Koizumi et al. BMC Cancer 2010, 10:423 http://www.biomedcentral.com/1471-2407/10/423 without pain at diagnosis of skeletal metastasis. There- fore, two types of events were used for the analysis: SRE and CSD. Deaths due to other causes were treated as censored. The starting point of the study was the date of diagnosis of skeletal metastasis. Thirdly, a Cox pro- portional hazards regression model was used to investi- gate whether the presence of pain was a significant factor using univariate and multivariate analysis. The proportionality of each hazard was confirmed by plot- ting a log-minus-log curve. Multivariate Cox propor- tional hazards regression analysis was conducted using a backward stepwise method (the statistical level of signifi- cance determined with the Wald test was set at P < 0.10). All analyses were performed using SAS version 8.2 (SAS Institute Inc., Cary NC). Finally, as a lead-time effect could influence the results, a similar analysis (sec- ond and third steps) using the initial surgery date as a starting point was also performed. In addition, the number of bone metastatic lesions (solitary or multiple) at the time of diagnosis of skeletal metastasis was an important factor. The importance of this factor was reported [14] as solitary bone metastasis was a significant favorable prognostic factor compared with multiple bone lesions at initial presentation. Strati- fied sub-analysis was conducted on groups of individuals with a solitary bone lesion or multiple bone lesions at their initial presentation of skeletal metastasis. This retrospective study was approved by the Institu- tional Review Board. Informed consent was not required as this was a retrospective study. Results Of 668 patients who developed skeletal metastasis, the pain status was unknown in 2 patients, therefore 666 patients were included in this study. Of the total num- ber of individuals, 396 (59.5%) did not complain of pain and 270 (40.5%) did. CEA values were available for 647 patients; 412 (63.7%) had a normal value and 235 (36.3%) were above the reference value. CA15-3 values were available for 649 patients; 391 (60.2%) had normal values and 258 (39.8%) were above the reference value. In 396 osseous metastasis patients without pain, 60 had elevated CEA without elevation of CA 15-3, 55 had ele- vated CA 15-3 without elevation of CEA, 80 presented with elevation of both CEA and CA 15-3, and in 201 patients both CEA and CA 15-3 were within normal limits. There were 187 (28.1%) patients who did not pre- sent with both bone pain and tumor marker elevation. The patients’ characteristics are presented in addi- tional file 1. With the exception of CA15-3 and number of metastatic lesions (solitary or multiple), there was no significant difference between patients with and without pain regarding the factors noted at the start of therapy. The mean period from surgery to diagnosis of skeletal Page 3 of 9 metastasis was 1262 days (median 1031 days) for patients without pain and 1268 days (median 921 days) for patients with pain. There was no statistical difference in the period from surgery to diagnosis between patients with or without pain (Mann-Whitney U test p = 0.785). The mean period from diagnosis of skeletal metastasis to development of SREs was 657 days (median 395 days) for patients without pain and 281 days (median 45 days) for patients with pain. There was a statistical dif- ference in the period from diagnosis to development of SREs between patients with and without pain (Mann- Whitney U test p < 0.0001). The numbers of patients with and without pain in each SRE are presented in additional file 2. The Kaplan-Meier curves for SREs in patients with and without pain are presented in figure 1. The starting point was the date of diagnosis of skeletal metastasis. Patients without pain had fewer SREs than those with pain (log-rank test: p < 0.001). Figure 2 shows Kaplan- Meier curves for survival (CSD) in patients with and without pain. Patients without pain survived longer than those with pain (log-rank test: p <0.001). The results of Cox’s proportional hazard analysis regarding SRE are given in additional file 3. Presence or absence of pain was a significant factor in both univariate analysis (base: no pain, hazard ratio = 2.331, 95% CI 1.930-2.815) and multivariate analysis (hazard ratio = 2.243, 95% CI 1.815-2.773). The results of Cox’s proportional hazard analysis regarding survival (CSD) are presented in addi- tional file 4. Presence or absence of pain was a signifi- cant factor in both univariate analysis (base: no pain, hazard ratio = 1.441, 95% CI 1.211-1.715) and multivari- ate analysis (hazard ratio = 1.535, 95% CI 1.263-1.866). Figures 3 and 4 show Kaplan-Meier curves for SRE and survival (CSD) in patients without and with pain; the starting point was the date of initial surgery. These analyses (from the date of surgery) were carried out to neglect the lead-time effect of diagnosis. Patients with- out pain had fewer SREs than those with pain (log-rank test: p < 0.001). Patients without pain survived longer than those with pain (log-rank test: p = 0.007). Cox’s proportional hazard analysis was performed for SRE and survival (CSD) from the date of surgery. The presence or absence of pain was a significant factor in both uni- variate analysis (base: no pain, hazard ratio = 1.691, 95% CI 1.402-2.040) and multivariate analysis (hazard ratio = 1.646, 95% CI, 1.343-2.018) with regard to SRE. Regard- ing survival (CSD), presence or absence of pain was a significant factor in both univariate analysis (base: no pain, hazard ratio = 1.268, 95% CI, 1.066-1.509) and multivariate analysis (hazard ratio = 1.365, 95% CI, 1.128-1.651). The Kaplan-Meier curves for patients with or without pain are presented in figure 5. Patients with a solitary