oncology but did not receive NAC. 30 (79%) had comorbidities that impacted this decision with 15 (39%) ineligible based on impaired renal function. CONCLUSIONS: Despite the relatively high rates of medical oncology referral and NAC use in this cohort, there are still opportunities to improve the efficiency of this practice. Quality improvement initiatives could optimize the referral of patients with ≥T2 bladder cancer for consideration of cisplatin-based NAC and establish an important quality metric in the management of these patients.

Comparison of readmission and early revision rates as a quality metric in total knee arthroplasty using the Nationwide Readmission Database.

BACKGROUND: After release of the Comprehensive Care for Joint Replacement bundle, there has been increased emphasis on reducing readmission rates for total knee arthroplasty (TKA). The potential for a separate, clinically-relevant metric, TKA revision rates within a year following surgery, has not been fully explored. Based on this, we compared rates and payments for TKA readmission and revision procedures as metrics for improving quality and cost. METHODS: We utilized the 2013 Nationwide Readmission Database (NRD) to examine national readmission and revision rates, the reasons for revision procedures, and associated costs for elective TKA procedures. As data are not linked across years, we examined revision rates for TKA completed in the month of January by capturing revision procedures in the subsequent following 11-month period to approximate a 1-year revision rate. Diagnosis and procedure codes for revision procedures were collected. Average readmission and revision procedure costs were then calculated, and the cost distributed across the entire TKA population. RESULTS: We identified 20,851 patients having TKA surgery. The mean unadjusted 30- and 90-day TKA readmission rates were 3.4% and 5.8%, respectively. In contrast, the mean unadjusted 3-month and approximate 1-year reoperation rates were 1.0% and 1.6%, respectively. The most common cause for revision was periprosthetic joint infection, which accounting for 62% of all reported revision procedures. The mean payment for 90-day readmission was roughly half ($10,589±$11,084) of the mean inpatient payment for single reoperation procedure at 90 days ($20,222±$17,799). Importantly, nearly half (46%) of all 90-day readmissions were associated with a reoperation event within the first year. CONCLUSIONS: Readmission following TKA is associated with a 1-year reoperation in approximately half of patients. These reoperations represent a significant patient burden and have a higher per episode cost. Early reoperation may represent a more clinically relevant target for quality improvement and cost containment.

Reframing Financial Incentives Around Reducing Readmission After Radical Cystectomy.

OBJECTIVE: To better understand the financial implications of readmission after radical cystectomy, an expensive surgery coupled with a high readmission rate. Currently, whether hospitals benefit financially from readmissions after radical cystectomy remains unclear, and potentially obscures incentives to invest in readmission reduction efforts. MATERIALS AND METHODS: Using a 20% sample of national Medicare beneficiaries, we identified 3544 patients undergoing radical cystectomy from January 2010 to November 2014. We compared price-standardized Medicare payments for index admissions and readmissions after surgery. We also examined the variable financial impact of length of stay and the proportion of Medicare payments coming from readmissions based on overall readmission rate. RESULTS: Medicare patients readmitted after cystectomy had higher index hospitalization payments ($19,164 readmitted vs $18,146 non-readmitted, P = .03) and an average readmission payment of $7356. Adjusted average Medicare readmission payments and length of stay varied significantly across hospitals, ranging from $2854 to $15,605, and 2.0 to 17.1 days, respectively (both P <.01), with longer length of stay associated with increased payments. After hospitals were divided into quartiles based on overall readmission rates, the percent of payments coming from readmissions ranged from 5% to 13%. CONCLUSION: Readmissions following radical cystectomy were associated with increased Medicare payments for the index hospitalization, and the readmission payment, potentially limiting incentives for readmission reduction programs. Our findings highlight opportunities to reframe efforts to support patients, caregivers, and providers through improving the discharge and readmission processes to create a patient-centered experience, rather than for fear of financial penalties.

Dynamic readmission prediction using routine postoperative laboratory results after radical cystectomy.

