The purpose of this study is to evaluate the efficacy of a form of cognitive behavioral therapy (CBT) in a population of individuals with PTSD and common related problems (depression, anxiety or sleep disorders, pain, psychosocial stressors, low social support, substance use disorder). Half of the participants will receive a cognitive behavioral therapy with specific modules for the treatment of related problems (CBT-E) and the other half of participants will receive therapy without specific modules (CBT-C).

The main assumption is that participants treated in the CBT-E condition will present a lower level of symptom intensity of PTSD and a higher remission rate than those in the CBT-C condition during the post-treatment assessment. As a secondary objective, an analysis of the different parameters of effectiveness of the two forms of CBT (e.g., average number of sessions required to reach remission, treatment strategies used) will be performed.

Post-traumatic stress disorder (PTSD) will affect nearly one in ten Canadians in their lifetime and more than 75% of them will develop at least one other related problem (i.e. depression). Results from a previous study suggest that CBT offering a flexible number of sessions and support for common related problems can improve the effectiveness of CBT. However, no studies have demonstrated the superior efficacy of flexible CBT compared to CBT targeting only PTSD to individuals with PTSD and common related problems.

A randomized parallel-stratified single-blind controlled trial will be used. Randomization will be performed through www.randomized.net, a comprehensive internet-based randomization service for clinical trials, with multiple Coordinating Center users (each with their own role). A total of 134 individuals will be randomized to one of two therapy conditions. Following the initial assessment, between 8 and 32 sessions with a psychologist will be offered to participants upon achievement of remission of the diagnosis of PTSD. Discontinuation of treatment will be decided in collaboration with the participant and will be considered on the basis of achieving a non-clinical severity level of symptoms of PTSD on the PCL-5 (i.e., score <33 and absence or non-clinical level of severity of each of the symptoms). The participants will be re-evaluated 3 times after their therapy (1 week, 3 months and 6 months post-therapy) on primary and secondary measures. A 1 year post-therapy follow-up will also be provided for the primary measure only.

Planned analysis. The statistical analysis will respect the "intent-to-treat" principle: the results of all randomized participants will be included in the groups in which they were originally assigned. The significance level is fixed at .05 and bilateral tests will be used. With regards to missing data, first the type of missing data will be determined (e.g. missing completely randomly). Thereafter, a multiple imputation strategy (3 to 5 imputations) will be used to replace the data and carry out the various analysis planned. This data replacement strategy is recognized as being robust and efficient in addition to resolving uncertainty due to missing data. Generalized linear models will be used to execute group comparisons (CBT-C vs CBT-E) on the main measurements using the pre-treatment (T0) result as a covariate. The same strategy will be used for comparisons of the secondary variables and related problems. An analysis of the different parameters of effectiveness of the two forms of CBT (e.g. average number of sessions required to achieve remission, treatment strategy used) will be carried out.

Subgroup analysis. No subgroup analysis is expected. Exploratory analysis will be conducted to determine if the results are generalizable to victims of both sexes and to the two major categories of traumatic exposures characterizing the sample recruited, namely violent criminal acts and workplace accidents.

Sample size and calculations of treatment effects. Based on the treatment effect size observed in a pilot study on the effects of CBT-E, similar research, and on the use of the GPower power calculation software, the sample size required to obtain statistical power of .80 with 3 follow-ups, a significance level at .05 and bilateral testing for ANCOVA is 53 participants per group. A sample of 106 participants will allow for detections of differences in means between the two groups in terms of symptoms intensity of PTSD. It will also be adequate to verify the other assumptions and objectives of this project. Based on data from our recent studies on the efficacy of CBT-C, and the literature on similar therapies (Bradley et al., 2005), the attrition rate is estimated to be 28% for participants who will be randomized in CBT-C. For CBT-E, the attrition rate observed during a pilot project was 14% (3 dropouts out of 21 participants). Given these attrition rates, recruitment for each of the therapies to achieve the required statistical power is calculated as follows: CBT-C = 53 participants * attrition (28%) = 74; CBT-E = 53 participants * attrition (14%) = 60). Consequently, a total of 134 participants will be randomized to one of the two conditions of therapy.

Selection bias: The assignment to the intervention will be made only after the participant has agreed to participate in the study. Bias in statistical analysis: In order to minimize biases, the analysis will be carried out in a single-blind manner, the statistician will not know which intervention refers to the code assigned to each of them in the database. Bias resulting from missing data and dropouts: All analysis will follow the "intent to treat" principle, in order to minimize biases stemming from missing data and drop-outs during the intervention.

Conducting a clinical trial on comparing the effectiveness of CBT with or without specific modules for patients with PTSD and related problems will increase the efficacy of CBT, innovate in the delivery of CBT, and improve the training of current and future clinicians. The results could also encourage victims' compensation boards to adjust their practices and thus improve the health of the beneficiaries.

Inclusion Criteria:

• Exposed to a traumatic event at an adult age
• Primary PTSD diagnosis according to the DSM-5 criteria
• At least one PTSD related problem (depressive, anxiety or sleep disorders, pain, psychosocial stressors, inadequate social support, substance use disorder)
• Being fluent in French.

Exclusion Criteria:

• Schizophrenia diagnostic, current or past psychotic episodes, bipolar disorder, organic cerebral disorder or intellectual deficiency
• Presence of a trouble linked to a substance developed prior to the traumatic event
• Physical condition preventing participation in the study (e.g. cerebral trauma)
• Presence of active suicide ideas.