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

ObjectivesTo 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).MethodsTwo-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).ResultsThe receiver operating characteristic (ROC) analysis established a best-fit cutoff value for the quantifiers D₉₀ and V₁₀₀ of 147 Gy and 92%, respectively. The Kaplan–Meier analysis of bRFS at the cutoff value of D₉₀ = 147 Gy 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 >147 Gy and D₉₀ ≤147 Gy 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 = 140 Gy (p = 0.050) and D₉₀ = 160 Gy (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.ConclusionsThe cutoff value of D₉₀ = 147 Gy 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 = 152.9 Gy, median V₁₀₀ = 92.5%).