Confirmatory factor analysis of brief pain inventory (BPI) functional interference clusters in patients with bone metastases.

To determine which of the previously proposed functional interference cluster models is most appropriate in patients with bone metastases and to determine if the cluster structures identified at baseline differed between responders and non-responders following palliative radiotherapy. METHODS: The confirmatory test data set consists of breast and prostate cancer patients treated with palliative radiotherapy between May 2003 to January 2007. Worst pain and functional interference scores were assessed using Brief Pain Inventory at baseline, 4, 8 and 12 weeks post radiation treatment. The baseline cluster structure of the confirmatory dataset was compared to each of the previously proposed baseline cluster models. Maximum likelihood CFA was used to account for possible correlation amongst the factor components. A MIMIC model was used to determine the invariance of the cluster models between responders and non-responders during follow-up. RESULTS: A total of 169 eligible patients were analysed. There were 91 male and 78 female patients with a median age of 68 years. The median KPS was 70. A single 8 Gy and 20 Gy in 5 fractions were used in 97% of all analysed patients. The RTOG model, in which relationships with others and sleep comprised the mood-related interference cluster and walking ability and normal work comprised the physical-interference cluster, provides the best fit for the sample data. The follow-up cluster structure is not similar across the responder groups indicating that cluster structures shift following radiation treatment, as evidenced by pain response. CONCLUSION: Although differing slightly this analysis confirms pretreatment symptom clusters exist for patients with bone metastases from breast or prostate cancer based on the RTOG 9714 data. This could help formulate symptom management interventions at initial diagnosis. Symptom clusters dissolve or change after treatment which may be a function of the treatment or population and requires further study.