Parameters predicting for prostate specific antigen response rates at one year post low-dose-rate intraoperative prostate brachytherapy

Abstract

Purpose: To develop a model for prostate specific antigen (PSA) values at one year among patients treated with intraoperatively planned ¹²⁵I prostate brachytherapy (IOPB).

Material and methods: Four hundred and deven patients treated with IOPB for prostate adenocarcinoma were divided into four groups: those with PSA values ≥ 3 ng/ml; < 3 and ≥ 2; < 2 and ≥ 1 or PSA < 1 between 10.5 and 14.5 months post implantation (1yPSA). Ordinal regression analysis was then performed between patient, tumor, and treatment characteristics. 1yPSA values were also compared with toxicity outcomes.

Results: Median 1yPSA was 0.77 (0.04-17.36). Thirty-two patients (8%) had a PSA ≥ 3; 35 (9%) had PSA < 3, ≥ 2; 87 (21%) had PSA < 2, ≥ 1, and most patients 254 (62%) had PSA < 1. PSA response was independent of gland volume, Gleason score, clinical stage, seed activity, V₉₀, V₂₀₀, D₉₀, or number of needles and seeds used. Older patients had significantly lower 1yPSA; median ages 65.1 (46.5-81.0), 62.1 (50.4-79.5), 60.5 (47.1-80.3), and 58.1 (45.1-74.2) years for each of the 1yPSA groups respectively (p < 0.001). Also, both implant V₁₅₀ (p < 0.001) and initial PSA values (p = 0.04) were predictive of 1yPSA values. There was no correlation between 1yPSA values and toxicity encountered.

Conclusions: PSA response at 1 year post IOPB appears to be dependent on patient age, initial PSA, and implant V₁₅₀. Our results provide reassurance that parameters other than biochemical failure influence 1yPSA values.

Keywords: LDR; PSA; brachytherapy; prostate cancer; seeds.

Fig. 1

Cumulative probability cross sections for 1yPSA groupings for varying A) age, B) initial PSA, and C) V₁₅₀. Each plotted line represents the probability that a patient with the given variable characteristic (e.g., age 40 vs. 80) and static characteristics (e.g., initial PSA 5.8 and V₁₅₀ of 76.6) will have a final PSA in the given ordinal group or any higher group. Hence, there is always a 100% probability that a patient will be in group one or higher. However, as seen in our example, there are 96% and 10% chances that the 40 and 80 year old’s will be in groups 2, 3, or 4. Hence, there are 4% and 90% chances of 40 and 80 year old’s having PSA < 1. Plot A) fixed PSA of 5.8 and V150 of 76.6. Plot B) fixed V₁₅₀ of 76.6 and age of 63.3. Plot C) fixed initial PSA of 5.8 and age of 63.3

Fig 2

Time to PSA nadir < 0.2 ng/ml in months (from the time that the 1yPSA was measured) as a function of 1yPSA grouping. Patients with PSA at 1 year < 1 ng/ml had significantly shorter times to nadir than all other patient groups 60

Fig. 3

Linearized regression modeling of relationship between 1yPSA and time to meet A) PSA nadir of < 0.2 ng/ml, and B) PSA nadir of < 0.5 ng/ml. These times are in months and are from the time of measurement of the 1yPSA