--- title: 'Three-strategy decision tree in R - HVE' author: "The DARTH workgroup" output: html_document: default pdf_document: default keep_tex: yes self_contained: no --- Developed by the Decision Analysis in R for Technologies in Health (DARTH) workgroup. If you use this code as a basis for your own work, please cite our papers (see suggestions below) and in your acknowledgments for both the code and publications please add: **The authors thank the Decision Analysis in R for Technologies in Health (DARTH) workgroup for providing the coding framework in R.** \newpage ```{r setup, include=FALSE} knitr::opts_chunk$set(echo = TRUE, warning = FALSE, message = FALSE, eval = T) ``` Change `eval` to `TRUE` if you want to knit this document. ```{r} rm(list = ls()) # clear memory (removes all the variables from the workspace) ``` # 00 Case example Viral encephalitis can be caused by herpes viruses (HVE) or other viruses (OVE). Herpes viruses cause approximately 52% of cases of viral encephalitis. Without treatment, the risk of complications (death or severe sequelae) for HVE is 71%; for OVE, the risk is only 1%. A drug, vidarabine, decreases the likelihood of complications due to HVE from 71% down to 36%. However, among OVE patients, treatment with vidarabine is associated with severe side effects, increasing the risk of complications from the 1% baseline to 20%. It is possible to obtain a definitive diagnosis of HVE by means of a brain biopsy, but the procedure itself also has a 5% probability of inducing complications. You are tasked with evaluating the healthcare costs and benefits associated with three possible management strategies (no treatment, vidarabine treatment, or brain biopsy followed by vidarabine treatment for those who are diagnosed with HVE). Benefits will be measured in terms of quality-adjusted life-years (QALYs). The healthcare cost of a case of viral encephalitis without complications is \$1,200; however, if complications occur, the cost rises to \$9,000. The cost of vidarabine treatment is \$9,500, while a brain biopsy costs \$25,000 per procedure. A patient who recovers from viral encephalitis without complications has an average of 20 remaining QALYs; however, a patient who experiences complications has an average of 19 remaining QALYs. Since a brain biopsy is an unpleasant procedure, patients who undergo it also experience a one-time loss of 0.01 QALYs regardless of the outcome of the biopsy. For this exercise we assumed a hypothetical CE threshold of 10000/QALY Parameters are summarized in Table 1 and the tree structure is shown in the figure below. ```{r, echo = F, warning = F, message = F, out.width = '100%', fig.cap = 'Schematic representation of the Sick-Sicker model', fig.align = 'center'} if (!require(knitr)) install.packages('knitr') include_graphics("figures/decision_tree_HVE.png") ``` # 01 Load packages ```{r, warning = FALSE, message = FALSE} if (!require('pacman')) install.packages('pacman'); library(pacman) # use this package to conveniently install other packages # load (install if required) packages from CRAN p_load("dplyr", "devtools", "scales", "ellipse", "ggplot2", "lazyeval", "igraph", "ggraph", "reshape2", "knitr", "stringr", "dampack") # load (install if required) packages from GitHub p_load_gh("DARTH-git/darthtools") ``` # 02 Load functions ```{r} # all functions are in the darthtools package so no need for additional functions ``` # 03 Define parameter input values ```{r} v_names_str <- c("No Tx", "Tx All", "Biopsy") # names of strategies, no treatment, treatment and biopspy - Tx = treatment n_str <- length(v_names_str) # number of strategies wtp <- 100000 # willingness to pay