Automated Bone Scan Index to Optimize Prostate Cancer Working Group Radiographic Progression Criteria for Men With Metastatic Castration-Resistant Prostate Cancer

The study sought to quantify the total increase in tumor burden represented by prostate cancer working group progression criteria, and to determine the interval increase that best associates with overall survival. An absolute increase of 0.6 in aBSI from the first follow-up scan results in the highest association with survival in patients with metastatic castration resistant prostate cancer. INTRODUCTION: Radiographic progression-free survival (rPFS) by Prostate Cancer Working Group (PCWG) cr iter ia is a radiographic endpoint. The automated bone scan index (aBSI) quantifies osseous disease burden on bone scintigraphy as a percentage of total skeletal weight. Using the aBSI, we sought to quantify increase in tumor burden represented by PCWG progression criteria, and to determine the interval increase that best associates with overall survival (OS). PATIENT AND METHODS: Retrospective analysis of trials using androgen receptor axis–targeted drugs for metastatic castration resistant prostate cancer patients (mCRPC). aBSI increase in bone disease was assessed from baseline scan to time-to-progression (per PCWG cr iter ia). Threshold for time to aBSI increase were explored and the association between each time-to-threshold and OS was computed. RESULTS: A total of 169 mCPRC patients had bone scans available for aBSI analysis. Of these, 90 (53%) had progression in bone meeting PCWG criteria. Total aBSI increase in patients meeting PCWG cr iter ia was 1.22 (interquartile range [IQR]: 0.65–2.49), with a median relative increase of 109% (IQR: 40%−377%). Median aBSI at baseline was 3.1 (IQR: 1.3–7.1). The best association between OS and time- to-progression occurred with an absolute increase in aBSI equal to 0.6 (Kendall’s tau 0.52). CONCLUSION: An absolute increase of 0.6 or more in aBSI from the first follow-up scan results in the highest association with OS in patients with mCRPC. The rPFS by PCWG, identified progression at nearly twice this tumor burden, suggesting that aBSI may be used to further develop the PCWG cr iter ia without degrading its association with OS.