This trial evaluates the feasibility, acceptability, and preliminary efficacy of self-help internet-based Acceptance and Commitment Therapy (ACT) intervention on meaning-related distress as well as secondary outcomes in patients following hematopoietic cell transplantation (HCT). Randomized controlled trial (RCT) will be enhanced with a single-case experimental design (SCED).

Recruitment will take place at a single center, after elective admission to the bone marrow transplantation and oncohematology unit due to HCT before the start of conditioning treatment. Recruitment will take place on average on the 2nd day after admission. Every two days, the transplant coordinator and physician (members of the research team) will review the lists of patients enrolled for HCT. Those who meet the inclusion criteria will be initially informed of the purpose of the study and invited for an extensive briefing by a recruiter (member of the research team). Patients will also be allowed to ask any remaining questions about the aim of the study and the study procedures. After receiving an extensive briefing, all patients who give written informed consent will proceed with baseline. Data will be collected via a self-reported survey on a mobile device. Clinical data will be obtained from the medical records. All participants will receive written user instructions for the daily sessions and measurements.

In RCT, participants will be randomly assigned in a double-blinded manner to ACT intervention and education conditions at a ratio of 1:1 by a trial coordinator (member of the research team). Randomization will be stratified by type of transplant. Participants and researchers analyzing data will be blind to the allocation of the participants to the conditions. Participants allocated to the ACT intervention will receive online ACT-based intervention within 14 days after hospital discharge. Each day's intervention will consist of an educational and practical part (standard ACT activity) followed by a debrief. On some days, participants will also receive additional exercise (optional). The whole intervention will be tailored to the context of the disease and treatment. During the same period, participants allocated to the education will receive an online guide to post-HCT recommendations. The intervention/education will be discontinued by participants at any time without any negative consequences. During the intervention/education, participants will fill in a short questionnaire assessing potential mediator variables each day, at the beginning of every online session. Participants will receive daily reminders about the intervention/education. Also, direct technical support will be available 24/7. If participants fail to complete study assessments, motivational reminders will be sent by email. If participants drop out or stop using the intervention, they will be asked for the reason(s) why they decided to quit the intervention and/or study.

In SCED, all participants will take part in the online ACT intervention and daily surveys at the 2nd and 3rd post-intervention assessments.

Feasibility will be examined via attrition and adherence rates, as well as questions about intervention engagement. Acceptability will be measured by intervention satisfaction and evaluation (attractiveness and easiness).