Phenotype definition: post-RT pain

All women enrolled in the study filled out the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-39/Radiation Therapy Oncology Group (RTOG) 0413 protocol QOL questionnaire at baseline and post-RT, which contains four pain severity items (i.e., pain at its worst, least, average during the past 4 weeks, and now) from the Brief Pain Inventory (BPI). A pain score was determined as the mean of these four pain severity items (from 0 = no pain to 10 = the worst imaginable pain) as suggested by the BPI developers [15], and moderate to severe pain (pain score ≥ 4) was considered clinically relevant [16, 17]. Therefore, cases were defined as those that had a pain score ≥ 4 at post-RT (n = 326), and the reference group included those with a pain score < 4 at post-RT (n = 786).