Area of Expertise
- Artificial Intelligence
- Business Analytics
- Health Care Management
- Machine Learning
- Operations Management
- Optimization Under Uncertainty
- Supply Chain Management
Applies optimization and decomposition methods, machine learning, stochastic modeling and queueing theory, and optimal control to model complex, large-scale systems in health care, supply chain management, inventory, and logistics.
Babier, A., Chan, T., Diamant, A., Mahmood, R. and McNiven, A. (2020), "The Importance of Evaluating the Complete Knowledge-Based Automated Planning Pipeline", European Journal of Medical Physics, 72, 73-79 .Keywords
We determine how prediction methods combine with optimization methods in two-stage knowledge-based planning (KBP) pipelines to produce radiation therapy treatment plans. We trained two dose prediction methods, a generative adversarial network (GAN) and a random forest (RF) with the same 130 treatment plans. The models were applied to 87 out-of-sample patients to create two sets of predicted dose distributions that were used as input to two optimization models. The first optimization model, inverse planning (IP), estimates weights for dose-objectives from a predicted dose distribution and generates new plans using conventional inverse planning. The second optimization model, dose mimicking (DM), minimizes the sum of one-sided quadratic penalties between the predictions and the generated plans using several dose-objectives. Altogether, four KBP pipelines (GAN-IP, GAN-DM, RF-IP, and RF-DM) were constructed and benchmarked against the corresponding clinical plans using clinical criteria; the error of both prediction methods was also evaluated. The best performing plans were GAN-IP plans, which satisfied the same criteria as their corresponding clinical plans (78%) more often than any other KBP pipeline. However, GAN did not necessarily provide the best prediction for the second-stage optimization models. Specifically, both the RF-IP and RF-DM plans satisfied the same criteria as the clinical plans 25% and 15% more often than GAN-DM plans (the worst performing plans), respectively. GAN predictions also had a higher mean absolute error (3.9 Gy) than those from RF (3.6 Gy). We find that state-of-the-art prediction methods when paired with different optimization algorithms, produce treatment plans with considerable variation in quality.
Chan, T., Diamant, A. and Mahmood, R. (2020), "Sampling from the Complement of a Polyhedron: An MCMC Algorithm for Data Augmentation", Operations Research Letters, 48(6), 744-751.
We present an MCMC algorithm for sampling from the complement of a polyhedron. Our approach is based on the Shake-and-bake algorithm for sampling from the boundary of a set and provably covers the complement. We use this algorithm for data augmentation in a machine learning task of classifying a hidden feasible set in a data-driven optimization pipeline. Numerical results on simulated and MIPLIB instances demonstrate that our algorithm, along with a supervised learning technique, outperforms conventional unsupervised baselines.
Babier, A., Chan, T., Diamant, A., Mahmood, R. and McNiven, A. (2020), "Knowledge-Based Automated Planning with 3-D Generative Adversarial Neural Networks", Medical Physics Journal , 47(2), 297-306.Keywords
To develop a knowledge‐based automated planning pipeline that generates treatment plans without feature engineering, using deep neural network architectures for predicting three‐dimensional (3D) dose. Our knowledge‐based automated planning (KBAP) pipeline consisted of a knowledge‐based planning (KBP) method that predicts dose for a contoured computed tomography (CT) image followed by two optimization models that learn objective function weights and generate fluence‐based plans, respectively. We developed a novel generative adversarial network (GAN)‐based KBP approach, a 3D GAN model, which predicts dose for the full 3D CT image at once and accounts for correlations between adjacent CT slices. Baseline comparisons were made against two state‐of‐the‐art deep learning–based KBP methods from the literature. We also developed an additional benchmark, a two‐dimensional (2D) GAN model which predicts dose to each axial slice independently. For all models, we investigated the impact of multiplicatively scaling the predictions before optimization, such that the predicted dose distributions achieved all target clinical criteria. Each KBP model was trained on 130 previously delivered oropharyngeal treatment plans. Performance was tested on 87 out‐of‐sample previously delivered treatment plans. All KBAP plans were evaluated using clinical planning criteria and compared to their corresponding clinical plans. KBP prediction quality was assessed using dose‐volume histogram (DVH) differences from the corresponding clinical plans. The best performing KBAP plans were generated using predictions from the 3D GAN model that were multiplicatively scaled. These plans satisfied 77% of all clinical criteria, compared to the clinical plans, which satisfied 67% of all criteria. In general, multiplicatively scaling predictions prior to optimization increased the fraction of clinical criteria satisfaction by 11% relative to the plans generated with nonscaled predictions. Additionally, these KBAP plans satisfied the same criteria as the clinical plans 84% and 8% more frequently as compared to the two benchmark methods, respectively. We developed the first knowledge‐based automated planning framework using a 3D generative adversarial network for prediction. Our results, based on 217 oropharyngeal cancer treatment plans, demonstrated superior performance in satisfying clinical criteria and generated more realistic plans as compared to the previous state‐of‐the‐art approaches.
