Relationship between Day 0 dosimetric parameters and biochemical relapse-free survival in patients treated with transperineal permanent prostate interstitial brachytherapy with ¹²⁵I seeds

Abstract 

Objectives

To determine the relationship between dosimetric parameters obtained on postimplantation Day 0 and biochemical relapse-free survival (bRFS) in patients treated with ¹²⁵I transperineal interstitial permanent prostate brachytherapy (TIPPB).

Methods

Two-hundred twenty men with low-risk (n=155, 70.4%), low-volume intermediate-risk (n=63, 28.7%), or high-risk (n=2, 0.9%) prostate cancer were treated with TIPPB between December 2000 and June 2006. Seventy-four (33.6%) patients received short-term (3–6 months) androgen suppression therapy before TIPPB. The median followup for patients free of biochemical failure was of 37.9 months (range, 24.0–84.5 months).

Results

The receiver operating characteristic (ROC) analysis established a best-fit cutoff value for the quantifiers D₉₀ and V₁₀₀ of 147Gy and 92%, respectively. The Kaplan–Meier analysis of bRFS at the cutoff value of D₉₀=147Gy using the ASTRO, nadir+2, and combined (ASTRO and nadir+2) definitions showed a trend toward statistical significance for the ASTRO (p=0.076) and nadir+2 (p=0.064) definitions and a statistically significant correlation for the combined definition (p=0.033). The corresponding 7-year bRFS for the D₉₀ >147Gy and D₉₀ ≤147Gy subsets using the ASTRO, nadir+2, and combined definitions were 96.5% vs. 89.7% (ASTRO, p=0.076); 93.7% vs. 70.5% (nadir+2, p=0.064); and 94.4 vs. 75.5% (combined, p=0.033). The V₁₀₀ (%) cutoff value of 92% predicted by the ROC analysis was not significant. Among other cutoff values, only D₉₀=140Gy (p=0.050) and D₉₀=160Gy (p=0.098) showed a trend toward statistical significance when the nadir+2 and the ASTRO definitions were used. The rest of dosimetric, tumor, and patient parameters did not show statistical correlation with bRFS in the Kaplan–Meier analysis.

Conclusions

The cutoff value of D₉₀=147Gy obtained on postimplantation Day 0 showed a trend toward significant correlation with bRFS when the standard ASTRO and nadir+2 definitions were used and a weak but statistically significant correlation with bRFS as per the nonstandard combined definition in a series of patients with predominantly low-risk disease (70.4%) treated at high radiation doses (median D₉₀=152.9Gy, median V₁₀₀=92.5%).

Keywords: Brachytherapy, Prostate cancer, ¹²⁵I, Monotherapy, Implant quality, Quantifiers, D₉₀, V₁₀₀ (%)