OBJECTIVE: To determine if the addition of electronic health record data enables better risk stratification and readmission prediction after radical cystectomy. Despite efforts to reduce their frequency and severity, complications and readmissions following radical cystectomy remain common. Leveraging readily available, dynamic information such as laboratory results may allow for improved prediction and targeted interventions for patients at risk of readmission. METHODS: We used an institutional electronic medical records database to obtain demographic, clinical, and laboratory data for patients undergoing radical cystectomy. We characterized the trajectory of common postoperative laboratory values during the index hospital stay using support vector machine learning techniques. We compared models with and without laboratory results to assess predictive ability for readmission. RESULTS: Among 996 patients who underwent radical cystectomy, 259 patients (26%) experienced a readmission within 30 days. During the first week after surgery, median daily values for white blood cell count, urea nitrogen, bicarbonate, and creatinine differentiated readmitted and nonreadmitted patients. Inclusion of laboratory results greatly increased the ability of models to predict 30-day readmissions after cystectomy. CONCLUSIONS: Common postoperative laboratory values may have discriminatory power to help identify patients at higher risk of readmission after radical cystectomy. Dynamic sources of physiological data such as laboratory values could enable more accurate identification and targeting of patients at greatest readmission risk after cystectomy. This is a proof of concept study that suggests further exploration of these techniques is warranted.

Characterising 'bounce-back' readmissions after radical cystectomy.

OBJECTIVE: To examine predictors of early readmissions after radical cystectomy (RC). Factors associated with preventable readmissions may be most evident in readmissions that occur within 3 days of discharge, commonly termed 'bounce-back' readmissions, and identifying such factors may inform efforts to reduce surgical readmissions. PATIENTS AND METHODS: We utilised the Healthcare Cost and Utilization Project's State Inpatient Databases to examine 1867 patients undergoing RC in 2009 and 2010, and identified all patients readmitted within 30 days of discharge. We assessed differences between patients experiencing bounce-back readmission compared to those readmitted 8-30 days after discharge using logistic regression models and also calculated abbreviated LACE scores to assess the utility of common readmissions risk stratification algorithms. RESULTS: The 30-day and bounce-back readmission rates were 28.4% and 5.6%, respectively. Although no patient or index hospitalisation characteristics were significantly associated with bounce-back readmissions in adjusted analyses, bounce-back patients did have higher rates of gastrointestinal (14.3% vs 6.7%, P = 0.02) and wound (9.5% vs 3.0%, P < 0.01) diagnoses, as well as increased index and readmission length of stay (5 vs 4 days, P = 0.01). Overall, the median abbreviated LACE score was 7, which fell into the moderate readmission risk category, and no difference was observed between readmitted and non-readmitted patients. CONCLUSION: One in five readmissions after RC occurs within 3 days of initial discharge, probably due to factors present at discharge. However, sociodemographic and clinical factors, as well as traditional readmission risk tools were not predictive of this bounce-back. Effective strategies to reduce bounce-back readmission must identify actionable clinical factors prior to discharge.

Lineage tracing suggests that ovarian endosalpingiosis does not result from escape of oviductal epithelium.

Most high-grade serous carcinomas are thought to arise from Fallopian tube epithelium (FTE), but some likely arise outside of the tube, perhaps from ectopic tubal-type epithelium known as endosalpingiosis. Importantly, the origin of endosalpingiosis is poorly understood. The proximity of the tubal fimbriae to the ovaries has led to the proposal that disruptions in the ovarian surface that occur during ovulation may allow detached FTE to implant in the ovary and form tubal-type glands and cysts. An alternative model suggests that cells present in ectopic locations outside the Müllerian tract retain the capacity for multi-lineage differentiation and can form glands with tubal-type epithelium. We used double transgenic Ovgp1-iCreERT2 ;R26RLSL-eYFP mice, which express an eYFP reporter protein in OVGP1-positive tissues following transient tamoxifen (TAM) treatment, to track the fate of oviductal epithelial cells. Cohorts of adult mice were given TAM to activate eYFP expression in oviductal epithelium, and ovaries were examined at time points ranging from 2 days to 12 months post-TAM. To test whether superovulation might increase acquisition of endosalpingiosis, additional cohorts of TAM-treated mice underwent up to five cycles of superovulation and ovaries were examined at 1, 6, and 12 months post-TAM. Ovaries were sectioned in their entirety to identify endosalpingiosis. Immunohistochemical staining for PAX8, tubulin, OVGP1, and eYFP was employed to study endosalpingiosis lesions. Ovarian endosalpingiosis was identified in 14.2% of TAM-treated adult mice. The endosalpingiotic inclusion glands and cysts were lined by secretory and ciliated cells and expressed PAX8, tubulin, OVGP1, and eYFP. Neither age nor superovulation was associated with a significant increase in endosalpingiosis. Endosalpingiosis was also occasionally present in the ovaries of pre-pubertal mice. The findings imply that ovarian endosalpingiosis in the mouse does not likely arise as a consequence of detachment and implantation of tubal epithelium and other mechanisms may be relevant. © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