Diamant, A., Johnston, D. and Quereshy, F. (2019), "Why Do Surgeons Schedule Their Own Surgeries?", Journal of Operations Management, 63(5), 262-281.
Surgery is a knowledge intensive, high‐risk professional service. Most hospitals give surgeons considerable autonomy in deciding which patients to operate on and when. In theory, this allows surgeons the operational flexibility to prioritize surgeries based on intimate knowledge of their patient’s clinical needs. At odds with this strategy is the operations management literature, which favors the standardization and centralization of scheduling focused on achieving the efficient use of all resources, such as operating room capacity. Unfortunately, a little is known as to how surgeons customize their schedules and why they value such control. To this end, we conduct an exploratory qualitative study of the scheduling behavior of surgeons at a large Canadian teaching hospital. We identify significant differences between surgeons as to their priorities when scheduling. Two constructs are formative in surgeon decision‐making: the timeliness of treatment for their patients and idiosyncratic personal priorities. Our work has implications for achieving surgeon support for initiatives to standardize and centralize routines for patient scheduling. Accordingly, we formulate propositions that address the conditions under which such efforts will achieve the desired balance between flexibility and efficiency.
Baron, O. and Diamant, A. (2019), "Double-Sided Matching Queues: Priority and Impatient Customers", Operations Research Letters, 47(3), 219-224.
We analyze a double-sided queue with priority that serves patient customers and customers with zero patience (i.e., impatient customers). In a two-sided market, high and low priority customers arrive to one side and match with queued customers on the opposite side. Impatient customers match with queued patient customers; when there is no queue, they leave the system unmatched. All arrivals follow independent Poisson processes. We derive exact formulae for the stationary queue length distribution and several steady-state performance measures.
Carrasco, A., Cire, A., Diamant, A. and Yunes, T. (2019), "A Network‐Based Formulation for Scheduling Clinical Rotations", Production and Operations Management, 28(5), 1186-1205.
We investigate the scheduling practices of a medical school that must assign a cohort of students to a series of clinical rotations, while respecting both operational and quality‐of‐service requirements. Students become available to start clerkship progressively throughout the year and can complete rotations at hospitals in different geographic regions. Each hospital may offer a subset of the clinical rotations, with different start dates, capacities, and cost rates. We propose a novel network‐flow model based on decision diagrams, a graphical structure that compresses the state space of a dynamic program, to model feasible schedules. We demonstrate that our network model has several interesting structural features, is computationally superior as compared to a classical mixed‐integer linear program, and can be used to generate useful insights that can aid in managerial decision‐making. Using a dataset collected from the American University of the Caribbean, we perform a counterfactual analysis which shows that had our scheduling approach been implemented, a cost reduction of approximately 19% on average could have been achieved. To understand how assignment decisions can affect future costs, we develop a discrete‐event simulation of the licensing examination and clerkship scheduling process. We then compare our exact scheduling approach with current practice and achieve an average cost reduction of 25%. We also show that this cost reduction is robust with respect to estimation and forecast uncertainty, specifically, the licensing exam failure rate and the future cohort size.
Diamant, A., Milner, J., Quereshy, F. and Xu, B. (2018), "Inventory Management of Reusable Surgical Supplies", Health Care Management Science, 21(3), 439-459.Keywords
We investigate the inventory management practices for reusable surgical instruments that must be sterilized between uses. We study a hospital that outsources their sterilization services and model the inventory process as a discrete-time Markov chain. We present two base-stock inventory models, one that considers stockout-based substitution and one that does not. We derive the optimal base-stock level for the number of reusable instruments to hold in inventory, the expected service level, and investigate the implied cost of a stockout. We apply our theoretical results to a dataset collected from a surgical unit at a large tertiary care hospital specializing in colorectal operations. We demonstrate how to implement our model when determining base-stock levels for future capacity expansion and when considering alternative stockout protocols. Our analysis suggests that the hospital can reduce the number of reusable instrument sets held in inventory if on-site sterilization techniques (e.g., flash sterilization) are employed. Our results will guide future procurement decisions for surgical units based on costs and desired service levels.
Diamant, A., Milner, J. and Quereshy, F. (2018), "Dynamic Patient Scheduling for Multi-Appointment Health Care Programs", Production and Operations Management, 27(1), 58-79.Keywords
We investigate the scheduling practices of a multidisciplinary, multistage, outpatient health care program. Patients undergo a series of assessments before being eligible for elective surgery. Such systems often suffer from high rates of attrition and appointment no‐shows leading to capacity underutilization and treatment delays. We propose a new scheduling model where the clinic assigns patients to an appointment day but postpones the decision of which assessments patients undergo pending the observation of who arrives. In doing so, the clinic gains flexibility to improve system performance. We formulate the scheduling problem as a Markov decision process and use approximate dynamic programming to solve it. We apply our approach to a dataset collected from a bariatric surgery program at a large tertiary hospital in Toronto, Canada. We examine the quality of our solutions via structural results and compare them with heuristic scheduling practices using a discrete‐event simulation. By allowing multiple assessments, delaying their scheduling, and by optimizing over an appointment book, we show significant improvements in patient throughput, clinic profit, use of overtime, and staff utilization.
