We identified five types of third-wave psychotherapy for cancer caregivers investigated over the last 20 years. The most common intervention was MBSR, followed by MBCT and ACT. MBSR has a flexible structure, takes less time than other psychotherapies and is widely used in nonclinical populations [12]. For this reason, MBSR was the most popular intervention in previous studies on caregivers of elderly individuals [18].
The delivery of third-wave psychotherapy interventions has evolved over time. We noticed a growing trend of intervention using digital devices after 2017. In this review, third-wave psychotherapy has been delivered in various ways, such as voice or video calls, web pages, and mobile applications, moving beyond previous face-to-face delivery methods. This is in contrast to a previous mindfulness intervention study conducted on palliative caregivers in 2016 in which 90% of interventions were provided face-to-face [32]. Interventions using digital devices are accessible at any time and place, making them tremendously advantageous for cancer caregivers, who have a sizable temporal burden [33]. In our review, the participants of two studies using mobile applications [20, 29] positively evaluated the intervention based on its ease of access.
In addition to the modes of delivery, the structure of digital psychotherapy needs to be considered. The ineffectiveness of digital health interventions could be derived from the insufficient structure of the therapy when compared to traditional face-to-face treatment [34]. Cognitive‒behavioral therapy draws out human emotions from unmeasurable areas, observes behavior, and manipulates the configuration of behavior to verify its effectiveness through re-executable experiments; as such, the structure of the treatment is emphasized [11]. Two interventions using mobile applications in our review were less structured programs that did not adhere to essential steps in CBT (i.e., case conceptualization) and were not effective when the outcomes of the intervention and control groups were compared.
One notable disadvantage of interventions using digital devices is that they imply less interaction with the therapist. Cognitive‒behavioral therapy, which values the relationship with the therapist, still lacks sufficient evidence regarding mobile or web-based interventions that do not require the user to communicate directly with the therapist [33]. Two interventions using the web page and the mobile app without interactions with the therapist failed to demonstrate effectiveness on caregivers [20, 29], except for a positive effect in the domain of mindfulness [29].
Involving the therapist, even indirectly, in the intervention can be one solution in this case. The results of the intervention by Köhle et al. [27] using a webpage indicate that the scores for positive mental health, psychological flexibility, self-compassion, sense of mastery, and relational communication style were higher than those for interventions involving personalized feedback via email. Therefore, it is essential to establish an effective intervention while ensuring ease of use through advanced technology when planning future studies.
In previous systematic reviews of web-based and mobile applications targeting health care workers, digital devices were not yet an adequate substitute for face-to-face interventions [35]. However, digital technology may be more suitable for helping individuals as a complement to face-to-face therapy for managing relatively mild emotional distress [35, 36]. We should continue experimenting with digital technologies and find practical uses for them.
Most of the interventions targeted patient–caregiver dyads. Some patient–caregiver dyad programs reported promising outcomes, and therefore, dyad intervention has been emphasized in the literature [37, 38]. The studies included in our review also displayed some positive results. Hsiao et al. [28] found that during the group MBSR session, depression and stress measured by salivary cortisol levels within 45 minutes after waking up were significantly reduced, and sleep quality, QoL, and mental well-being were improved. Milbury et al. [22] reported a significant group effect of the decrease in depression in patient–caregiver dyads who received couples-based mindfulness meditation intervention.
Some interventions indicated improvement in patient health outcomes more clearly than caregiver outcomes. MBSR interventions for patient–caregiver dyads by Schellekens et al. [25] and Kubo et al. [29] found that the QoL of patients measured by the Global Quality of Life subscale of the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire (EORTC QLQ) and Functional Assessment of Cancer Therapy General Scale (FACT-G) was significantly improved, whereas the partners' QoL measured as distress or burden was not influenced by the interventions. Likewise, dyad MBCT interventions by Milbury et al. [22] did not improve the psychological health of the caregivers but had a statistically significant positive effect on the patient's cognitive and cancer-related symptoms. Given that patients’ distress interacts with that of family members [39], improving patient outcomes can ultimately be beneficial to caregivers.
Whether dyad interventions help improve the quality of relationships is debatable. Hsiao et al. [28] reported that immature attachment signaled by, for example, anxiety and avoidance in relationships decreased in relationships between partners; however, Price-Blackshear et al. [30] found that dyadic adjustment and relationship quality worsened after the intervention in the dyad meditation group. A study by Price-Blackshear et al. [30] implied that coordination and relationship quality in individual meditation groups improved over time. These results may indicate that caregivers also desire to receive independent support in caregiving settings [32].
Most studies were interested in depression, which was also one of the most measured outcomes in previous psychosocial intervention reviews [4]. In our review, the interventions of Hsiao et al. [28] and Milbury et al. [21] both demonstrated positive effects of third-wave psychotherapy on caregivers’ depression. The prevalence of depression in cancer caregivers is over 40% [26], which calls for the use of appropriate intervention approaches. Mindfulness-based interventions have demonstrated effectiveness in reducing the severity of depression in a wide range of individuals, with or without the disease [40].
All 12 studies were randomized trials, but only one study estimated and secured an appropriate sample size. The results should thus be interpreted with caution, as the quality of the study has not been evaluated, and improvements in methodology are recommended before conclusions can be drawn about the efficacy of third-wave psychotherapy for caregivers of cancer patients. Moreover, there are not enough data on the long-term outcomes of interventions.
There are some limitations of this review. We included only experimental studies that quantitatively confirmed the results of the interventions. In addition, the search period was limited to the last 20 years based on the start of active clinical application of third-wave psychotherapy, and data from the 1980s to 2000, when third-wave psychotherapy was first introduced, were not included.