Cesium-131 (131Cs) has recently been introduced into the market by Isoray Medical Inc. (Richland, WA). Due to its short half life (9.7 days) and higher average energy (30.4 keV), 131Cs potentially has some advantages over routinely used isotopes 125I and 103Pd for low-dose-rate prostate brachytherapy. The purpose of this study was to independently generate a 131Cs activity to prostate volume nomogram based on our stranded-seed modified peripheral loading implant technique and to compare this nomogram to existing 131Cs nomograms. Eighty consecutive patients who had recently undergone 131Cs prostate brachytherapy on an institutional phase II protocol were selected for this study. The patients were treated using a modified peripheral loaded stranded-seed technique. Plans determining the number of seeds and needle loading were generated using the Variseed treatment-planning system (Varian Medical Systems, Inc., Palo Alto, CA). Approximately 30 % of the seeds were implanted within the PTV in the periprostatic tissue. The prescribed dose was 115 Gy for all patients. Patients were implanted with 131Cs (Model Cs-1) seeds with an activity of 2.0 U/seed (3.135 mCi/seed). The mean prostate volume was 35.1 ± 8.7 cc (Range: 18.2 – 63.2 cc) and the mean number of planned sources was 81 ± 12 seeds (Range: 56 – 118 seeds), with an average total activity of 161.9 ± 23.6 U (Range: 112 – 236 U). For the range of prostate volumes in this study, the activity in U could be determined using a best fit power law equation of 22.1 (prostate volume)0.5616 with an R2 of 0.93 or using a linear fit equation of 2.613 (prostate volume) + 70.221 with an R2 of 0.93. Our nomogram was approximately 2 to 6% higher compared to the Isoray unpublished nomogram and was approximately 15% lower for smaller prostates (20 cc) and 15% higher for larger prostates (60 cc), with identical activity for prostate volumes of 40 cc, compared to the Cesium Advisory Group’s published nomogram. We have characterized the amount of activity per volume necessary to achieve consistent high quality implants using 131Cs stranded-seeds with a modified peripheral loaded technique. The differences seen between nomograms, reflects the different treatment planning and implant styles (such as loose seeds versus stranded seeds, planning target volume margins, etc.). The nomogram acts as a secondary dosimetry check to independently verify the accuracy of the treatment planning system determined dose calculations.