The trial is a parallel, two-arm, randomised pilot trial (Ottawa, Ontario, Canada). Participants will be recruited from the Mood and Anxiety Program of the Royal Ottawa Mental Health Centre, which is a specialized tertiary care centre, and from two psychiatrists working in community.
Procedure Participants will be referred to the study by the treating psychiatrist, then screened in a telephone interview by the research coordinator; the eligibility criteria will be finally assessed by a psychiatrist involved in the study and by the research coordinator. The psychiatrist will administer the Structured Clinical Interview (SCID) for DSM-IV-TR (First et al. 2002) and collect the information related to clinical variables: previous hospitalizations, history of suicide attempts, comorbidity with a current anxiety disorder, age at onset of the mood disorder, number of previous major depressive episodes, duration of the current episode, severity of the current depressive episode, use of alcohol or street drugs, family history of affective disorders, and number of chronic medical conditions. The research coordinator will assess the severity of depression using the HAMD-17. If the criteria of eligibility to the study were satisfied, participants will be asked to sign an informed written consent. Individuals meeting study criteria will be randomly assigned by computer-generated numbers to treatment with either I-MBCT or CBT (1:1). Participants included in the trial will be followed by the referring clinician. Concurrent use of psychotropics will be allowed. Concomitant treatment with any form of structured "bona fide" psychotherapy is not permitted during the study.
Participants randomized to I-MBCT receive 12 weekly 60- minute sessions of I-MBCT according to the protocol of Segal et al. (2002), integrated with the "theme, rationale, intention and practice" skills (TRIP) protocol (Woods et al. 2016, 2019). The original MBCT protocol, created for eight group sessions lasting two hours, is adapted to twelve individual sessions.
Participants randomized to CBT receive 12 weekly 60-minute individual sessions of standard CBT strategies following Beck et al. (1979). Participants receive a copy of Greenberger and Padesky's Mind over Mood (The Guilford Press, 1995) for use during the intervention.
Data Analyses The statistical analysis will be performed on the overall sample of patients who will start the treatment. As this was a feasibility study, a formal sample size calculation was not required. We applied the "rule of thumb" recommended by Julious (2005), suggesting a minimum of 12 participants per treatment arm, which optimizes the gain in precision around the mean. Pre-treatment demographic and clinical characteristics will be compared between I-MBCT and CBT treatment using univariate analyses (t-test for continuous variables and Fisher test for categorical variables).
The primary outcome will be the HAM-D. The changes in depressive and anxiety symptoms, rumination, emotion dysregulation, and negative automatic thoughts during MBCT and at follow-up will be tested through linear mixed-effects models (LMM, SPSS Mixed procedure). Interaction between time and treatment will be use to compare the changes during MBCT and CBT. Multiple regression analyses will be used to test the value of clinical variables in predicting the changes in severity of depressive symptoms during psychotherapy, adjusting for age, gender and baseline depressive severity .
To evaluate the strength of the changes in outcome variables (depressive and anxiety symptoms, rumination, emotion dysregulation, and negative automatic thoughts) during I-MBCT and CBT, within-group effect sizes of the change pre- and post-treatment using Cohen's delta for repeated measures will be used.
