Abstract
Background/Aim: The risk of genitourinary toxicity during radiotherapy for prostate cancer was found to be lower for bladder volumes ≥200 ml. An app that reminds patients daily to drink water might be helpful. Before being investigated in patients, an app should be tested in healthy volunteers.
Materials and Methods: Thirty healthy volunteers were included in this prospective study and asked to test the app and affirm (=satisfaction) or negate nine statements. These statements belonged to the sections ‘Download and installation’ (two statements), ‘Navigation’ (two statements), or ‘Content/functions’ (five statements). If a satisfaction rate was <60%, the app was to be considered not useful. If it was <80%, the app needed optimization. iPhone users (n=18) were compared to Android users (n=12).
Results: Satisfaction rates (participants affirming a statement) were 90.0% (27 out of 30 participants) and 86.7% (26 out of 30 participants) regarding the two statements belonging to the Download and installation section. Regarding the two statements of the Navigation section, satisfaction rates were 100% (28 out of 28) and 96.6% (28 out of 29), respectively. For the Content/functions section, satisfaction rates were 79.3% (23 out of 29 participants) for the statement: “The app reminded me at the selected times”. For the other four statements, satisfaction rates were each 100% (25 out of 25, 27 out of 27, 28 out of 28, and 29 out of 29 participants). Significant differences between iPhone and Android users were not observed. When looking at the subgroups of iPhone and Android users, two additional aspects were identified that needed modifications.
Conclusion: Although the new reminder app was mainly rated usable, some modifications were required. Our findings highlight that a pre-study in healthy volunteers is important.
Introduction
Moderately hypofractionated radiotherapy (mHF-RT) is frequently used in the treatment of prostate cancer, particularly in low- or intermediate-risk situations (1). A common dose-fractionation regimen includes 20 fractions of 3.0 Gy over 4 weeks. Many patients with high-risk cancer receive conventionally fractionated radiotherapy (CF-RT), often with total doses of 76-80 Gy in 38 to 40 fractions over approximately 8 weeks. Both mHF-RT and CF-RT may cause genitourinary toxicity, including acute cystitis. In three previous studies, the risk of experiencing genitourinary toxicity was lower when the bladder volume was 200 ml or more (2-4). Drinking protocols have been developed to improve the bladder-filling status during a course of radiotherapy (5-10). To comply with such a protocol may be challenging for the mostly elderly patients. A mobile application that reminds the patients to drink water before each session of radiotherapy might be helpful in this context. Such an app has already been designed. However, before it can be tested in patients, it appears reasonable to evaluate its usability in a cohort of healthy volunteers. Therefore, this prospective study was performed, which included 30 healthy volunteers who tested the new reminder app and rated its practicability and functionality. Based on the results of this study, the app might require minor or major modifications.
Materials and Methods
This prospective study included 30 healthy volunteers and was part of the protocol of a prospective clinical trial involving patients irradiated for prostate cancer (11). It had received approval from the responsible Ethics Committee in Lübeck, Germany (2024-519_1). Healthy volunteers were recruited from the social environment of the members of the study team. Twenty-three of them were women, and seven were men. The median age was 41 years (range=21–70 years). The study participants had to download a reminder app, which was designed by the Lübeck-based company Nextlabel OHG. After receiving the link for the download of the app by e-mail from Nextlabel OHG, the participants tested the app and completed a questionnaire. They were asked to affirm or negate nine statements regarding the usability of the reminder app.
When starting to use the app, the participants were able to choose between different languages, namely English, German, and Danish. Subsequently, the app provided some information regarding the importance of drinking water prior to each radiotherapy session. Moreover, the participants received instructions regarding the amount of water they were required to drink (300 ml) and the recommended time of water intake (45 min before the radiotherapy session). They were able to set the time they would like to be reminded for each day. After they were reminded, the participants had to confirm by pressing a button that the fluid had been taken as required. For every confirmed intake, they received a piece of a puzzle as a reward. The intent was to complete the puzzle. Selected screenshots of the app in use are shown in Figure 1.
Selected screenshots of the reminder app.
The questionnaire included a total of nine statements assigned to one of three sections. The sections were named ‘Download and installation’, ‘Navigation’, and ‘Content/functions’. These sections included two, two, and five statements, respectively.
The nine statements that had to be affirmed (Yes) or negated (No) are summarized in Figure 2. Moreover, the participants were allowed to add their own comments related or not to any of the three sections. The rate of participants who affirmed a statement represented the corresponding satisfaction rate. If this rate was <60%, the reminder app was to be rated as not being useful. If the rate was between 60% and <80%, the app was to be regarded as useful but requiring further optimization.
English version of the questionnaire for the healthy volunteers, comprising three sections and nine statements to be affirmed or negated.
In addition to the evaluation by the entire cohort, iPhone users (n=18) were compared to Android users (n=12) with respect to the nine statements. These comparisons were performed with Fisher’s exact tests, and a value of p<0.05 was regarded as indicating a statistically significant difference.
