Low Dose Rate Brachytherapy Monotherapy at the Mayo Clinic

The Mayo Clinic reported the 10-year oncological results on 974 consecutive low and intermediate risk patients treated with low dose rate brachytherapy (I-125 seeds) monotherapy from 1998 to 2013.

Patients were treated as follows:

• 90% of the prostate received 159 Gy (median dose) of 1-125 seeds
• 26% received some ADT (mainly to shrink the prostate)

Patient characteristics (number (%)) were as follows:

• Gleason 6: 783 (80%)
• Gleason 3+4 :153 (16%)
• Gleason 4+3:  38 (4%)
• Stage T 2b or 2c: 24 (2.5%)
• PSA ≥ 10: 93 (10%)
• Low risk: 693 (71%)
• Intermediate risk: 281 (29%)

While they did not routinely collect data on the percent of positive biopsy cores, they did define an "unfavorable intermediate risk" cohort as having Gleason 4+3 or multiple intermediate risk factors.

With median follow-up of 6 years, there were only 45 cases of biochemical failure, and 10 deaths from any cause. The 10-year biochemical recurrence-free survival was:

• 85% overall
• 90% among low risk men
• 74% among intermediate risk men

The following hazard ratios were significant on multivariate analysis:

• Gleason score 4+3: 7.0
• Use of ADT: 0.3
• PSA (per unit increase): 1.17
• Unfavorable intermediate risk : 3.75

Predominant Gleason pattern 4 also affected the rate of distant metastases and prostate cancer-specific survival. Use of ADT did not. Local recurrence was only 2%.

Radiation dose was consistently escalated, so the effect of dose differences failed to meet statistical significance. Patient selection has changed over the years. Low risk men are routinely steered towards active surveillance. Multiparametric MRI is now used to stage intermediate risk candidates in order to find those where cancer is likely to have escaped the prostate capsule or penetrated the seminal vesicles -- those patients may be offered multimodal radiation with both external beam therapy and a brachy boost. While use of monotherapy was rare among those with GS 4+3, it is probably much rarer today. Monotherapy seems to be sufficient in favorable intermediate risk men.

This study had similar results to those reported by Cleveland Clinic (see this link). It also affirms that monotherapy is all that's needed for favorable intermediate risk (see this link), and that brachy boost therapy is needed for unfavorable risk patients (see this link). These are reflected in current guidelines (see this link). The use of ADT beyond cytoreduction does not seem to be necessary, at least in high risk men receiving brachy boost therapy (see this link). This study did not address the toxicity of brachytherapy, which should be discussed with one's brachytherapist.

With thanks to Brian Davis for allowing me to read the full text of his study.