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Ki-67 Staining Index Predicts Distant Metastasis and Survival in Locally Advanced Prostate Cancer Treated With Radiotherapy

An Analysis of Patients in Radiation Therapy Oncology Group Protocol 86-10

¹ Department of Pathology, Baylor College of Medicine, Houston, Texas; ² American College of Radiology Headquarters, Philadelphia, Pennsylvania; ³ Department of Pathology, Latter Day Saints Hospital, Salt Lake City, Utah; ⁴ Department of Pathology, Karmanos Cancer Institute and Wayne State University, Detroit, Michigan; ⁵ Department of Radiation Oncology, University of California at San Francisco, San Francisco, California; ⁶ Radiation Oncology Center, Sutter Cancer Center, Sacramento, California; ⁷ University of Michigan, Ann Arbor, Michigan; ⁸ Massachusetts General Hospital, Boston, Massachusetts; and ⁹ Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, Pennsylvania

Purpose: Proliferative activity defined by Ki-67 staining index (SI) has been correlated with progression and prognosis in a number of malignant tumors including prostate cancer. However, few studies have examined Ki-67 SI in pretreatment diagnostic material from patients treated with definitive radiotherapy. In a prior study, we found that a Ki-67 SI of >3.5% was associated with poorer patient outcome. The goals of this analysis were to validate the prognostic value of Ki-67 SI and this cut point.

Experimental Design: Of 456 assessable patients in Radiation Therapy Oncology Group Protocol 86-10, diagnostic material from 108 patients was available for Ki-67 analysis using MIB-1 antibody. Sixty patients were treated with external beam radiotherapy (EBRT) alone, and 48 patients were treated with short-term androgen deprivation + EBRT. Median follow-up was 9 years for those living. The relationship of Ki-67 with distant metastasis (DM), disease-specific survival (DSS), and overall survival (OS) was examined.

Results: The median Ki-67 SI was 7.1% (range, 0.2–45.5%). The 7.1% cut point was associated with DM and DSS; however, the 3.5% cut point was as strong a determinant and was the focus of this analysis. In Cox proportional hazards regression, Ki-67 SI was independently associated with DM and DSS. When the Ki-67 SI was 3.5% and >3.5%, the 5-year risk of DM was 13.5% and 50.8% (P = 0.0005), respectively, and the 5-year risk of DSS was 97.3% and 67.7% (P = 0.0039), respectively. No association of Ki-67 SI with OS was observed.

Conclusions: Higher Ki-67 SI was significantly associated with a greater risk of DM and DSS in locally advanced prostate cancer after definitive EBRT or AD + EBRT.

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