Prognostic Value of Automated Bone Scan Index (aBSI) in Patients with mCRPC Undergoing Three vs. Six Cycles of 223Ra Therapy

<b>Background/Objectives</b>: In patients with metastatic castration-resistant prostate cancer (mCRPC) and osseous metastases only, ²²³Radium therapy represents a valuable therapeutic option. Bone scintigraphy (BS) is typically performed to assess metastasis load, with the BS-derived automated bone scan index (aBSI) used for response assessment. This study aimed to evaluate the prognostic value of aBSI in patients receiving three or six cycles of ²²³Ra therapy. <b>Methods</b>: We included patients that were diagnosed with extensive osseous tumor load on BS, had no visceral or nodal metastases, had undergone ²²³Ra therapy. The aBSI prior to and following three or six cycles of therapy, total tumor volume (TTV), SUV_max, and overall survival were analyzed. <b>Results</b>: This study included 49 mCRPC patients (mean age: 70 ± 9 years) with 42 (85.7%) receiving six and 7 (14.3%) receiving three cycles. After three cycles, the mean aBSI (<i>p</i> = 0.369), TTV (<i>p</i> = 0.902), and SUV_max (<i>p</i> = 0.149) remained unchanged. After six cycles, the mean aBSI (<i>p</i> = 0.247) and TTV (<i>p</i> = 0.784) were unchanged, while SUV_max decreased significantly (<i>p</i> = 0.001). The aBSI did not significantly correlate with the mean aBSI (six cycles: χ² = 1.823, <i>p</i> = 0.177; three cycles: χ² = 0.308, <i>p</i> = 0.579). <b>Conclusions</b>: Although quantitative changes in TTV and aBSI did not significantly correlate with each other, their respective absolute values consistently indicated stable disease burden under therapy. This highlights its potential as a useful tool for monitoring disease burden while indicating that aBSI alone is insufficient for predicting overall survival.