NEW ORLEANS—Radioactive seed implantation alone continues to prove effective in low-risk prostate cancer, but optimal dose is important in achieving a good outcome, according to a report from Mount Sinai School of Medicine, New York, presented at the American Society for Therapeutic Radiology and Oncology (abstract 55).
"Low-risk patients with optimal dose implants can achieve a 94% biochemical freedom from failure rate at 8 years. Despite optimal dose, intermediate- and high-risk patients do not appear to be optimally treated with seed implants alone," said Marisa A. Kollmeier, MD, of the Department of Radiation Oncology at Mount Sinai.
The study included 243 patients with clinically localized prostate cancer treated with permanent radioactive seed implantation. At 1 month, they underwent CT-based dosimetric analysis. Follow-up ranged from 61 months to 135 months (median, 75).
At presentation, 49% of patients had stage T2a or lower prostate cancer (n = 120), and 52% had stage T2b to T2c (n = 123). Most patients (78%) had a Gleason score of 6 or less, and 61% had an initial PSA level of 10 ng/mL or less (median, 8.2 ng/mL).
Implant dose was defined as the D90, or dose delivered to 90% of the prostate volume on postimplant dosimetry. Based on prior analyses, patients were divided into the following dose groups using two isotopes—optimal D90: 125I greater than or equal to 140 TG-43 Gy or 103Pd 100 Gy or more; suboptimal: D90: 125I less than 140 TG-43 Gy or 103Pd less than 100 Gy.
A dose of 160 Gy was prescribed for the 138 patients receiving 125 I and a dose of 115 Gy was prescribed for the 105 patients receiving 103Pd. Forty percent of patients also received hormonal ablation at least 3 months before and 2 to 3 months following seed implantation. No patient had external beam radiation. Biochemical failure was defined by the ASTRO definition.
Patients were divided into three risk group categories:
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Low: Stage T2a or lower, Gleason score 6 or less, or PSA 10 ng/mL or less (n = 75)
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Intermediate: Stage T2b, Gleason score 7, or PSA 10.1 to 20 ng/mL (n = 70)
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High: Presence of two or more intermediate-risk factors or Gleason score 8 or higher or PSA greater than 20 ng/mL (n = 98)
At 5 years, 56 patients developed a PSA failure, with a median time to failure of 24.3 months. When stratified according to risk groups, 84% of failing patients fell within the intermediate- and high-risk groups. At 8 years, freedom from biochemical failure rates were 88% for the low-risk group, 81% for intermediate risk, and 65% for high risk.
Several prognostic factors were statistically significant. By stage, 85% of patients stage T2a or less were free from biochemical failure at 8 years, compared with 69% of patients stage T2b or higher. By grade, 81% of patients presenting with a Gleason score of 6 or less were free from biochemical failure vs 67% with a Gleason score of 7 and 53% with a score of 8 and above, Dr. Kollmeier reported.
Initial PSA was also a significant prognostic factor. At 8 years, freedom from biochemical failure was seen in 80% of patients with a PSA of 10 ng/mL or less; 86% with a PSA 10.1 to 20 ng/mL; and 45% with a PSA greater than 20 ng/mL.
Dose Optimization
When analyzed according to dose group, 82% of patients with optimal dose implants were free from failure at 8 years, vs 68% with suboptimal doses. Freedom from biochemical failure at 8 years in the low-risk group was 94% for those receiving an optimal dose vs 75% for those receiving a suboptimal dose (see Figure), Dr. Kollmeier reported. All of these patients were treated with brachytherapy alone.
In the intermediate-risk and high-risk groups, brachytherapy dose optimization did not make a significant difference, though the percentages were more favorable for the optimal dose groups. In the intermediate-risk patients, 83% of optimal-dose patients were free from failure, compared with 79% receiving suboptimal doses; in high-risk patients, these figures were 70% and 59%, she said.
"In the multivariate analysis, initial PSA, Gleason score, and dose group were significant predictors of outcome. Both implant dose and risk group stratification are important in the optimization of brachytherapy," she concluded.