Results
The statements were rated by a median of 29 (range=25-30) study participants. In addition to the rating of the statements, 17 additional individual comments were made. Six comments were related to Download and installation, three comments to Navigation, and eight comments to Content/functions.
Considering the entire cohort, satisfaction rates were sufficiently high for the Download and installation section and the Navigation section (Table I). For the Content/ functions section, satisfaction rates were perfect (100%) with respect to four statements. However, the satisfaction rate was less than 80% regarding the statement “The app reminded me at the selected times” (Table I). Thus, the app will need modifications in this respect. The most common topics addressed in the 17 individual comments were different problems with downloading or installing the app (six comments), non-receipt of push notification (four comments), and the setting of the times for being reminded to drink water (four comments).
Rates of affirmation (yes) and negation (no) of the nine statements related to the usability of the reminder app.
Significant differences between iPhone and Android users were not observed (Table II). However, in the subgroup evaluations, the satisfaction rate was less than 80% for two statements. These were “There were no delays in the installation.” for iPhone users (satisfaction rate=77.8%) and “The app reminded me at the selected times.” for Android users (satisfaction rate=75.0%).
Rates of affirmation (yes) and negation (no) of the nine statements by iPhone and Android users of the app.
Discussion
Definitive local radiotherapy is a standard treatment for non-metastatic prostate cancer (1). This treatment can lead to genitourinary toxicity, particularly in cases with inappropriate filling (2-4). In a previous study of 80 patients irradiated with 70.2 Gy in 39 fractions, the proportion of patients reporting pain on urination was 0% when the initial bladder volume was at least 180 ml versus 24% when it was less than 180 ml (2). In a prospective study of patients treated with CF-RT and a median total dose of 80 Gy (n=193), keeping the bladder volume above 180 ml resulted in less acute genitourinary toxicity (3). In another study of patients treated with CF-RT and total doses of 76 to 78 Gy to the prostate with/without 45 Gy to the pelvic lymph nodes, acute genitourinary toxicity was significantly increased when the bladder volume was less than 200 ml (4).
These studies suggested that bladder filling should be kept at 200 ml or more, which may be facilitated with the use of specific drinking or bladder-filling protocols (5-10, 12-17). However, some patients may find it difficult to comply with such a protocol. Therefore, an app was created to remind patients to drink water 45 min before their radiotherapy session (11). In previous studies, patients with prostate cancer who had a bladder volume of less than 200 ml or less than 250 ml, respectively, were found to have higher mean numbers of radiotherapy sessions with a critical bladder volume of less than 200 ml (18-20). Therefore, these patients would benefit in particular from the reminder app.
However, before the app can be tested in clinical trials with patients, its practicability and functionality should be evaluated by healthy volunteers. In the present prospective study, 30 healthy volunteers tested and rated nine aspects related to the usability of the app. Seven aspects achieved a satisfaction rate of ≥90%, of which five aspects achieved 100% (Table I). A satisfaction rate of <80% (79.3%) was found only for the aspect “The app reminded me at the selected times”. According to the pre-defined limits, this aspect of the app was considered to require modifications. When looking at the subgroups of iPhone and Android users, two additional aspects were identified that needed modifications. These aspects included “There were no delays in installation” for iPhone users and “The app reminded me at the selected times” for Android users (Table II). In addition, improvement of the app was required according to the additional individual comments, e.g. with respect to receipt of push notifications, and the setting of the times for reminders. The results of this study were summarized in a report to the manufacturer of the app, who made corresponding changes. Afterwards, the reminder app was improved and subsequently considered ready to be tested in clinical trials involving patients irradiated for prostate cancer.
Study limitations. Despite the value of the results of this study for optimization of the reminder app, the limitations of the present study should be considered. These include the comparably small sample size, the fact that many participants were women, and the fact that most of them were considerably younger than the majority of patients with prostate cancer (1). Moreover, selection biases were introduced because participants were required to possess a smart phone and were recruited from the social environment of the members of the study team.
Conclusion
The new reminder app was mainly rated usable. Modifications were required regarding one aspect when considering the entire cohort. The subgroup evaluation revealed that modifications were needed for one aspect each in the groups of iPhone and Android users. These findings illustrate that evaluation of the usability of a new app in healthy volunteers is quite important.
Acknowledgements
The study was partially funded by the Interreg Deutschland-Danmark program (code 01-1-23 2).
Footnotes
Authors’ Contributions
All Authors participated in designing the study and contributed to collection of the data that were analyzed by D.R. The manuscript was drafted by D.R., and subsequently reviewed and approved by all Authors.
Conflicts of Interest
The Authors belong to project partners of the Interreg-project HeAT. Otherwise, no conflicts of interest in relation to this study exist.
Artificial Intelligence (AI) Disclosure
No artificial intelligence (AI) tools, including large language models or machine learning software, were used in the preparation, analysis, or presentation of this manuscript.
- Received February 26, 2026.
- Revision received March 20, 2026.
- Accepted March 31, 2026.
- Copyright © 2026 The Author(s). Published by the International Institute of Anticancer Research.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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