threshold # Probabilities p_HVE <- 0.52 # prevalence of HVE p_HVE_comp <- 0.71 # complications with untreated HVE p_OVE_comp <- 0.01 # complications with untreated OVE p_HVE_comp_tx <- 0.36 # complications with treated HVE p_OVE_comp_tx <- 0.20 # complications with treated OVE p_biopsy_death <- 0.005 # probability of death due to biopsy # Costs c_VE <- 1200 # cost of viral encephalitis care without complications c_VE_comp <- 9000 # cost of viral encephalitis care with complications c_tx <- 9500 # cost of treatment c_biopsy <- 25000 # cost of brain biopsy c_death_biopsy <- 0 # cost of dying from brain biopsy # QALYs q_VE <- 20 # remaining QALYs for those without VE-related complications q_VE_comp <- 19 # remaining QALYs for those with VE-related complications q_loss_biopsy <- 0.01 # one-time QALY loss due to brain biopsy q_death_biopsy <- 0 # remaining QALYs for those who died during biopsy ``` # 04 Create and run decision tree model ## 04.1 Create vectors of weights for each strategy ```{r} # Create vector of weights for each strategy v_w_no_tx <- c( p_HVE * p_HVE_comp , # HVE, complications p_HVE * (1 - p_HVE_comp) , # HVE, no complications (1 - p_HVE) * p_OVE_comp , # OVE, complications (1 - p_HVE) * (1 - p_OVE_comp)) # OVE, no complications v_w_tx <- c( p_HVE * p_HVE_comp_tx , # HVE w/tx, complications p_HVE * (1 - p_HVE_comp_tx) , # HVE w/tx, no complications (1 - p_HVE) * p_OVE_comp_tx , # OVE w/tx, complications (1 - p_HVE) * (1 - p_OVE_comp_tx)) # OVE w/tx, no complications v_w_biopsy <- c( p_biopsy_death , # biopsy death # no biopsy death., HVE w/tx, complications (1 - p_biopsy_death) * p_HVE * p_HVE_comp_tx , # no biopsy death., HVE w/tx, no complications (1 - p_biopsy_death) * p_HVE * (1 - p_HVE_comp_tx) , # no biopsy death., OVE, complications (1 - p_biopsy_death) * (1 - p_HVE) * p_OVE_comp , # no biopsy death., OVE, no complications (1 - p_biopsy_death) * (1 - p_HVE) * (1 - p_OVE_comp)) ``` ##04.1.1 Extra code to demonstrate the use of "clones" Often in decision trees certain branches either repeat themselves or are shared across multiple strategies, perhaps with some differences in their input parameters. For example, in viral encephalitis, the complications of HVE are shared among all three strategies. To avoid coding the same section of the tree multiple times, we can construct a function that describes the structure of this section of the tree (called a subtree), and has as input the parameters that are related to that section of the tree. This is comparable, with what is in specialized software like TreeAge or Amua called a clone. In the code chunk below, we first show how we can develop the clone function. And next how to use the function as part of a tree. Finally, we demonstrate that we get identical results compared to a tree with no clones. ```{r} # Develop a function to calculate the vector of weights for the _hve_complication section of the decision tree calculate_hve_complication <- function(p_HVE, p_HVE_comp, p_OVE_comp){ # p_HVE: the probability of having p_HVE # p_HVE_comp: the probability of getting a complication when you have an HVE infection # p_OVE_comp: the probability of getting a complication when you have an OVE infection v_weights <- c( p_HVE * p_HVE_comp , # HVE, complications p_HVE * (1 - p_HVE_comp) , # HVE, no complications (1 - p_HVE) * p_OVE_comp , # OVE, complications (1 - p_HVE) * (1 - p_OVE_comp)) return (v_weights) # return the vector of weights } # Create vector of weights for each strategy using the function v_w_no_tx_clone <- calculate_hve_complication(p_HVE = p_HVE, p_HVE_comp = p_HVE_comp, p_OVE_comp = p_OVE_comp) v_w_tx_clone <- calculate_hve_complication(p_HVE = p_HVE, p_HVE_comp = p_HVE_comp_tx, p_OVE_comp = p_OVE_comp_tx) v_w_biopsy_clone <- c(p_biopsy_death, (1 - p_biopsy_death) * calculate_hve_complication(p_HVE = p_HVE, p_HVE_comp = p_HVE_comp_tx, p_OVE_comp = p_OVE_comp)) # Check if the clone gives identical results all.equal(v_w_no_tx_clone, v_w_no_tx) all.equal(v_w_tx_clone, v_w_tx) all.equal(v_w_biopsy_clone, v_w_biopsy) ``` ## 0.4.2 Create vectors of effects ```{r} # Create vector of outcomes being complications for each strategy v_comp_no_tx <- c(1, # HVE, complications 0, # HVE, no complications 1, # OVE, complications 0) # OVE, no complications v_comp_tx <- c(1, # HVE, complications 0, # HVE, no complications 1 , # OVE, complications 0) # OVE, no complications v_comp_biopsy <- c(1, # biopsy complications 1, # no biopsy comp., HVE w/tx, complications 0, # no biopsy comp., HVE w/tx, no complications 1, # no biopsy comp., OVE, complications 0) # no biopsy comp., OVE, no complications # Create vector of outcomes (QALYs) for each strategy v_qaly_no_tx <- c(q_VE_comp , # HVE, complications q_VE , # HVE, no complications q_VE_comp , # OVE, complications q_VE) # OVE, no complications v_qaly_tx <- c(q_VE_comp , # HVE, complications q_VE , # HVE, no complications q_VE_comp , # OVE, complications q_VE) # OVE, no complications # loss due to biopsy v_qaly_biopsy <- c(q_death_biopsy - q_loss_biopsy, # biopsy complications q_VE_comp - q_loss_biopsy, # no biopsy comp., HVE w/tx, complications q_VE - q_loss_biopsy, # no biopsy comp., HVE w/tx, no complications q_VE_comp - q_loss_biopsy, # no biopsy comp., OVE, complications q_VE - q_loss_biopsy ) # no biopsy comp., OVE, no complications ``` ## 0.4.3 Create vectors of costs ```{r} # Create vector of costs for each strategy # cost of no treatment v_cost_no_tx <- c(c_VE_comp , # HVE, complications c_VE , # HVE, no complications c_VE_comp , # OVE, complications c_VE) # OVE, no complications # cost of treatment v_cost_tx <- c(c_VE_comp + c_tx, # HVE, complications c_VE + c_tx, # HVE, no complications c_VE_comp + c_tx, # OVE, complications c_VE + c_tx) # OVE, no complications # cost of biopsy procedure v_cost_biopsy <- c(c_death_biopsy + c_biopsy, # cost of death (zero) c_VE_comp + c_tx + c_biopsy, # no biopsy comp., HVE w/tx, complications c_VE + c_tx + c_biopsy, # no biopsy comp., HVE w/tx, no complications c_VE_comp + c_biopsy, # no biopsy comp., OVE, complications c_VE + c_biopsy ) # no biopsy comp., OVE, no complications ## STAR MATERIAL ## # Extra funfact about coding # When you like to add a specific cost to all elements of a vector you can also add the costs before the vector. The vector for the cost of the biopsy for example could also be coded as follows: # cost of biopsy procedure #v_cost_biopsy <- c_biopsy + # c(c_death_biopsy, # cost of death (zero) # c_VE_comp + c_tx, # no biopsy comp., HVE w/tx, complications # c_VE + c_tx, # no biopsy comp., HVE w/tx, no complications # c_VE_comp , # no biopsy comp., OVE, complications # c_VE) # no biopsy comp., OVE, no complications # that is because on the background R will create replicates of the scalar (c_biopsy) to match the size of the vector you are adding it to #################### ``` ## 0.4.4 Calculate the expected effects (complications, utilities) and costs ```{r} # Calculate expected complications for each strategy total_comp_no_tx <- v_w_no_tx %*% v_comp_no_tx total_comp_tx <- v_w_tx %*% v_comp_tx total_comp_biopsy <- v_w_biopsy %*% v_comp_biopsy # Calculate total utilities for each strategy total_qaly_no_tx <- v_w_no_tx %*% v_qaly_no_tx total_qaly_tx <- v_w_tx %*% v_qaly_tx total_qaly_biopsy <- v_w_biopsy %*% v_qaly_biopsy # Calculate total costs for each strategy