New onset alcohol use disorder following bariatric surgery.

BACKGROUND: Bariatric surgery is the most effective treatment for morbid obesity; however, there may be significant unanticipated psychosocial effects following surgery. Prior studies have identified a threefold increase in the incidence of alcohol use disorder (AUD) after Roux-en-Y gastric bypass (RYGB). With sleeve gastrectomy (SG) now comprising over 50% of primary bariatric operations, the degree to which patients who undergo SG develop AUD remains unknown. We sought to characterize the patients and incidence of AUD following SG compared to RYGB. METHODS: This study used prospectively collected data from a state-wide quality collaborative. The presence of AUD was determined using the Alcohol Use Disorders Identification Test for Consumption (AUDIT-C), with a score ≥ 4 in men and ≥ 3 in women suggestive of AUD. We used bivariate Chi-square tests for categorical variables and independent samples t tests for continuous variables. We used multivariable logistic regression to identify patient characteristics that may predispose patients to development of AUD at 1 and 2 years after surgery. RESULTS: The overall prevalence of AUD in our population (n = 5724) was 9.6% preoperatively, 8.5% at 1 year postoperatively, and 14.0% at 2 years postoperatively. The preoperative, 1-year, and 2-year prevalence of AUD for SG were 10.1%, 9.0%, and 14.4%, respectively. The preoperative, 1-year, and 2-year postoperative prevalence of AUD for RYGB were 7.6%, 6.3%, and 11.9%, respectively. Predisposing patient factors to AUD development included higher educational level (p < 0.01) and higher household income (p < 0.01). CONCLUSIONS: This is first large, multi-institutional study of AUD following SG. The prevalence of alcohol use disorder in patients undergoing SG and RYGB was similar pre- and postoperatively. The majority of patients developed AUD following their second postoperative year. Understanding the timing and incidence of alcohol use disorder in patients undergoing SG-the most commonly performed bariatric operation in the United States-is critical to providing appropriate counseling and treatment.

Predictors and Cost of Readmission in Total Knee Arthroplasty.

BACKGROUND: The Comprehensive Care for Joint Replacement bundle was created to decrease total knee arthroplasty (TKA) cost. To help accomplish this, there is a focus on reducing TKA readmissions. However, there is a lack of national representative sample of all-payer hospital admissions to direct strategy, identify risk factors for readmission, and understand actual readmission cost. METHODS: We used the Nationwide Readmission Database to examine national readmission rates, predictors of readmission, and associated readmission costs for elective TKA procedures. We fit a multivariable logistic regression model to examine factors associated with readmission. Then, we determined mean readmission costs and calculated the readmission cost when distributed across the entire TKA population. RESULTS: We identified 224,465 patients having TKA across all states participating in the Nationwide Readmission Database. The mean unadjusted 30-day TKA readmission rate was 4%. The greatest predictors of readmission were congestive heart failure (odds ratio [OR] 2.51, 95% confidence interval [CI] 2.62-2.80), renal disease (OR 2.19, 95% CI 2.03-2.37), and length of stay greater than 4 days (OR 2.4, 95% CI 2.25-2.61). The overall median cost for each readmission was $6753 ± 175. Extrapolating the readmission cost for the entire TKA population resulted in the readmission cost being 2% of the overall 30-day procedure cost. CONCLUSIONS: A major focus of the Comprehensive Care for Joint Replacement bundle is improving cost and quality by limiting readmission rates. TKA readmissions are low and comprise a small percentage of total TKA cost, suggesting that they may not be the optimal measure of quality care or a significant driver of overall cost.

