The various factors influencing participation in breast cancer medicinal research identified in the nine studies were placed by the authors into three categories: person-related, physician-related and study-related.
The person-related category comprised health, psycho-social and demographic reasons. A younger age (demographic reason) was identified as a factor influencing willingness to participate [15, 17, 18].
Regardless of whether study participants were younger or older, they frequently had high subjective perceptions of their risk of breast cancer [16, 18], although their objective risks, assessed according to Gail scores, in one of the two studies, were relatively low on average [17, 19, 20]. Another study also showed women who had participated in medication studies assessed their breast cancer risk subjectively much higher than women who had not participated, though both groups did not differ according to their Gail scores . This suggests that, in future studies, subjective perceptions of risk should be addressed. Also, in ovarian cancer investigations, potential subjects' higher personal risk perception and concern raised the probability of making use of screening [21, 22]. These results suggest that women's participation in such studies depends more on subjective risk than objective risk. Though subjective breast cancer risk in two studies in this review correlated with the women's willingness to participate [17, 19], in another medication study, women's participation was more likely when they were less personally concerned about breast cancer ; however, this last-mentioned study does not reveal whether relatives of the study participants had suffered from breast cancer or not, which could increase the subjects' concern, and thus interest, in participating in a medication study. One study showed women who had first and second degree relatives with breast cancer requested information about the medication study twice as often as women who did not have this diagnosis among their relatives (OR 2.35, 95% CI, 0.99-5.57).
Studies show a negative correlation between the concern regarding breast cancer and satisfaction with the physician's consultation . Nevertheless, satisfaction with physicians' consultation and communication processes (physician-related reason) was mentioned as reason for participation/non-participation in medication studies [16, 18], indicating that patient-clinician relationships play a decisive role in patients' willingness to participate in a study.
Identification of potential differences in consultations in the medication studies was not possible [11, 13, 16] because only one study reported the content of the conversations . Therefore, not all women in the studies might have been informed about the same things. To avoid this distortion in future studies, the use and documentation of conversation manuals in these consultations seems advisable.
The main study protocol-related reason against participation was additional time needed [12, 18, 19].
The randomization procedure was mentioned in two studies as reason against participation [11, 19]; willingness for randomization was mentioned in one study as an influencing factor for participation . In cancer research, lack of understanding of the principle of randomization has been researched as barrier to subject participation [24, 25]. Another study showed that, among those who initially decided against participation in randomized studies, more than half ultimately consented to participation after they had received more detailed information regarding the randomization process . Though this connection was not identifiable from studies in this review, our results showed that randomization could influence participation in medication studies. Informing potential study subjects of the reasons for randomization could therefore promote their participation.
Fear of possible side-effects (a treatment-related reason) was also frequently mentioned as reason for non-participation [11, 15, 18, 19], suggesting that the probabilities for possible side-effects should be explained extensively during recruitment.
The review shows that the willingness to participate in the theoretical scenarios was considerably higher (58%; range: 25-75%) [11, 13, 16] than in studies that were actually, or yet to be, conducted (27%; range: 1.5-55%) [12, 14, 15, 17–19]. High willingness to participate in hypothetical scenarios is also seen in other studies [26–28]. Two studies collected their data retrospectively [12, 19], with the risk of recall bias in the results. Two other studies counteracted this bias by collecting would-be participants' relevant reasons immediately after consent or rejection of participating in their respective study [15, 18]. This procedure could also prevent such distortion in future studies.
A limitation of this review concerns its ten-year time frame. Although more full texts might have been included if our criteria allowed older investigations, the primary goal of this review was to identify current studies; we therefore restricted this study to the past decade.
The strengths of this review include its use of an extensive assessment scheme, allowing comprehensive quality evaluation of the respective articles, using consistent criteria. This scheme also could function as a kind of checklist, thus reducing the probability of forgetting any items in the assessment. Another strong point was that six databases were searched, allowing wide coverage of possible publications, as a result, of the articles gleaned from the references of all the full texts, only one was found that had not been part of the original database literature research. Inclusion of only high-quality studies is a further strength, as all studies had to show a high quality, of at least 75%, to be included in this review. Two studies did not meet this requirement and were therefore not included in the results [29, 30].
Physicians' viewpoints as to why women with breast cancer or breast cancer risk choose or decline to participate in medication studies is being researched, both with regard to general cancers  and breast cancer [24, 32], but not specifically with medication studies.
In sum, this review shows that the reasons for participation/non-participation in medication studies are multifactorial. Moreover, while factors affecting patient participation in medication studies are obviously useful to know in planning and realizing future investigations, few such insights are currently available, apparently due to the small number of relevant studies; further quantitative and qualitative research is needed.