Bone scan index at baseline as a tool for predicting hematologic toxicity in metastatic castration-resistant prostate cancer patients eligible for radium-223 treatment.

Abstract

e16513 Background: The prognosis of castration-resistant prostate cancer with skeletal metastasis (CRPCSM) is poor. The ALSYMPCA trial led to the approval of radium-223 (Ra223) in such patients. Factors that could predict hematologic toxicities associated with Ra223 remain poorly defined. We analyzed the utility of bone scan index (BSI) at baseline (BSI-B) as a predictive marker for such toxicities. Methods: This is a retrospective study of CRPCSM patients without visceral metastasis who received Ra223 at Roswell Park Cancer Institute from 2013 to 2015. BSI was defined utilizing the estimation of a numerical index (max, 70.8% for diffuse skeletal involvement) that expresses the fractional involvement of each bone by tumor. BSI-B values were classified into < first quartile ( < Q1), between first and third quartile (Q1-Q3), and > third quartile ( > Q3). The associations between BSI-B and different hematologic parameters [hemoglobin (Hgb), platelets (Plt), absolute neutrophil count (ANC)] over the Ra223 treatment duration were evaluated using linear mixed models. Results: A total of 79 patients were included in this analysis. The median Gleason score was 8 (range: 4-10) and the median age at first Ra223 was 71 years. Seventy percent of patients received 5 or more doses of Ra223. There was a significant association between BSI-B and Hgb (p = 0.005) and Plt (p = 0.011) levels at baseline and at all time points. The > Q3 BSI-B was associated with a greater drop in Plt from an elevated baseline with subsequent treatments of Ra223 [drop from a mean of 293.5±21.3 x109/L (n = 19) to 132.2±48.8 x109/L (n = 9) after sixth Ra223]; while such a decline was not observed for lower BSI-B (p < 0.001). The ANC decreased with each Ra223 in patients with > Q3 BSI-B but not in the ones with lower BSI-B (p = 0.003). Conclusions: Our work demonstrates that hematologic parameters at baseline and during Ra223 treatment are associated with higher BSI-B values, raising the possibility of BSI-B as a valuable tool for predicting the risk of cytopenias before initiating Ra223 in CRPCSM patients. Prospective validation is needed to confirm the utility of our findings.