Index and Nonindex Pain Endpoints in Radiation Therapy for Painful Tumors: A Secondary Analysis of a Prospective Observational Study

Purpose Improving pain interference in daily activities, rather than mere pain reduction, is a desirable endpoint for palliative radiation therapy. The association between pain response and pain interference has been studied almost exclusively in patients with painful bone metastases (PBMs), whereas nonindex pain has scarcely been explored in palliative radiation therapy. We investigated whether index and nonindex pain endpoints are associated with pain interference changes in patients with both PBMs and painful non-bone-metastasis tumors (PNTs). Methods and Materials Brief pain inventory data collected at baseline and at 2 months post-treatment were used to calculate differences in pain interference scores. Pain response in terms of the index pain was assessed using the international consensus endpoint. Patients were diagnosed with predominance of other pain (POP) if nonindex pain of malignant or unknown origin was present and had a greater pain score than the index pain. Results Of 302 patients, 127 (42%) had PBMs and 175 (58%) had PNTs. The median pain interference score, which is based on the mean of the 7 subscale items, decreased to a greater extent among responders than among nonresponders (PBM group: -3.43 vs -0.57 [P = .005]; PNT group: -2.43 vs -0.29 [P < .001]). Moreover, patients without POP experienced a greater reduction in their median pain interference score than did those with POP (PBM group: -2.71 vs +0.43 [P = .004]; PNT group: -2.00 vs +1.57 [P = .007]). The Jonckheere-Terpstra test showed a significant trend across 4 pain response categories in patients with PBMs and those with PNTs (P < .001 for both). Conclusions The index and nonindex pain endpoints were positively and negatively associated with improvement in pain interference, respectively. There was no apparent difference between patients with PBMs and PNTs in terms of the associations of these endpoints with pain interference.