ESTRO 2025 - Abstract Book

S1875

Clinical - Urology

ESTRO 2025

31

Digital Poster Predicting the Treatment Effect of Ra-223 Therapy for Metastatic Prostate Cancer Based on Alpha-Particle Range and Pre- Treatment Type Yasuo Oguma 1 , Makoto Hosono 2 , Kaoru Okajima 1 , Yutaro Wada 1 , Eri Inoue 1 , Yukinori Matsuo 2 1 Radiology, Nara Hospital, Kindai University, Ikoma, Japan. 2 Radiation Oncology, Kindai University, Osaka-Sayama, Japan Purpose/Objective: Alpha particles emitted by Ra-223 penetrate only a few layers of tumor cells. We hypothesize that Ra-223 therapy is more effective for smaller metastases. Our objective is to clarify the relationship between treatment efficacy and the intensity of bone metastasis. Material/Methods: The treatment effect of Ra-223 therapy was evaluated by the change rate of PSA during Ra-223 administration. A predictive model for PSA change rate was constructed using multiple regression analysis for patients who received at least one dose of Ra-223 (every 4 weeks, maximum of 6 doses) between 2016 and 2023, in a retrospective study. The change rate was calculated as (post-dose/pre-dose). Blood biochemical tests were collected within 1 month prior to 1st administration and 1 month after completion. To determine the intensity of metastatic tumors, the Bone Scan Index (BSI; Bone navi, PDR pharma co. ltd.), PSA, and ALP were used as factors. PSA doubling time (PSADT) was used as an index of proliferation rate. Other factors included Gleason Score (GS) and type of prior or concurrent treatment (taxane, abiraterone, enzalutamide). Factors related to bone metastasis intensity (BSI, ALP, and PSA) were analyzed separately because they were weakly associated with each other. PSA change rate, BSI, ALP, and PSA were set to the natural logarithm in order to make them normally distributed. Results were considered statistically significant if p<0.05. Results: Pre and post PSA data were available for 61 out of 64 eligible patients, with 12 patients (20%) showing a decrease and 49 patients (80%) showing an increase. One results of the analysis using BSI as an indicator of bone metastasis are presented in the table below. The prior or concurrent use of abiraterone and GS showed statistical significance, whereas other indices of metastatic intensity such as BSI, PSA and ALP (not listed) did not reach statistical significance.

Regression coefficient estimates

95%CI

SE

t

p

(Intercept)

-6.804

-11.218 -2.391 2.169 -3.136 0.004

GS

0.660

0.204

1.116 0.224 2.944 0.006

ln(BSI)

-0.043

-0.360 0.274 0.156 -0.275 0.785

Prior or concurrent use of abirateron

1.660

0.460

2.860 0.590 2.814 0.008

Prior use of ENZ

0.967

-0.046 1.980 0.498 1.943 0.061

Prior use of TAX

0.223

-0.791 1.237 0.498 0.447 0.658

PSADT

-0.003

-0.008 0.002 0.002 -1.284 0.208

Conclusion: The efficacy of Ra-223 treatment, as measured by PSA change rate, was found to be significantly associated with prior treatment and GS, regardless of the bone metastasis intensity.

Keywords: Ra-223, prostate cancer, bone metastasis

References: Radiation . 2022; 2(3):273-284. https://doi.org/10.3390/radiation2030021