The intervention group will be assessed with the study surveys at baseline (T1), will participate in the online MBI for the next 6 weeks, and will be subsequently reassessed with the study surveys at 7 weeks (T2) and at 11 weeks (T3) of follow-up.

A wait-list control group will be used with the objective that all participants have the opportunity of receiving the online MBI and at the same time allow comparison of the effectiveness of the program with a group without intervention. Given that treatment-related stress has an important negative impact on patients' wellbeing, it is appropriate to offer the control group the possibility of receiving the intervention if its effectiveness is demonstrated.

The control group will be assessed with the study surveys at baseline (T1), will not participate in the online MBI for the next 6 weeks, and will be subsequently reassessed with the study surveys at the same timepoints as the intervention group at 7 weeks (T2) and at 11 weeks (T3) of follow-up. Upon completion of recruitment and follow-up of all participants, the study will end, and the control group will be able to receive the online MBI according to their preference.

The online MBI sessions will be available in a website created specifically for this purpose, where each pre-recorded session will be uploaded so that each patient can begin the intervention at the same time as their chemotherapy regimen. Each session will last approximately 1 hour, for a total of 6 sessions. The topics that will be covered in these sessions include: mindfulness overview, stress and suffering, emotional regulation, meditation, management of stressful situations, self-compassion and acceptance. Participants will watch 1 session per week following the order that will be indicated on the website; patients will be encouraged to watch the sessions during their chemotherapy infusions. During these sessions, patients will be instructed in mindfulness practice and will perform targeted exercises to promote complete consciousness, including mindful body awareness (body scan) and meditation.

Measured through the Perceived Stress Scale (PSS). The PSS is a self-report instrument that assesses the level of perceived stress over the past month. It consists of 10 items that are rated using a 5-point Likert scale (0: Never; 1: Almost never; 2: Sometimes; 3: Fairly often; 4: Very often). The total score is obtained by adding the scores of the 10 items, obtaining a total value between 0-40, where higher scores indicate a higher level of perceived stress. A moderate stress level is considered between 14-26 points, and a high stress level is considered when obtaining >27 points.