High-grade serous carcinomas arise in the mouse oviduct via defects linked to the human disease.

Recent studies have suggested that the most common and lethal type of 'ovarian' cancer, i.e. high-grade serous carcinoma (HGSC), usually arises from epithelium on the fallopian tube fimbriae, and not from the ovarian surface epithelium. We have developed Ovgp1-iCreERT2 mice in which the Ovgp1 promoter controls expression of tamoxifen-regulated Cre recombinase in oviductal epithelium - the murine equivalent of human fallopian tube epithelium (FTE). We employed Ovgp1-iCreERT2 mice to show that FTE-specific inactivation of several different combinations of tumour suppressor genes that are recurrently mutated in human HGSCs - namely Brca1, Trp53, Rb1, and Nf1 - results in serous tubal intraepithelial carcinomas (STICs) that progress to HGSC or carcinosarcoma, and to widespread metastatic disease in a subset of mice. The cancer phenotype is highly penetrant and more rapid in mice carrying engineered alleles of all four tumour suppressor genes. Brca1, Trp53 and Pten inactivation in the oviduct also results in STICs and HGSCs, and is associated with diffuse epithelial hyperplasia and mucinous metaplasia, which are not observed in mice with intact Pten. Oviductal tumours arise earlier in these mice than in those with Brca1, Trp53, Rb1 and Nf1 inactivation. Tumour initiation and/or progression in mice lacking conditional Pten alleles probably require the acquisition of additional defects, a notion supported by our identification of loss of the wild-type Rb1 allele in the tumours of mice carrying only one floxed Rb1 allele. Collectively, the models closely recapitulate the heterogeneity and histological, genetic and biological features of human HGSC. These models should prove useful for studying the pathobiology and genetics of HGSC in vivo, and for testing new approaches for prevention, early detection, and treatment. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Arid1a inactivation in an Apc- and Pten-defective mouse ovarian cancer model enhances epithelial differentiation and prolongs survival.

Inactivation of the ARID1A tumour suppressor gene is frequent in ovarian endometrioid (OEC) and clear cell (OCCC) carcinomas, often in conjunction with mutations activating the PI3K-AKT and/or canonical Wnt signalling pathways. Prior work has shown that conditional bi-allelic inactivation of the Apc and Pten tumour suppressor genes in the mouse ovarian surface epithelium (OSE) promotes outgrowth of tumours that reflect the biological behaviour and gene expression profiles of human OECs harbouring comparable Wnt and PI3K-AKT pathway defects, although the mouse tumours are more poorly differentiated than their human tumour counterparts. We found that conditional inactivation of one or both Arid1a alleles in OSE concurrently with Apc and Pten inactivation unexpectedly prolonged the survival of tumour-bearing mice and promoted striking epithelial differentiation of the cancer cells, resulting in morphological features akin to those in human OECs. Enhanced epithelial differentiation was linked to reduced expression of the mesenchymal markers N-cadherin and vimentin, and increased expression of the epithelial markers Crb3 and E-cadherin. Global gene expression profiling showed enrichment for genes associated with mesenchymal-epithelial transition in the Arid1a-deficient tumours. We also found that an activating (E545K) Pik3ca mutation, unlike Pten inactivation or Pik3ca H1047R mutation, cannot cooperate with Arid1a loss to promote ovarian cancer development in the mouse. Our results indicate that the Arid1a tumour suppressor gene has a key role in regulating OEC differentiation, and paradoxically the mouse cancers with more initiating tumour suppressor gene defects had a less aggressive phenotype than cancers arising from fewer gene alterations. Microarray data have been deposited in NCBI's Gene Expression Omnibus (GSE67695).