Chan, T., Babier, A., Diamant, A., Mahmood, R. and McNiven, A. (2018), "Automated Treatment Planning in Radiation Therapy using Generative Adversarial Networks", Proceedings of Machine Learning Research, 85, 484-499.
Knowledge-based planning (KBP) is an automated approach to radiation therapy treatment planning that involves predicting desirable treatment plans before they are then corrected to deliverable ones. We propose a generative adversarial network (GAN) approach for predicting desirable 3D dose distributions that eschews the previous paradigms of site-specific feature engineering and predicting low-dimensional representations of the plan. Experiments on a dataset of oropharyngeal cancer patients show that our approach significantly outperforms previous methods on several clinical satisfaction criteria and similarity metrics.
Cleghorn, M., Diamant, A., Jackson, T., Milner, J., Okrainec, A., Sockalingam, S. and Quereshy, F. (2015), "Patient and Operational Factors Affecting Wait Times in a Bariatric Surgery Program in Toronto", Canadian Medical Association Journal, 3(3), E331-E337.
Increasing rates of obesity have led to growing demand for bariatric surgery. This has implications for wait times, particularly in publicly funded programs. This study examined the impact of patient and operational factors on wait times in a multidisciplinary bariatric surgery program. Methods: A retrospective study was conducted involving patients who were referred to a tertiary care centre (University Health Network, Toronto Western Hospital, Toronto) for bariatric surgery between June 2008 and July 2011. Patient characteristics, dates of clinical assessments and records describing operational changes were collected. Univariable analysis and multivariable log-linear and parametric time-to-event regressions were performed to determine whether patient and operational covariates were associated with the wait time for bariatric surgery (i.e., length of preoperative evaluation). Results: Of the 1664 patients included in the analysis, 724 underwent surgery with a mean wait time of 440 (standard deviation 198) days and a median wait time of 445 (interquartile range 298−533) days. Wait times ranged from 3 months to 4 years. Univariable and multivariable analyses showed that patients with active substance use (β = 0.3482, p = 0.02) and individuals who entered the program in more recent operational periods (β = 0.2028, p).
Afeche, P., Diamant, A. and Milner, J. (2014), "Double-Sided Batch Queues with Abandonment: Modeling Crossing Networks", Operations Research, 62(5), 1179-1201.Keywords
We study a double-sided queue with batch arrivals and abandonment. There are two types of customers, patient ones who queue but may later abandon, and impatient ones who depart immediately if their order is not filled. The system matches units from opposite sides of the queue based on a first-come first-served policy. The model is particularly applicable to a class of alternative trading systems called crossing networks that are increasingly important in the operation of modern financial markets. We characterize, in closed form, the steady-state queue length distribution and the system-level average system time and fill rate. These appear to be the first closed-form results for a double-sided queuing model with batch arrivals and abandonment. For a customer who arrives to the system in steady state, we derive formulae for the expected fill rate and system time as a function of her order size and deadline. We compare these system- and customer-level results for our model that captures abandonment in aggregate, to simulation results for a system in which customers abandon after some random deadline. We find close correspondence between the predicted performance based on our analytical results and the performance observed in the simulation. Our model is particularly accurate in approximating the performance in systems with low fill rates, which are representative of crossing networks.
Cleghorn, M., Diamant, A., Jackson, T., Milner, J., Okrainec, A., Sockalingam, S. and Quereshy, F. (2014), "Analysis of Patient Attrition in a Publicly Funded Bariatric Surgery Program", Journal of the American College of Surgeons, 219(5), 1047-1055.
Obesity is a global epidemic, and several surgical programs have been created to combat this public health issue. Although demand for bariatric surgery has grown, so too has the attrition rate. In this study we identify patient characteristics and operational interventions that have contributed to high attrition in a multistage, multidisciplinary bariatric surgery program. Study Design
A retrospective study was conducted of 1,682 patients referred for bariatric surgery at the University Health Network in Toronto, Canada, from June 2008 to July 2011. Demographic information, presurgical assessment dates, and records describing operational changes were collected. Several penalized likelihood and mixed effects multivariable logistic regression models were used to determine whether patient characteristics, operational changes, and previous experience affected program completion and intermediate transitions between assessments. Results
Although the majority of attrition appears to be the result of patient self-removal, males (odds ratio [OR] 0.511, 95% CI 0.392 to 0.663, p < 0.001), and individuals with active substance use (OR 0.223, 95% CI 0.096 to 0.471, p < 0.001) were less likely to undergo surgery. Operational practices had a detrimental effect on program completion (OR 0.590, 95% CI 0.456 to 0.762, p < 0.001). Conversely, patients with a BMI > 40 kg/m2 (OR 1.756, 95% CI 1.233 to 2.515, p = 0.002) and those who lived within 25 to 300 km of the center (OR > 1.633, p < 0.001) were more likely to undergo surgery.
Courses TaughtCoordinating Demand and Supply
Models & Applications in Operational Research