total_cost_no_tx <- v_w_no_tx %*% v_cost_no_tx total_cost_tx <- v_w_tx %*% v_cost_tx total_cost_biopsy <- v_w_biopsy %*% v_cost_biopsy # vector of expected complications v_total_comp <- c(total_comp_no_tx, total_comp_tx, total_comp_biopsy) # vector of total QALYs v_total_qaly <- c(total_qaly_no_tx, total_qaly_tx, total_qaly_biopsy) # vector of total costs v_total_cost <- c(total_cost_no_tx, total_cost_tx, total_cost_biopsy) # Name outcomes names(v_total_comp) <- v_names_str # names for the elements of the total comp vector names(v_total_qaly) <- v_names_str # names for the elements of the total QALYs vector names(v_total_cost) <- v_names_str # names for the elements of the total cost vector df_output <- data.frame(Strategy = v_names_str, Cost = v_total_cost, Effect = v_total_qaly, Complications = v_total_comp) # model output df_output ``` # 05 Cost-Effectiveness Analysis For the cost-effectiveness we make use of the `calculate_icers` function from the `dampack` package. You can find more information about this package by typing `??calculate_icers` in your Console. ```{r} # create the transition probability matrix for NO treatment decision_tree_HVE_cea <- dampack::calculate_icers(cost = df_output$Cost, effect = df_output$Effect, strategies = df_output$Strategy) decision_tree_HVE_cea ``` ## 05.1 Plot frontier of Decision Tree ```{r} plot(decision_tree_HVE_cea, effect_units = "QALYs", label = "all") ``` ##05.2 Estimate net monetary benefit The code below estimates the net monetary benefit (NMB). This is a metric used to evaluate the economic value of an intervention in monetary values. It is calculated by subtracting the total costs from the total monetary benefits generated by the project. To express the benefits in monetary terms the benefits are multiplied with the willingness-to-pay threshold. The use of a NMB can help to determine whether the gains (in monetary terms) from an interventions exceed its costs. A positive NMB indicates that the benefits outweigh the costs, while a negative NMB suggests the opposite. ```{r} # calculate vector of nmb v_nmb <- v_total_qaly * wtp - v_total_cost names(v_nmb) <- v_names_str # names for the elements of the nmb vector which.max(v_nmb) # Which strategy gives the highest net monetary benefit? ``` # Acknowlegdement: For this work we made use of the template developed by the Decision Analysis in R for Technologies in Health (DARTH) workgroup: . The notation of our code is based on the following provided framework and coding convention: Alarid-Escudero, F., Krijkamp, E., Pechlivanoglou, P. et al. A Need for Change! A Coding Framework for Improving Transparency in Decision Modeling. PharmacoEconomics 37, 1329–1339 (2019). . - Alarid-Escudero F, Krijkamp EM, Enns EA, Yang A, Hunink MGM Pechlivanoglou P, Jalal H. An Introductory Tutorial on Cohort State-Transition Models in R Using a Cost-Effectiveness Analysis Example. Medical Decision Making, 2023; 43(1). (Epub). - Alarid-Escudero F, Krijkamp EM, Enns EA, Yang A, Hunink MGM Pechlivanoglou P, Jalal H. A Tutorial on Time-Dependent Cohort State-Transition Models in R using a Cost-Effectiveness Analysis Example. Medical Decision Making, 2023; 43(1). Other work from DARTH can be found on the website: # Copyright for assignment work Copyright 2017, THE HOSPITAL FOR SICK CHILDREN AND THE COLLABORATING INSTITUTIONS.All rights reserved in Canada, the United States and worldwide. Copyright, trademarks, trade names and any and all associated intellectual property are exclusively owned by THE HOSPITAL FOR Sick CHILDREN and the collaborating institutions. These materials may be used, reproduced, modified, distributed and adapted with proper attribution.