Converting between the International Prostate Symptom Score (IPSS) and the Expanded Prostate Cancer Index Composite (EPIC) urinary subscales: modeling and external validation

Background: Prostate-related quality of life can be assessed with a
variety of different questionnaires. The 50-item Expanded Prostate Cancer
Index Composite (EPIC) and the International Prostate Symptom Score (IPSS)
are two widely used options. The goal of this study was, therefore, to
develop and validate a model that is able to convert between the EPIC and
the IPSS to enable comparisons across different studies. 
Methods: Three hundred forty-seven consecutive patients who had previously
received radiotherapy and surgery for prostate cancer at two institutions
in Switzerland and Germany were contacted via mail and instructed to
complete both questionnaires. The Swiss cohort was used to train and
internally validate different machine learning models using fourfold
cross-validation. The German cohort was used for external validation.
Results: Converting between the EPIC Urinary Irritative/Obstructive
subscale and the IPSS using linear regressions resulted in mean absolute
errors (MAEs) of 3.88 and 6.12, which is below the respective previously
published minimal important differences (MIDs) of 5.2 and 10 points.
Converting between the EPIC Urinary Summary and the IPSS was less accurate
with MAEs of 5.13 and 10.45, similar to the MIDs. More complex model
architectures did not result in improved performance in this study. The
study was limited to the German versions of the respective questionnaires.
Conclusions: Linear regressions can be used to convert between the IPSS
and the EPIC Urinary subscales. While the equations obtained in this study
can be used to compare results across clinical trials, they should not be
used to inform clinical decision-making in individual patients. Trial
registration This study was retrospectively registered on
clinicaltrials.gov on January 14th, 2022, under the registration number
NCT05192876.