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Abstract Background Trauma-focused treatments for posttraumatic stress disorder (PTSD) are commonly delivered either once or twice a week. Initial evidence suggests that session frequency affects treatment response, but very few trials have investigated the effect of session frequency. The present study’s aim is to compare treatment outcomes of twice-weekly versus once-weekly sessions of two treatments for PTSD related to childhood trauma, imagery rescripting (ImRs) and eye movement desensitization and reprocessing (EMDR). We hypothesize that both treatments will be more effective when delivered twice than once a week. How session frequency impacts treatment response, whether treatment type moderates the frequency effect, and which treatment type and frequency works best for whom will also be investigated. Methods The IREM-Freq trial is an international multicenter randomized clinical trial conducted in mental healthcare centers across Australia, Germany, and the Netherlands. We aim to recruit 220 participants, who will be randomized to one of four conditions: (1) EMDR once a week, (2) EMDR twice a week, (3) ImRs once a week, or (4) ImRs twice a week. Treatment consists of 12 sessions. Data are collected at baseline until one-year follow-up. The primary outcome measure is clinician-rated PTSD symptom severity. Secondary outcome measures include self-reported PTSD symptom severity, complex PTSD symptoms, trauma-related cognitions and emotions, depressive symptoms, dissociation, quality of life, and functioning. Process measures include memory, learning, therapeutic alliance, motivation, reluctance, and avoidance. Additional investigations will focus on predictors of treatment outcome and PTSD severity, change mechanisms of EMDR and ImRs, the role of emotions, cognitions, and memory, the optimization of treatment selection, learned helplessness, perspectives of patients and therapists, the network structure of PTSD symptoms, and sudden treatment gains. Discussion This study will extend our knowledge on trauma-focused treatments for PTSD related to childhood trauma and, more specifically, the importance of session frequency. More insight into the optimal session frequency could lead to improved treatment outcomes and less dropout, and in turn, to a reduction of healthcare costs. Moreover, the additional investigations will broaden our understanding of how the treatments work and variables that affect treatment outcome. Trial registration Netherlands Trial Register NL6965, registered 25/04/2018.

Background: It is unclear whether the evidence-based treatments for PTSD are as effective in patients with CA-PTSD. Objective: We aimed to investigate the effectiveness of three variants of prolonged exposure therapy. Method: We recruited adults with CA-PTSD. Participants were randomly assigned to Prolonged Exposure (PE; 16 sessions in 16 weeks), intensified Prolonged Exposure (iPE; 12 sessions in 4 weeks followed by 2 booster sessions) or a phase-based treatment, in which 8 sessions of PE were preceded by 8 sessions of Skills Training in Affective and Interpersonal Regulation (STAIR+PE; 16 sessions in 16 weeks). Assessments took place in week 0 (baseline), week 4, week 8, week 16 (post-treatment) and at a 6-and 12-month follow-up. The primary outcome was clinician-rated PTSD symptom severity. Results: We randomly assigned 149 patients to PE (48), iPE (51) or STAIR+PE (50). All treatments resulted in large improvements in clinician assessed and self-reported PTSD symptoms from baseline to 1-year follow-up (Cohen’s d > 1.6), with no significant differences among treatments. iPE led to faster initial symptom reduction than PE for self-report PTSD symptoms (t135 = −2.85, p = .005, d = .49) but not clinician-assessed symptoms (t135 = −1.65, p = .10) and faster initial symptom reduction than STAIR+PE for self-reported (t135 = −4.11, p < .001, d = .71) and clinician-assessed symptoms (t135 = −2.77, p = .006, Cohen’s d = .48) STAIR+PE did not result in significantly more improvement from baseline to 1-year follow-up on the secondary outcome emotion regulation, interpersonal problems and self-esteem compared to PE and iPE. Dropout rates did not differ significantly between conditions. Conclusions: Variants of exposure therapy are tolerated well and lead to large improvements in patients with CA-PTSD. Intensifying treatment may lead to faster improvement but not to overall better outcomes. The trial is registered at the clinical trial registry, number NCT03194113, https://clinicaltrials.gov/ct2/show/NCT03194113